Download Chapter 1: Introduction and Background

Chapter 1
Plan for this chapter:
1. Background on the Atmosphere
2. The Circulation of the Atmosphere
3. Middle Atmospheric Chemistry
4. HALOE and SOFIE Satellite Measurements
5. Thesis Plan
1. Introduction and Background
The Earth's atmosphere is a nitrogen-oxygen mix (about 78%-21%) with traces of
other gases and aerosols (suspended liquid or solid particles) (see Table 1). These gases
are generally well-mixed by circulation processes which dominate dissipation in the
turbosphere, below about 120 km; but, it is not completely homogenous (Houghton, p.
58).
I was probably looking more for differences than similarities:
“The amount of mixing in the lower atmosphere ensures that no significant diffusive
separation occurs between the heavy atmospheric constituents (e.g. argon) and the light
ones (e.g. hydrogen). For practical purposes, therefore, the main constituents of the
lower atmosphere are uniformly mixed apart from those which are involved in phase
changes (e.g. water vapour) or chemical changes( e.g. ozone).”
“The lower boundary of the region above which molecular diffusion dominates is ~120
km altitude and is known as the turbopause (sometimes known as the homopause: the
region above, where molecular diffusion is dominant, is sometimes known as the
homosphere).” [Hougthon, p. 58]
(In addition to regional differences in humidity, and city-localized smog, there was a
catastrophic incident of outgassing carbon dioxide from Lake Nyos in Camaroon Africa.)
Trace gases present in very small proportions are measured in small mixing ratio units
like ppmv (parts per million by volume). Generally, the atmosphere is given structure by
gravitational influences (by earth, moon and sun), the solar energy input from the sun
(especially with respect to the earth's shape and orientation with respect to the sun), and
the earth's rotation.
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Table 1. Fractional abundance of important
atmospheric gases [Houghton p226]
Nitrogen
Oxygen
Argon
Carbon dioxide
Neon
Helium
Krypton
Xenon
Hydrogen
Methane
Nitrous oxide
Carbon monoxide
0.78083
0.20947
0.00934
0.00033
18.2 E-6
5.2 E-6
1.1 E-6
0.1 E-6
0.5 E-6
2 E-6
0.3 E-6
0.1 E-6
The state of the atmosphere is described by its properties, such as volume,
pressure, temperature, composition (gas and aerosol) and velocity, with respect to
location. A profile is a description of properties with respect to location, particularly
altitude. Various physical influences and chemical processes act to maintain or change
individual and collective properties of the atmosphere. The amount of any particular
substance in a parcel of air depends on the initial amount, plus changes due to circulation
of material (gas) in and out of the parcel and sources and sinks for that particular
material. For example, the composition in a particular location is maintained or changed
according to:
current state = previous state + sources – sinks + inflow – outflow,
where the sources and sinks are due to physical or chemical processes and the flow is due
to air circulation.
Some of the fundamental influences on the atmosphere are the earth’s gravity,
rotation, tilt and orbit, incoming solar radiation (insolation), and to a lesser extent the
tidal influences from the sun and moon. The daily and annual cycles of rotation and orbit
are the most important and obvious. The near-circular orbit produces a near-symmetry in
the atmosphere such that the two hemispheres follow the same seasonal cycles with a
half-year delay. There are some influences that vary over time, but so slowly compared
to the time period for the data used for this work that they can be considered constant.
Among those that could be significant are changes in the sun’s output over its 22-year
sunspot cycles and the monthly lunar tides (Britt, 2003, lunar tides?). While considering
what physical influences act upon the atmosphere, it is important to consider whether
those influences are constant or variable. The 22-year sunspot cycle is well-known and is
on a timescale similar to the 14-year HALOE data set. The spots are darker because they
are cooler. This suggests a difference in energy that reaches the earth—at least in the
distribution of visible wavelengths. If there is a measurable effect, then one would
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naturally anticipate the effect would be most distinct between solar extremes—in this
case, between solar min and solar max which are about a quarter of the full solar cycle
apart.
Recent solar minima have shown an increasing trend for Total Solar Irradiance (Britt,
2003, acrim.com) though previously the first five years of data showed a downward trend
[Willson, 1986].
Solar and lunar tides observably affect the oceans of water on the earth’s surface, so it
follows that the same forces which act on the matter of the liquid oceans will act upon the
atmosphere as well (and they act on the solid earth too).
maybe: “… it is more accurate to state that as far as the conditions of ice particle
formation are concerned, the summertime mesopause region is in a rather chaotic state of
motion. It is now understood that most of this activity is due to gravity waves and tides
with only a minor influence from planetary waves.” [Thomas, 1991, p. 559a]
The question then becomes one of magnitude of the differences in influence and of the
atmosphere’s response.
“The H2O dissociation rate is chiefly determined by the Lyman- radiation above
roughly 70 km altitude. …. The solar Lyman- radiation varies strongly with the solar
activity approximately by a maximum factor of two between solar activity minimum
and maximum …” [Sonneman, 2005]
“The Active Cavity Radiometer Irradiance Monitor (ACRIM) I instrument was the
first to clearly demonstrate that the total radiant energy from the sun was not a
constant. However, the solar variability was so slight (0.1% of full scale) that
continuous monitoring by state-of-the-art instrumentation was necessary. It is theorized
that as much as 25% of the anticipated global warming of the earth may be solar in
origin. In addition, seemingly small (0.5%) changes in the TSI output of the sun over a
century or more may cause significant climatological changes on earth.” [NASA,
ACRIMSAT]
“Stratospheric ozone is affected by solar cycle variations through changes in the UV
fluxes which affect the photo-dissociation of chemical species. Changes in stratospheric
dynamics resulting from solar cycle variations, are another possible cause and
consequence of ozone variation. The impact on the total ozone column through 10 such
variations has been demonstrated through both observations and model studies
(Brasseur, 1993;
Jackman et al., 1996;
Zerefos and Crutzen, 1975;
van Loon and Labitzke, 1994;
Zerefos et al., 1997;
Hood, 1997;
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Haigh, 1994;
Shindell et al., 1999).”[Isaksen, 2008]
For the lunar tides
Earth’s gravity is probably the simplest to understand influence on the
atmosphere. It holds the gases close to the earth. At each point in the atmosphere, the
overlying air mass weighs down on it, making the pressure greatest at the deepest points
of the earth’s surface and making it decrease with altitude like an exponential ‘decay’.
(This does not define a top, but some models extend to at least 1000 km [e.g. MSISE-90
mentioned in the thermal profile figure.].) Gases naturally tend to form layers according
to their relative densities (by dissipation processes) when settling under the influence of
gravity; however, the earth's rotation and insolation drive air circulation and make mixing
of gases a dominant process in the lower ~120 km, a region so called the turbosphere
(Houghton, p. 58). Apparently [Brasseur 1986] & Solomon put it about 100 km on p.
126. “In the vicinity of the homopause (transition between the homosphere and the
heterosphere near 100 km), vertical mixing (which lead to a uniform mixing ration of
major gases such as N2 or O2 below 100km) is gradually replaced by diffusive separation
of atmospheric constituents according to their molecular mass. As a result, the lightest
compounds (such as the hydrogen and helium atoms) are transported upwards and their
relative abundances increase with altitude. Conversely, the heaviest species (such as N2
or O2) are transported downwards and, as a result, their mixing ratio decreases very
rapidly with height. Figure 3.37 shows an example of the vertical profiles derived
for several compounds between the surface
and 1000 km altitude.”
[Wayne, p. 40]
“The region of transition in an atmosphere between turbulent mixing and molecular
diffusion is known as the turbopause (or, sometimes, homopause.)
The rotation of the earth’s surface topography contributes to the circulation of the
atmosphere and generates waves within it. Insolation drives (contributes to) atmospheric
heating and circulation through absorption (by gas and aerosol) and photochemistry and
through evaporation of surface water, but insolation varies according to location on the
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earth’s curved surface and the earth’s rotation and orientation with respect to the sun.
Injection processes such as volcanism and the burning of incoming meteoric material
affect the atmosphere by changing its composition directly.
The earth's gravitational pull on the atmosphere produces a density distribution
that is like an exponential decay because any part of the atmosphere is pressed upon by
the weight of all overlying atmosphere. The air tends to layer according to the densities
of its constituent gases through dissipation, but this happens more in the upper
atmosphere than the lower 100 km, which is called the turbosphere because it is where
circulation processes work to mix the air. Also, the familiar tidal influences the sun and
moon exert on the oceans act on the atmosphere (and solid earth) too.
The earth's rotating surface tends to pull (shear) the nearby layers of atmosphere
along with it, and topographic textures generate disturbances that propagate as waves
through the atmosphere. Each individual molecule of the rotating atmosphere would tend
to continue on a straight path away from the rotation axis in a line tangent from the
earth's surface, but gravity holds it back (and collision with earthward moving molecules
from higher altitudes knocks it back too). The combination of rotation and gravity should
produce equator-ward flow near the surface, and pressure effects should produce return
flows at higher altitudes (reference
I tried to reason out how things would work mechanically, but maybe it would be better
to refer to Brewer-Dobson circulation
The simplest conceptual model of an atmosphere would be one with minimal influence
factors, for example, imagine a lonely planet with no sun nor moon that does not rotate.
If gravity only acts on the atmosphere, there would not be much to circulate it.
A next step in developing this concept model is to add in another single influence.
Incorporating rotation, the atmosphere’s molecules would tend to move in a straight line
by inertia, away from the rotation axis. (A wet dog or cat uses this principle to dry by
shaking its head and propelling the water away.) Gravity pulls the atmosphere towards
the earth’s center—not just towards the axis. The outward motion of inertia and axial
inward component of gravity work partly against each other but there is no counteracting
force for the component of gravity towards the equatorial plane through the earth’s
center. As air moves towards the equator and piles up, it should increase the pressure.
Air; that has moves away from the poles will have decreased the pressure there. Wind
flows from higher towards lower pressure. Since air has been moving along the surface
towards the equator, the only place left for a return flow is at a higher altitude. Coriolis
effects will complicate the flow, of course, and that would be dependent on rotation
speed.
[Houghton p. 70]
This does not explain the causal mechanism of Brewer-Dobson circulation, but it must
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have been empirical at least.
“Although on photochemical theory it would be expected that the maximum ozone would
occur in tropical regions where there is maximum solar radiation and a minimum in polar
regions, in fact the reverse is the case. To account for this Brewer (1949) proposed what
has become known as the Dobson-Brewer circulation in which air enters the stratosphere
through upward motion in the tropics. Ozone-rich air is then transported polewards and
downwards so that a large concentration of ozone occurs at high latitudes in the lower
stratosphere where its photochemical lifetime is very long. Observations of the ozone
mixing –ratio provide an important tracer of these motions.”
This covers pressure effects for poleward motion:
[Wayne, p. 66]
talks of temperature differences between land and sea and day flow from land to sea and
night flow from sea to land.
“On a global scale, exactly the same kind of motions occur, with hot air rising near the
equator and being forced towards the cooler poles by the higher pressure in the higher
temperature regions. Each hemisphere has its own circulation cell—named after its
discoverer, a Hadley cell…”
Perhaps Brewer-Dobson circulation is partly caused by a portion of Hadley circulation
that survives beyond the tropopause cold trap.
Energy from the sun is another driving influence on the earth’s circulation. It
heats the earth’s surface, land and ocean, and evaporates water. The heating of air
increases its volume and reduces its density, which makes the air lighter and rise with
respect to cooler heavier surrounding (or overlying) air. Additionally, water vapor
displaces ‘normal’ dry air, and moist air rises through buoyancy because its molecular
mass is only 18 g/mol where dry air’s mass is 29 g/mol. As air rises in the atmosphere, it
expands and so tends to cool. The temperature decreases with height in the troposphere,
and when radiative heating from the surroundings, sun or surface and heating from
internal chemical reactions cannot balance the cooling, water vapor cools to condensation
and relinquishes its latent heat. So, higher in the troposphere, the air becomes relatively
cooler and drier, thus denser, and convection slows.
Most of the moisture has rained out by the time the rising air reaches the
tropopause, a cold layer of atmosphere atop the air circulation cells that maintains a water
vapor level of about 3 ppmv and where the temperature is a local minimum (about 210 K
average from U.S. Standard Atmosphere 1962, 180K minimum from HALOE
temperature plots). Vertical motion slows very much because the rising air has reduced
advantage from lowered water vapor buoyancy and it must compete against warmer air
higher in the stratosphere. Consequently, much of the tropospheric airflow is diverted
horizontally and usually poleward (reference).
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Okay, ‘poleward’ is just wrong for the Ferrell cell. Probably I was fixated here on the
Hadley cell. Diverted horizontally would be because the air has to go somewhere and it
has a harder time going up… continuity
[Wayne, p. 66] talks of temperature differences between land and sea and day flow from
land to sea and night flow from sea to land.
“On a global scale, exactly the same kind of wind motions occur, with hot air rising near
the equator and being forced towards the cooler poles by the higher pressure in the
higher temperature regions. Each hemisphere has its own circulation cell—named after
its discoverer, a Hadley cell…”
stronger support:
“Because temperature increases with altitude in the stratosphere, warmer air overlays
colder air. As a result of this temperature structure, convection never happens in the
stratosphere. If we could displace an air parcel to a higher altitude in the stratosphere, it
would be colder than its surroundings. Cold air is more dense than warm air, and the
parcel would sink back to its original location, though it would overshoot slightly because
of its momentum.” [NASA SEES website, 2000]
“We also not that the stratosphere throughout possesses a highly stable stratification with
values of potential temperature far higher than is typical in the troposphere. Vertical
motion is therefore severely inhibited. … Clearly, however, in the winter hemisphere heat
is being transported from low to high latitudes by dynamical processes. ” [Houghton, p.
157] On the next page it says that a simple meridional circulation cell would not
conserve momentum nor necessarily transport enough heat during winter. [Houghton, p.
159]
[Wayne, p. 67] not as good support:
“Hadley circulation might well consist of two cells each encompassing a hemisphere
from equator to pole, and with the flows directly north-south. For Earth, the real Hadley
circulation only extends within the tropics (up to the ‘horse latitudes’), and it has a strong
westerly component aloft, and a corresponding easterly component on the return surface
flow. Both the westerly directional component and the limited span of the Hadley cell
are a consequence of planetary rotation. Atmospheric gases posess mass, and if they
rotate more or less with the planet they therefore posess angular momentum. North-south
motions imply a change in radius of rotation, decreasing to near zero at the poles. Yet
angular momentum must be conserved, and the atmosphere achieves this conservation by
developing zonal motion (that is in the direction of rotation of the Earth).
The increase in temperature in the stratosphere results from absorption by various gases
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of solar energy and heat energy produced by and reflected from the earth and surrounding
parts of the atmosphere.
Incoming solar radiation (insolation) heats the atmosphere directly as gases in the
atmosphere absorb particular wavelengths (e.g. greenhouse gases, photochemistry
reactants) and indirectly through the heating of the earth’s surface and aerosols in the
atmosphere. Light energy is also redistributed by reflection and scattering from aerosols
and by absorption and re-emission by certain gases, like the (certain polyatomic)
greenhouse gases do for infrared energy.
Greenhouse gases are so named is because they absorb infrared wavelengths of
heat radiated from the earth’s surface and re-emit the infrared photons both upwards and
downwards: since part of the energy is returned towards the surface, the gas helps the
earth retain warmth, like a greenhouse. However, the same greenhouse gases can absorb
and re-emit incoming solar radiation as well, and in that case the net effect is to partly
shield the surface (and lower regions of the atmosphere) because part of the energy is
emitted upwards. The upward-directed energy might participate in some processes in
higher parts of the atmosphere or be radiated out to space, which is a cooling process.
How much and where the atmosphere is heated by chemistry or sunlight processes
(photoabsorption) depends on how and where the gases are distributed in altitude. The
major greenhouse gases in the earth’s atmosphere are water vapor, carbon dioxide,
methane, and ozone.
Of the earth’s 342 W m –2 of visible insolation (1370 W m –2 of all wavelengths
[Houghton, p2]), 107 W m –2 is reflected by clouds, aerosol, atmosphere and surface; 67
W m –2 is absorbed directly by the atmosphere and 168 W m –2 is absorbed directly by the
surface. About 235 W m –2 of the visible insolation is ‘processed’ by the earth and
atmosphere and leaves as infrared. [Lunine Fig 14.4 based on Trenberth et al 1996]
Heating through photoabsorption has an effective depth range, and it depends
upon the incident light spectrum and intensity, the distribution (concentrations and
locations) of gases and aerosols, and the absorption response (e.g. how well the gases and
aerosols absorb, reflect, or scatter certain wavelengths of light). The filtering effects of
the atmosphere and the strong absorption and re-radiation of the earth’s surface (and
circulation and pressure profile + phase change + gravity waves) help to produce a
distinctly layered temperature structure as shown in Figure 1.
The thermal profile (how temperature varies with respect to location, particularly
altitude) allows characterization of layers according to whether the temperature generally
decreases or increases with altitude. Between these layers are (thinner) boundary layers
where the temperature ‘pauses’ around a local minimum or maximum. These commonlyused layer names are, from the surface upward: the troposphere and tropopause, the
stratosphere and stratopause, the mesosphere and mesopause, and the thermosphere.
(The thermosphere, thermopause and exosphere extend beyond the turbosphere). The
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different thermal gradients promote differences in air circulation in the layers,
particularly with respect to vertical motion.
The thermal profile in Figure 1 is an average of a global model reduced to the one
dimension of altitude. Added in circles is a model profile local to a particular latitude,
longitude and time. (The atmosphere is fluid, so the profile can change in response to the
many influences and processes at work.) In general, the tropopause is lower near the
poles and higher near the equator (reference).
It seems there was a different thermal profile diagram that had two heights for the
tropopause. I had noted equatorial height about 18 km and polar about 10 km, but I
didn’t note the reference.
Here is a nice diagram on the NASA SEES website that is even more informative:
[NASA SEES, 2000]
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Figure 1. Standard temperature model with a local temperature overlay(MSIS-E-90)
(U.S. Standard Atmosphere 1962 chart from http://history.nasa.gov/SP-367/chapt2.htm )
Overlay (circles) MSIS-E-90 model data from http://ccmc.gsfc.nasa.gov/modelweb/
(choose MSISE-90 Model [info, ftp, RUN ] to get to the menu at
http://ccmc.gsfc.nasa.gov/modelweb/models/msis.html. Use the default parameters,
except for using stepsize=1: January 1, 2000, UT 1.5, geographic coordinate: 55
latitude, 45 longitude; height 100, start 0, stop 1000, step 1; output {O, N2, O2, Total,
Tn, T_exos}.)
Sunlight also affects the atmosphere more directly. Certain wavelengths of
sunlight are absorbed by the atmosphere: some drive photochemical reactions or
photodissociate compounds, and some are simply absorbed and reemitted (as particular
infrared wavelengths are by greenhouse gases). Some energy is simply absorbed to
produce heat, like onto aerosols of meteoric or desert dust. Of course, the absorbed
energy is depleted from the sunlight that reaches deeper into the earth’s atmosphere, even
more so when the absorber is a more abundant part of the atmosphere’s composition or
denser as the atmosphere is at lower altitudes. So, the upper parts of the atmosphere
(closer to the light source) act as a filter for the lower parts of atmosphere. This would
naturally produce a layering even in an initially uniform-composition atmosphere, and
that layering depends upon the intensity distribution of the incoming wavelengths and the
wavelength response of the absorbing gases or aerosols. The resultant changes to
atmospheric composition indirectly affect equilibrium in non-photosensitive chemical
reactions too, by changing relative concentrations of certain reactants and/or products.
Circulation of the compounds into or out of particular regions of the atmosphere will
influence the chemistry as well.
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The earth is spheroid in shape and orbits the sun, nearly circularly, with a tilted
rotational axis (Figure 2). Consequently, the sunshine that reaches it is strongest and
most intense at the solar zenith point (e.g. the point where the sun’s rays reach the earth
perpendicularly to the surface) and its intensity diminishes towards the earth’s limb
(horizon), both because the surface is at an angle to the sunlight and because the sunlight
has had to traverse more absorbent atmosphere (at an angled approach) before reaching
the surface. As the earth rotates, the solar zenith point traces a path through a zone of
latitude, and so sets the position of the heat equator. Since the earth’s axis is tilted, the
solar zenith point and the heat equator cycle north and south across the tropics as the
earth orbits the sun over the course of each year… drawing the Hadley cell upwelling
along behind it.
Figure 2. Orientation of the Earth during a year.
Remarkably, for 7½ weeks (about 53 days) of summer the north pole receives
more solar energy in its 24-hour day than sunny Mexico does in a 12-hour day,
neglecting atmospheric shielding. During this period the relative earth-sun tilt exceeds
21 degrees and the pole receives more sunlight per day than any other place on earth.
The same occurs for the south pole in its summer. Figure shows the relative insolation
on a sphere calculated as the dot product of the local normal and sunward unit vectors at
1° latitude ×1° longitude patches (neglecting atmospheric influence) summed over 1°
latitude zones. The values represent the relative insolation totaled in the zone, and
equivalently that accumulated by a 1°×1° patch over a day’s rotation. (Multiply the
values by the solar flux of 1370 W/m2 [Houghton p 2] to scale them to (unfiltered) solar
intensity.) The dip in the graph locates a ‘cold zone’ that represents the edge of
night/twilight just outside the 24-hour-day illuminated area.
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Figure 3 Zonal insolation vs latitude at various tilt angles through year
While a location on the heat equator does receive the most direct sunshine each
day, it does not receive the most sunshine every day—that honor of the most sunshine in
a day is held by the earth’s poles, which for a period of time at the height of summer
(near solstice) receive more accumulated sunlight in their twenty-four-hour days than a
location on the heat equator does in its twelve hour day (about 27% more, neglecting
atmospheric absorption). Of course, the cost is a six-month cold and dark winter night.
Figure 3 demonstrates the relative variation of insolation as a function of latitude.
The factors that produce the earth’s climate are all variable: the obvious
variations are the daily and annual cycles that come from the earth’s rotational and orbital
periods made strong by the sun’s energetic influence, whereas the earth’s tilt with respect
to its orbital plane varies by about 2.5° over thousands of years. The sun too experiences
a 22-year solar cycle (an 11-year sunspot cycle without polarity) during which its output
changes. While the recent trend of solar output is disputed between 0.05% decadal
increase and an unspecified decrease, the energy input into the earth’s atmosphere and
surface can vary during the solar cycle [Britt 2003, Black 2005].
The seasons where one hemisphere tilts towards (and the other away from) the
sun are accompanied by development of a global mesospheric circulation cell
characterized by airflow upwards in the summer polar stratosphere and mesosphere,
circulation across the equator, and downwards in the winter polar region [Houghton, p.
157]. Though it is tempting to attribute the upward convection simply to a constantly
heated summer polar surface, the motion is instead explained as part of a response to
gravity waves [Thomas 1991, p 561a] propagating vertically upward through the
summer-altered atmosphere that “[exert] a net body force on the atmospheric flow,
tending to drive the flow velocity to that of the wave phase velocity [Lindzen, 1981]”
[Thomas 1991, p561a].
The temperature and density are factors that should affect the atmosphere’s
properties as a wave-propagating medium. The heat equator and the circulation cell
bands are also crowded towards higher latitudes in the summer hemisphere, perhaps
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increasing the activity and/or the effectiveness of the activity which produces the gravity
waves.
Warmed air is less dense than cool air at the same pressure (the volume increases
by heating approximately according to the ideal gas law PV=nRT), so it tends to be
buoyed up by cooler denser air which will descend according to the same principle and
gravity. The composition of air affects its density too: in particular, increased water
vapor content decreases the density of air from that of dry air. The atomic mass of dry air
is roughly 29 g/mol (80% * 2 N @ 14 g/mol + 20% * 2 O @ 16 g/mol), and water’s
atomic mass is 18 g/mol (2 H @ 1 g/mol + 1 O @ 16 g/mol). When water vapor mixes
into a volume of air, the water vapor molecules displace about as many dry air molecules
resulting in a lighter-than-dry-air mix, even at the same temperature. Additionally,
warmed air has a greater capacity for water vapor, enabling increased buoyancy. So
warmed moist air will rise, and cooler drier air will descend.
As an air parcel rises to higher altitudes in the atmosphere (esp. in the
troposphere), it expands due to lower air pressure. Its heat energy spreads out over the
larger volume, resulting in lower temperature (unless more heat energy is added). As the
air cools, it becomes more likely that water vapor will condense: The decrease in
temperature also decreases the saturation vapor pressure—the level of maximum water
vapor the air can hold at that temperature. As the ambient air pressure decreases for the
rising parcel so too does the partial pressure of water. When the saturation and vapor
pressures are equal, water changes phase: from gas to liquid or solid while cooling or the
reverse if heat is added (general atmospheric thermodynamics reference needed here
Houghton, p. 20).
The decreasing atmospheric pressure profile implies a ‘ceiling’ for the liquid
phase at water’s triple point pressure of ~0.006 atm (611.73 pascals or 0.0060373057 atm
above which water can be present only as vapor or ice. This altitude is around 34 km.)
Condensation reduces the portion of water vapor in the parcel, thus reducing both the
total volume of the parcel and its buoyancy. The parcel then becomes part of the cooler
denser air that descends towards the surface, completing a cyclic process due to
buoyancy.
When a buoyant parcel of air rises and the ambient air temperature decreases
faster than the parcel’s temperature does, its buoyancy is enhanced; conversely, if the
parcel rises into increasingly warmer air, it has to compete more with the surrounding air
and so it slows. The thermal gradient in the mesosphere decreases with altitude
somewhat like the troposphere, so we might expect strong buoyancy effects there as in
the troposphere, but they may be gentler because the pressure and density are much
lower.
2. The Circulation of the Atmosphere
The general circulation is the result of all influences that act on the atmosphere.
The earth’s rotation and the geographic (latitudinal) distribution of insolation produce
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strong latitudinal features in the atmosphere and its circulation. Many of these features
change seasonally due to the earth’s tilt and its change in orientation as it orbits the sun.
The earth rotates against the air and drags air near the surface with it, making the
wind come from the east. Higher layers of the atmosphere rotate with the earth too, but
lag farther behind the lower layers. A centrifugal effect tends to push air away from the
rotation axis (e.g. the air mass tends to continue in the same direction): outward (and
equatorward) along the surface in the polar regions, and outward (upward) at the equator
(reference
This is like something previous—maybe redundant
[Wayne, p. 67] (copied from above where it was not as good a support
“Hadley circulation might well consist of two cells each encompassing a hemisphere
from equator to pole, and with the flows directly north-south. For Earth, the real Hadley
circulation only extends within the tropics (up to the ‘horse latitudes’), and it has a strong
westerly component aloft, and a corresponding easterly component on the return surface
flow. Both the westerly directional component and the limited span of the Hadley cell
are a consequence of planetary rotation. Atmospheric gases posess mass, and if they
rotate more or less with the planet they therefore posess angular momentum. North-south
motions imply a change in radius of rotation, decreasing to near zero at the poles. Yet
angular momentum must be conserved, and the atmosphere achieves this conservation by
developing zonal motion (that is in the direction of rotation of the Earth).
). In terms of a local reference frame on the surface, this is the coriolis effect that
deflects winds to the right in the northern hemisphere and to the left in the southern
hemisphere. The centrifugal effect also works on the earth’s mass against gravity and
makes the surface into a slightly oblate spheroid, larger at the equator and flatter at the
poles than a sphere.
The troposphere has developed three main latitudinal zones of cellular air flow.
Generally the meridional component of the air flow forms a loop: moving upward at one
edge (latitude) of a cell, across the top, downward at the other edge, and then back along
the surface towards the starting latitude. The features can be mapped in a meridional
plane according to latitude and altitude (see Figure 4).
This is indeed what happens in the troposphere’s atmospheric cell zonal bands,
e.g. the tropical Hadley cells and the polar cells, but there are three such bands in each
hemisphere rather than the one anticipated (Figure 4). The third circulation cell type is
the Ferrell cell: it is a secondary feature driven by circulation in the surrounding cells.
(The relative planet size, rotation speed, and atmospheric density or ‘viscosity’ are major
factors in the number of circulation cell bands, but the earth’s water also plays a
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significant role in circulation and may factor into the number of cells too). Naturally, the
shear effect diminishes farther from the surface. The gravity wave disturbances too have
a range, they ‘break’ and release energy around 75 – 90 km depending on their
interaction with zonal winds and seasonal differences in them. [Thomas, 1991, p 561]
The Hadley Cell region is driven mainly by buoyancy and is centered in the
tropics near the heat equator, where the sunshine on the surface is most direct and strong.
Warmed moist air rises towards the tropopause and flows generally poleward. The air
cools and becomes less humid as water vapor condenses. Denser and heavier, it descends
again around 30º, mixing with air from higher latitudes. Air accumulates warmth and
water vapor as it returns toward the equator along the surface, and the cycle repeats.
“[The Hadley cell] is a closed circulation loop, which begins at the equator with
warm, moist air lifted aloft in equatorial low pressure areas to the tropopause and carried
poleward. At about 30°N/S latitude, it descends in a high pressure area. Some of the
descending air travels equatorially along the surface, closing the loop of the Hadley cell
and creating the Trade Winds.” (need reference for quote [ Wikipedia, Atmospheric
circulation])
Over the course of the year, the heat equator traverses the tropics and allows more
or less space for the other cell zones. In the Polar Cell regions, air tends to flow along the
surface away from the pole and to rise at the warmer latitudes on the outer part of the
polar cells.
“Though cool and dry relative to equatorial air, air masses at the 60th parallel are
still sufficiently warm and moist to undergo convection and drive a thermal loop. Air
circulates within the troposphere, limited vertically by the tropopause at about 8 km.
Warm air rises at lower latitudes and moves poleward through the upper troposphere at
both the north and south poles. When the air reaches the polar areas, it has cooled
considerably, and descends as a cold, dry high pressure area, moving away from the pole
along the surface but twisting westward as a result of the Coriolis effect to produce the
Polar easterlies.” (reference)[Wikipedia, Atmospheric circulation]
The Ferrell Cells are a secondary circulation feature, driven by the circulation in
the Hadley and Polar cells rather than directly by buoyancy and rotation influences. They
are located in the mid latitudes at about 30-60° on each side of the equator. A parcel of
air that flows towards the equator is constrained by the earth’s spheroid shape: it moves
away from the rotation axis and will rotate more slowly due to conservation of angular
momentum. Conversely, poleward airflow will tend to rotate faster. These are part of the
Coriolis Effect. So, prevailing surface wind patterns are from the east to west where air
flows away from the poles and towards the equator (on the way towards the heat
equator), as it is in the lower parts of the Hadley and Polar cells. Accordingly, surface
winds generally flow from the west in the Ferrell cell and are calm in the doldrums and
horse latitudes (see Figure 4).
15
Higher ‘return’ winds in each cell are influenced by the same Coriolis effects
according wind direction and distance from the axis, so they at least partially oppose the
prevailing wind direction at the surface. The same influences mentioned already are
present in varying degrees throughout the atmosphere, in the troposphere and higher
layers. Some are stronger than others due to the local conditions.
Figure 4. Tropospheric circulation (near equinox)
(http://en.wikipedia.org/wiki/Atmospheric_circulation#Hadley_cell)
There are airflow features in the atmosphere that are basically air circulation
along zones of latitude too, strongly longitudinal in direction. (A zone is defined as a
region around the globe defined between parallels of constant latitude. A meridian is a
great half-circle of constant longitude that extends from pole to pole across all latitudes.)
Jet streams are small regions of high-speed airflow typically at the poleward boundaries of the main
circulation cells near the tropopause, about 10 km high near the polar cell to about 15 km farther
equatorward (reference Wikipedia, Jet stream). Jet stream winds blow generally from west to east with
speeds of 25 – 110 m/s. A rotating band of air in some latitude zone around the globe will speed up if it
shrinks in radius towards the rotation axis, which it does if it moves towards a pole. A section of a rotating
band of air will do the same thing. (Though its relative direction with respect to the rotating surface may
be east to west, its actual direction is in the same direction as the earth’s rotation by item A above.) The jet
streams and cell boundaries are not rigidly fixed in latitude, but various disturbances and influences divert
the airflow in towards or away from the pole and back, making the boundaries wavy (see Figure 5). The
waves rotate westward around the pole like a planetary gear system and are called planetary waves or
Rossby waves [Houghton, p. 111 for ‘Rossby’
16
,westward? ].
“Rossby waves in the atmosphere are easy to observe as (usually 4-6) large-scale meanders of the jet
stream. When these loops become very pronounced, they detach the masses of cold, or warm, air that
become cyclones and anticyclones and are responsible for day-to-day weather patterns at mid-latitudes.”
[http://en.wikipedia.org/wiki/Rossby_wave]
The term ‘planet’ comes from a word in some language (maybe greek?) and means ‘wander’; so, planetary
means wandering. Ah, I found it!
“The word planet derives from the Greek word planetes, meaning “wanderer.” [Chaisson, E. McMillan, S.
Astronomy Today, Prentice Hall, NJ, 2002 p. 36]
Figure 5. Zonal wind flow in the troposphere and middle atmosphere. [m/s units]
[Houghton, p. 62, Fig 5.1(c): COSPAR, 1972]
Airflow over surface features (orography) of the earth generates disturbances that
propagate through the troposphere and to higher levels of the atmosphere. (Though this
does not depend explicitly on latitude, the disturbances may affect the latitudinal
structure through effects like Rossby waves.) For example, air that flows against a
mountain is pushed upwards and disturbs the overlying air. If the overlying air mass is
less dense and more buoyant, The uplifted air will tend to fall back to earth on the other
side of the mountain simply due to gravity.
It can compress the underlying air and ‘bounce’.
The disturbances can propagate upward through the atmosphere as gravity waves and
past the mountain as lee waves (references
[Houghton, p. 80] clouds & orography,
17
“Gravity waves represent only a minor component of the motion in the lower atmosphere.
Above 75 km, however, atmospheric motion is probably dominated by them. Gravity
waves include tidal motions, tides being a special case of gravity waves having a
particular horizontal scale and a particular period. These motions show up for instance in
the variations of ionized layers as well as in winds determined from the movement of
trails left by rockets or by meteors. Although mainly generated in the lower atmosphere,
gravity waves are propagated upwards, where as the density decreases their amplitude
increases. At levels above 75 km or so they are dissipated by viscous damping. Because
of this they contribute significantly to the energy budget of the upper mesosphere and
lower thermosphere. They also play a critical role in determining the momentum budget
in the region of the mesopause.” [Houghton, p. 110]
lee waves (picture) [Houghton, p. 110]
Compress is probably not the best word, but ‘bounce’ should be close.
Maybe I misunderstood something. I may have mistaken the lee waves picture to show
compression and bounce, but the following describes a similar scenario to what I tried to.
(The highest mountains are 8-9 km tall, lower than the stratosphere.)
“2.2.1 Static Stability -- Because temperature increases with altitude in the
stratosphere, warmer air overlays colder air. As a result of this temperature
structure, convection never happens in the stratosphere. If we could displace an
air parcel to a higher altitude in the stratosphere, it would be colder than its
surroundings. Cold air is more dense than warm air, and the parcel would sink
back to its original location, though it would overshoot slightly because of its
momentum. After overshooting, it would drop to a location where it would be
warmer than its surroundings. Warm air is less dense than cold air, and the
parcel would rise back to its original location, though it would once again
overshoot slightly. This process would continue in a series of vertical
oscillations about some equilibrium altitude where the parcel density and the
surrounding air (ambient) density were the same.”[NASA SEES]
[Thomas 1991, p558b.] distubances propagating upwards:[Thomas 1991, p561a “”]
). (Under the right conditions, the normally unseen water vapor in the uplifted air
condenses due to change in pressure and temperature into clouds that highlight the crests
of the lee waves, forming a progression of clouds in the mountain’s wake that resembles
ripples in the surface of a lake.)
Air against air can also generate disturbances: warm air flowing against a ‘soft’
mountain range of denser cold front air can be deflected upwards too. [Thomas 1991
references Garcia and Solomon 1985, saying that vertically propagating gravity waves
“are thought to be generated in the lower atmosphere in thunderstorms, frontal zones, and
flow over orography” .] Gravity waves carry energy and can influence circulation and
processes in the atmosphere above the troposphere (reference [Thomas 1991, p561a]).
18
“Middle atmospheric circulation is driven by solar heating and by momentum drag
resulting from gravity and planetary waves. These waves couple tropospheric climate to
the middle atmospheric circulation that in turn controls the transport of radiatively and
chemically active trace gases and exerts mechanical forcing on the troposphere below.”
[Summers, 1999]
“Exchange of momentum between the surface and the mesopause which occurs through
breaking gravity waves … operates in such a way so as to maintain this region of very
small zonal flow [Houghton, 1978]” [Houghton, p. 157]
The tropopause is higher in the tropical latitudes than at the poles due to greater
upwelling from rotational effects near the equator and greater insolation-driven buoyancy
near the heat equator (reference
I think I thought that this was why, but now I’m doubting it… the solid earth is oblate,
and that due to rotation. It seems rotation would also give the atmosphere some
oblateness. If the ozone layer is what defines the stratosphere, and it seems it does:
Wayne, p 58
“On Earth, the stratospheric inversion is a result of heating by absorption of solar
ultraviolet radiation in the ozone layer. Ozone is formed photochemically from O2 and
the 'layer' structure owes its existence to a peak in absorption and in reaction rates
(Wayne section 4.3.2). Too low in the atmosphere there are insufficient shortwavelength photons left to dissociate much O2, while too high there are insufficient O2
molecules to absorb much light and to associate with O atoms to make O3. A series of
chemical reactions concerned in the formation and destruction of ozone ultimately
releases the chemical energy of O2 dissociation, while the solar ultraviolet absorbed by
ozone itself is also liberated as heat. As a consequence, the heating in the stratosphere is
related to the ozone concentration profile (although modified according to the exact
mechanism of conversion of ultraviolet to heat energy). ... It is noteworthy that the ozone
layer absorbs wavelengths from the Sun that do not reach the Earth's surface, and that the
heating is achieved in situ rather than by absorption of re-radiated infra-red.”
So, it seems the location of the tropopause is determined by the stratosphere-generating
solar photolysis action on the oxygen and ozone distribution in the atmosphere. Rotation
should make the atmosphere to be oblate and to extend higher over the equator.
Upwelling may help do this too, literally raising the ozone over the heat equator and
sending it higher. If the sun’s ozone-photolytic wavelengths are absorbed higher rather
than lower, the tropopause should be higher.
[Houghton, p. 68-70] rephrased
Total column ozone is greater towards the poles than in tropical regions with maximal
values at high latitudes in spring; however, photochemical theory suggests maximum
ozone will be in the tropics where solar radiation is greatest. Brewer (1949) proposed
19
circulation to accommodate both of these apparently conflicting ideas: In DobsonBrewer circulation, air moves upward into the stratosphere in the tropics and then ozonerich air moves polewards and downwards so that a large concentration of ozone occurs
at high latitudes in the lower stratosphere where its photochemical lifetime is very long.
). The cold tropopause and the temperature gradient in the stratosphere brake the
upwelling due to buoyancy and act like a trap for water vapor. There is some exchange
between the troposphere and the stratosphere in lateral and vertical directions through the
tropopause, but the vertical motion is much slower in the stratosphere. Buoyancy
processes are not as dominant and do not generate as much circulation in the stratosphere
as they do in the troposphere. (This is because buoyant air has to compete in the
stratosphere against the buoyancy of the warmer overlying and surrounding air, so it
cannot rise as easily as it did in the troposphere.) The tropopause boundary layer
maintains a level of about 3 ppmv of water vapor across latitudes, well supplied and
mixed from the troposphere.
“Very dry air which has entered the stratosphere in the region of the tropical tropopause
is carried polewards through the rest of the stratosphere, all stratospheric air being found
to have a water-vapor content appropriate to saturated air at a temperature near to that of
the tropical tropopause.” [Houghton, p 159]
a. Circulation in stratosphere-mesosphere
In the stratosphere, air moves upward over the heat equator, poleward and
generally down towards the poles in what is known as the Brewer-Dobson circulation
(reference Houghton p. 68-70). This circulation is very slow, with a timescale of months
to years. The earth’s spin axis is tilted with respect to its orbit, so the relative earth-sun
tilt orientation changes according to the earth’s position in orbit, or equivalently,
according to the time of year. The solar zenith point and the heat equator are the point
and latitude zone on earth where sunshine is most direct and strongest, and they move
northward and southward according to the sun-earth orientation for the time of year.
The polar regions transition the twilight between 24-hour daylight and 24-hour
darkness as the solar zenith point crosses the geographic equator. Then the lightened and
darkened areas around the poles grow in size as the earth orbits toward solstice and
shrink again towards the twilight equinox. The result is annual cycles of seasonal change
for each hemisphere that are nearly the same due to the earth’s near-circular orbit but
with a six-month time difference.
Global cloud animations centered over Africa for June and other months show
that prevailing equatorial winds stay close to the equator (reference Gerhards, 2007).
Over Africa, clouds form around 0-10°N in July and just southwards of the equator by
late November. Figure 5shows the Brewer-Dobson upwelling around to 20°S degrees in
20
January (suggesting upwelling around 20°N six months later in July by symmetry). The
cloud formation in the animations (as a proxy for the Hadley cell center) seems to follow
the sun, but maybe not as far north and south as the Brewer-Dobson upwelling does in
the stratosphere. Perhaps the tropospheric response is moderated by the ocean surface or
hidden by circulation; or, perhaps it is the animation viewpoint, wherein the great African
desert doesn’t provide water for cloud formation where the heat equator would be. It
might be helpful to view clouds for all year and over other continents. Also, keep in
mind that the equatorial winds are strongly zonal and the Hadley cell circulation is
meridional.
The strong difference in polar insolation and effects of waves drive a mesospheric
circulation cell, pulling air upward from the summer polar stratosphere and sending it
downward into the winter polar vortex, countering and combining with Brewer-Dobson
circulation respectively (see Figure 6).
“Rising air in the summer mesosphere is cooled as it ascends and is balanced by
descending air in the winter mesosphere with its associated warming.” [Houghton, p 157]
At the mesopause, the zonal airflow component is small:
“… the magnitude of the zonal wind relative to the earth’s surface is about zero near 90
km – the mesopause level. Exchange of momentum between the surface and the
mesopause which occurs through breaking of gravity waves operates in such a way so as
to maintain this region of very small zonal flow. The overall structure of the mesosphere,
therefore, is determined by radiative exchange together with circulation which, in turn
satisfies the thermodynamic equation, the thermal wind relationship, and the requirement
for very small zonal flow in the region of the mesopause.” [Houghton, p 157 (references
to figures removed)]
Air in the mesopause rises or descends at the poles and has a meridional
component of airflow around the mesosphere:
“[some of the Gravity waves reaching the mesosphere from the tropospheric
disturbances exert force on the airflow, acting as drag on prevailing summer zonal winds
and reversing them from easterly to westerly in the lower thermosphere. … The wave
drag] also forces a mean equatorward wind of the order of 10 m s–1 in summer and a
similar poleward flow in winter. … The equatorward moving air, in combination with
the radiative forcing, induces vigorous upward motion (several centimeters per second)
as a result of mass continuity. The rising air cools adiabatically, resulting in very low
temperatures in summer. The opposite circulation occurs in the other hemisphere, and
downwelling warms the wintertime mesopause.” [Thomas, 1991, p 561a.]
At a flow of 1 cm/sec = 0.01 m/sec, air would traverse 864 m/day—it circulates
through the troposphere and partly mixes into the stratosphere (per 6). Also, there is an
extreme volume change by expansion/compression of the air as it travels vertically, so a
flow velocity of rarefied air corresponds to a much lower flow velocity of denser air.
21
Since a small supply of dense air in the lower stratosphere becomes a large volume in the
upper mesosphere, the circulation cell through the mesosphere can be closed with a small
(volume) lateral exchange between the lower parts of the stratosphere and/or troposphere.
At a flow of 1 cm/sec = 0.01 m/sec, air would traverse 864 m/day. Air pressure is
very low at mesospheric altitudes, on the order of a few pascals (51 km, 66 Pa), (71 km, 4
Pa) so little heat is transferred by conduction (leaving convection/radiation) as it
circulates through the troposphere and partly mixes into the stratosphere (per 6). Also,
there is an extreme volume change by expansion/compression of the air as it travels
vertically, so a flow velocity of rarefied air corresponds to a much lower flow velocity of
denser air. Since a small supply of dense air in the lower stratosphere becomes a large
volume in the upper mesosphere, the circulation cell through the mesosphere can be
closed with a small (volume) lateral exchange between the lower parts of the stratosphere
and/or troposphere.
Figure 6. Circulation in the stratosphere and mesosphere. Composited from images by NASA.
Studying Earth's Environment From Space. June 2000. (May 2008)
http://www.ccpo.odu.edu/SEES/index.html.
Decolored combined contour plots with flow vectors for water above 55 km and methane below.
b. The Polar Vortex
During the season when the sun recedes and the region cools, air circulating from
west to east forms a polar vortex in the lower stratosphere at that polar region. The
vortex acts to contain the air inside and to limit mixing with the rest of the stratosphere.
Wayne [p.184] likens the cold core of the southern winter vortex to a reaction vessel that
almost isolates the air inside from the rest of the atmosphere. Photochemistry diminishes
22
in the cold darkness of the polar winter vortex, and chemical reactions which do occur
can progress further towards depletion of reactants. Distinct compositional features
develop: for example, ozone levels over the Antarctic are reduced in the south polar
vortex and replenished when mixing resumes, seasonally forming and refilling an ozone
‘hole’. The vortex edge wanders, subject to the same disturbances that cause Rossby
waves (reference
I suppose I don’t have enough to support this presumption.
The vortex edge does move as seen in animations like
http://svs.gsfc.nasa.gov/vis/a000000/a003000/a003067/OMI_PV_320x240.mpg
http://svs.gsfc.nasa.gov/cgi-bin/advsearch.cgi?query=vortex&req=search
I associated the vortex edge, jet streams and Rossby waves as all being at or near the
outer boundary of the polar cell (and more jet streams at the outer boundary of the Ferrel
cell). I guess I have them confused: the polar cell is a tropospheric feature. I think the
vortex is a stratospheric feature
“The stratospheric aircraft measurements of 1987 have confirmed, beyond all doubt, that
there is anomalous chemistry going on within the vortex region.” [Wayne, p. 185]
Arctic circulation shown at http://research.iarc.uaf.edu/IPY-CTSM/index.php makes it
look like the vortex extends from troposphere to stratosphere, but they also have a link on
the left for mesosphere measurements
Jet streams appear to be 10-15 km high per Wikipedia, so they would be in the
troposphere.
).
The mesospheric circulation counters and contributes to Brewer-Dobson
circulation at the summer and winter polar regions respectively. In contrast to the
summer polar stratosphere, which is at its warmest and warmer than most of the global
stratosphere), the summer polar mesosphere is the coldest part of the atmosphere, and
reaches its coldest annual temperatures near 130 K around 85 km [Thomas, 1991 p 559560 (Thomas’ figure 2)]. The cold is maintained during several weeks of the summer,
and it is cold enough that moisture supplied in the expansion-chilled upwelling air freezes
into ice (onto ubiquitous condensation nuclei) and forms polar mesospheric clouds
(PMCs) (reference Thomas, 1991, p561a ; Hervig 2001). Such clouds remain constantly
illuminated by the summer sun and when dense enough, they reflect enough light back
towards the earth to be seen at night in lower latitudes where they are then called
noctilucent clouds.
23
The mesospheric air flow continues equatorwards at a rate of 10 m/s [Thomas,
1991 p 561a], and the ice particles it carries past the frosty cold region into warmer areas
absorb heat and sublimate back into water vapor. Polar mesospheric cloud season
continues until the atmospheric circulation no longer maintains sufficiently cold regions
and the remaining ice clouds sublimate and fade.
Meanwhile, in the winter polar regions, zonal winds rotate around the pole
forming a stable circumpolar vortex in the lower stratosphere. The vortex lasts around
eight months, from about Sep – Apr in the north and about May – Dec in the south.
Although, perhaps there are fewer disturbances due to the roominess allowed to the
winter polar circulation cell region by the retreat of the heat equator and the properties of
the winter-altered atmosphere. Air flows polewards and downward in the winter polar
portion of the mesospheric circulation cell, joining the stratospheric airflow from BrewerDobson circulation. Compression of the descending air can warm it a little, but it
descends into the constant dark and cold of polar winter. Darkness spans the entire
height of the stratosphere (up through 50 km altitude) 7.2° poleward of the day/night
terminator latitude zone and spreads from and retreats to the pole five months later, e.g.
10/08/2007-3/2/2008 at the north pole proper and six months afterwards for the south
pole.
The southern polar vortex is especially stable, probably due to the symmetry of
and sea-surround about the Antarctic. While it exists, it works like a container around a
region of stratosphere and limits mixing with other parts of the stratosphere. This makes
it a very good example of how the atmosphere can develop regions featuring particular
composition. As the sun sets and disappears, photochemical reactions diminish and
cease. Chemistry continues within the confines of the vortex, supplied by mesospheric
and stratospheric inflow of atmospheric constituent compounds, aerosols and water vapor
into the vortex (reference Houghton, p. 157
Wayne, p. 185). Though inflow may be vigorous (to match the summer pole upwelling),
outflow from the vortex is very slow since air has compressed, and products accumulate
in the ‘container’ over time. Heat energy produced by chemical reactions is not
constrained by the circulation and still can radiate away, leaving the air cold.
Naturally, the lower polar stratosphere is coldest during winter. Air flowing
downward from the mesosphere and upper stratosphere encounters the cold and higher
pressure. As the water vapor partial pressure rises to saturation levels, it condenses.
(Water’s triple point pressure is 611 Pa, so the corresponding altitude of about 34 km is a
ceiling above which water can be vapor or ice but not liquid.) The polar stratospheric
clouds (PSCs) that form are mixes of water ice or liquid with nitric acid and sometimes
sulfuric acid, depending on what water-soluble compounds accompany the inflow or are
held in the vortex. [Wayne, p. 187]
“PSC occur in winter at heights of 15-25 km” [Thomas, 1991, p. 561b]
24
3. Chemistry & compositional features
The composition of the atmosphere at any location depends in part on what
materials (gases, aerosols, etc) are delivered or removed by circulation and on what
compounds are produced or destroyed by chemistry and photochemistry. Reaction rates,
and thus heating contributions, depend upon the relative concentrations of the reactants
and products—the composition in the parcel of air.
Chemical processes depend locally upon relative concentrations of reactants, but
photochemical processes depend on geographic location (as well as altitude) because the
amount of insolation varies from place to place over the earth’s surface. The sun’s rays
are more perpendicular and more penetrating near the equator and oblique near the poles,
and the angle varies with season according to the earth’s tilt and orbital position.
At equinox the sun is directly over the equator at noon, and a one square meter
sunbeam lights one square meter of the earth’s surface. However, a one square meter of
sunbeam reaching the surface at a higher latitude at the same time is spread over more
area because the beam hits the surface at an angle. By this effect alone, the sunshine that
reaches the earth surface (and lowest levels of atmosphere) is strongest at the equator and
weaker towards the poles. Furthermore, the angled sunbeam travels a longer path
through the atmosphere and so loses energy by reflection, scattering or absorption in the
atmosphere. This is also why it is better to avoid excessive sun exposure on skin during
midday while the sunlight has a shorter path through the atmosphere. There is a
tremendous difference in insolation in the polar regions between the summer maximum
and winter zero, and we will see it has an important effect on (part of) the general
circulation.
The dominant controlling influence on composition is chemical production and
loss. Chemical reactions proceed at rates determined by the relative concentrations of
the reactant and product compounds in the air: photochemical reactions also depend
upon the wavelengths and amount of light available, so they have a depth range along the
light beam path according to the composition of the air and its absorptive properties
(general reference
Wayne Chapter 3, p 78 “Photochemistry and kinetics applied to atmospheres”?
Houghton section 5.5, p. 66 “Photochemical processes”?
Something about absorption & attenuation of light …attenuation length , optical
depth sounds like something from the Intro Atmosph. Phys class …
I learned about reaction rate dependence on concentration in chemistry class.
Photochemistry acts according to the spectral response of the compounds
(absorption & emission spectra)
“Only laboratory experiment can unequivocally identify what chemical species are
formed, and in what electronic states, and with what quantum efficiencies. Theory can,
however, offer guidance in two ways. First, the incident photons must have enough
energy to bring about any proposed change, so that thermochemical information can be
used to show whether a particular photodissociative channel is energetically possible.
25
Secondly, considerations such as the need to conserve quantum mechanical spin and
orbital angular momentum can indicate whether the channel is probable.” [Wayne, p. 86]
I have the notion that the photon must also be at the right energy to be absorbed, like in
an absorption or emission spectrum for the reactant and that a photon with the wrong
energy—even if it is a little more, like a little shorter wavelength, won’t cause a reaction,
or will be a lot less likely too. I’m thinking it may be like a resonance that allows energy
to transfer...
“The intensity of a photon beam, like that of a neutron beam, decreases exponentially
with distance through an absorbing material. The intensity is give by Equation 37-25,
where  is the cross section for absorption per atom. The important processes that
remove photons from a beam are the photoelectric effect at low energies, Compton
scattering at intermediate energies, and pair production at high energies.” [Tipler,
Physics, page 1020.]
[Brady & Humison, General Chemistry, 3rd. ed., 1982] talks about a photon exciting an
electron in an atom and E=h . This is what I’ve been thinking. Molecules have more
degrees of freedom and so they have molecular spectra in addition to the atomic spectra
(which maybe are modified)
). These influences are not all completely independent from each other, but they work in
concert throughout the atmosphere producing characteristic features in circulation and
composition (e.g. the polar vortex, the ozone layer).
a. Atmospheric Water Vapor
Atmospheric water vapor is an unseen component of the atmosphere and is
present well beyond where clouds of liquid water or ice commonly appear. Water vapor
is the primary contributor to the greenhouse effect which moderates the earth’s
temperature on the surface and in the atmosphere (general reference
greenhouse effect Wayne, p 41-53 H2O, CO2, O3 ; p. 48 chart indicates water is primary
: removing water alone leaves the least amount of trapped radiation remaining, 64%.
p. 49 “water vapour makes a sizeable contribution to atmospheric heating on earth”
). Water vapor and other greenhouse gases are so named is because they absorb infrared
wavelengths of heat radiated from the earth’s surface and re-emit the infrared photons
both upwards and downwards: since part of the energy is returned towards the surface,
the gas helps the earth retain warmth, like a greenhouse. However, the same gases can
absorb and re-emit incoming solar radiation as well, partly shielding the surface (and
lower regions of the atmosphere) by re-emitting part of the energy upwards. The
upward-directed energy might participate in some processes in higher parts of the
atmosphere or be radiated out to space, thus effecting cooling.
26
How much and where the atmosphere is heated by chemistry or light processes
(photoabsorption) like the greenhouse effect depends especially on the altitudinal
distribution in location and density of the participating gases (and aerosols)—this helps
explain the origin of the atmosphere’s thermal profile. Major greenhouse gases in the
earth’s atmosphere are water vapor, carbon dioxide, methane, nitrous oxide and ozone
(reference needed
Wikipedia, greenhouse gas. http://en.wikipedia.org/wiki/Greenhouse_gas).
Figure 7: spectroscopic absorption of various atmospheric gases
[http://en.wikipedia.org/wiki/Image:Atmospheric_Transmission.png]
Figure 8 Sun photometer channel wavelengths in relation to atmospheric spectra
[http://geo.arc.nasa.gov/sgg/SOLVE2-website/Presentations/SOSST05_RusselltalkS.ppt]
27
The dominant sources and sinks of water vapor in the stratosphere and
mesosphere are generally thought to be reasonably well understood (Allen et al., 1984;
Brasseur and Solomon, 1986; LeTexier et al., 1988). Water vapor enters the tropical
stratosphere though upward advection from the troposphere. The cold tropospheric
temperature acts as a valve to severely limit the amount of water vapor that can actually
enter the stratosphere. In addition, this upward advection carries tropospheric methane
into the stratosphere where it is oxidized to form approximately two water molecules for
every methane molecule. These two sources produce a water vapor profile with a mixing
ratio increasing with altitude in the stratosphere, peaking between 50-60 km altitude at a
value of 6-7 ppmv (Summers et al., 1997b). Above this altitude the photolysis of water
and its reaction with O(1D) leads to a decreasing water vapor mixing ratio with altitude
(reference
“Above ~ 60 km altitude the destruction of H2O by solar Lyman  photolysis is expected
to produce a rapid decrease of H2O abundance with altitude.” [Summers 1999]
“The ultimate source of odd-hydrogen is the photochemical destruction of H2O. This
occurs mostly by photolysis above about 60 km altitude and by
H2O + O(1D) →
OH + OH
( 1 )” [Conway, 2000, p 2613a]
“The OH density distribution obtained from one representative orbit of MAHRSI
observations (Fig. 1A) reflects the two principal photochemical sources of OH. In the
upper mesosphere (above ~65 km), OH is produced by photodissociation of H2O at
ultraviolet wavelengths (principally at the solar hydrogen Ly  emission line at 121.6
nm). Below ~65 km the dominant source of OH is the reaction O(1D) + H2O → 2 OH,
where O(1D) is a product of O3 photolysis” [Summers, 1997b]
).
The tropopause maintains water vapor levels at about 3 ppmv, but concentrations
are higher at higher altitudes. Upward vertical Brewer-Dobson circulation is slow,
accordingly slow is the input of water vapor into the stratosphere through the tropopause.
Oxidation of methane from surface sources, such as biological decomposition and
digestion (swamp gas and cow gas), adds about 3.2 ppmv of water vapor as well as heat
and carbon dioxide. Methane, which is also the principal component of natural gas fuel,
has a molecular mass of 16 g/mol and a vaporization temperature of 112K (-161.6 °C at
standard 1 atm). It is lighter than dry air like water vapor is, but methane remains
gaseous and rises unhindered by the tropopause’s cold. The methane combustion
processes are:
CH4 + O2
2 H2 + O2
→
→
CO + H2 + H2O
2 H2O
28
2 CO + O2
→
2 CO2
CH4(g) + 2 O2(g) → CO2 (g) + 2 H2O(l) + 890 kJ/mol
but the energy released should be reduced by the vaporization heat of water at the
temperature in the stratosphere where the reaction occurs. That relationship between
methane and water is easily inferred from the HALOE plots of water and methane
presented here, where the contours fit together like jigsaw puzzle pieces.
However, previous satellite observations (such as from HALOE’s first five years
and MAHRSI [Summers & Siskind 1997; Science vol 277 ]) have supported the
existence of additional water in a layer near 70 km where averaged values reach over 7
ppmv in tropical latitudes. This is more than the amount predicted by the standard
chemistry for that altitude, especially since solar Lyman-α photolysis was expected to
rapidly destroy water above 60 km. [Summers & Siskind 1999, p. 1837a]
“Since almost all CH4 is oxidized below ~60 km altitude, the H2O mixing ratio
should reach its maximum value near that altitude. Above ~60 km altitude the
destruction of H2O by solar Lyman α photolysis is expected to produce a rapid
decrease of H2O abundance with altitude.” [Summers & Siskind 1999, p. 1837a]
“The intense solar flux at 121.6 nm (the Lyman-alpha line) which penetrates deeply
into the upper mesosphere causes rapid solar photodissociation of water vapor
above about 75 km.” [Thomas 1991, p556b]
The difference in photolysis depths in these two quotes may be at least partly due
to the sun’s rays being more direct near (heat) equatorial latitudes and more filtered by
angled approach through the atmosphere towards polar latitudes. We cannot measure the
water abundance directly, but have to infer its sources.
Water may also be produced by heterogeneous chemistry, although direct
confirmation of this process in the atmosphere is very difficult. Heterogeneous chemistry
is the set of reactions among reactants and or materials of different phases, as opposed to
gas-phase only reactions (reference
Wikipedia: http://en.wikipedia.org/wiki/Heterogeneous_chemistry
Wayne book, p 10:
“Heterogeneous and catalytic processes in the atmosphere are not well identified
or understood, but that does not make them unimportant! A few examples are known.
For example, the rate of oxidation of SO2 increases several-fold as the air becomes more
nearly saturated with water. Sulphates can be formed by the reaction of sulphur dioxide
and ammonia in cloud droplets. When the water evaporates, ammonium sulphate aerosol
is left in suspension. The combined presence of soot particles and sulphur dioxide leads
to enhanced oxidation rates as well as a greatly increased health hazard. The following
section will describe how dependent we are on the presence of ozone in the stratosphere.
29
An ‘ozone hole’ has been evident during early Spring in the Antartctic for more than a
decade. It owes its origin to anomalous chemistry involving heterogeneous reactions
occurring on the surfaces of polar stratospheric clouds. Water-ice particles make up these
clouds, but there are indications that sulphate aerosols may also provide sites for surface
reactions in the stratosphere. In that case, massive volcanic eruptions may also result in
depletion of stratospheric ozone. The involvement of particles in tropospheric chemistry
is explored further in Section 5.3.9. Figure 1.1 summarizes some information about the
sources, lifetimes, and effects of aerosols of different sizes.” [Wayne, p. 10]
and another example that I found before finding page 10’s example:
“Liberation of the active chlorine from the reservoirs [HCl, ClONO2] is normally
rather slow. But it turns out that the two reservoir molecules can react together in the
presence of ice particles, such as those that make up the polar stratospheric clouds
ClONO2 + HCl → Cl2 + HNO3. The outcome is that molecular chlorine is
released as a gas, and the nitric acid remains in the ice particles, which can ultimately
transport water and nitric acid out of the vortex.” [Wayne, p. 188]
). Interaction with a material surface, such as that provided by dust from meteor ablation,
loosens the constraints of reaction between two species of gas by providing additional
reaction pathways, for example, a heterogeneous reaction can conserve spin by transfer to
or through a third ‘interacting participant’ in cases where the direct reaction would be
spin forbidden between two compounds.
“In the gas phase, [the reaction (1) O + H2 → H2O] is thermodynamically accessible
with an exothermicity of 4.8eV, and even though it is spin-forbidden, such reactions can
occur at rapid rates on surfaces, e.g. the reaction between N and NO to form N2O
[Halpern and Rosner, 1982]. The surface acts as a catalytic agent and both removes the
spin restrictions and stabilizes the reaction product. Incomplete energy accomodation is
also known to occur in surface reactions where for highly exothermic reactions the
energy release can appear as translational energy sufficient to desorb the product
molecule before surface accomodation can occur. The kinetic details of the surface
reaction, and the reaction probability, , for reaction (1) will depend strongly upon
several poorly known factors including the dust composition, sticking probability, surface
mobility, size of the particles (these particles may contain only ~100 molecules and may
be non-spherical), and the influence of other species which may also be adsorbed on the
dust.” [Summers, 1999]
Meteoric dust condenses from the evaporates of meteors that have burned up and
provides an abundant supplement to aerosol particles high in the atmosphere (about 1000
cm–3). About 44 metric tons of meteoric materials and space dust are pulled into the
earth’s atmosphere per day [Hunten et al, 1980, flux p.1343b attributed to Hughes 1978].
The vapors condense into solid particles that over time coagulate into larger and larger
particles that eventually grow too heavy to be kept aloft by air circulation (like BrewerDobson circulation). Tropical upwelling that counters sedimentation helps produce a
distribution of meteor dust, concentrating particles sized < 10 nm in a layer around 60-80
30
km (see Figure 9). It takes about thirty years for small 1nm particles to settle out of the
atmosphere, but the time decreases as the particle grows, to around three years for
particles that have grown to 10 nm. [Summers & Siskind, 1999, p.1839a, fig. 2]
Figure 9. Transport velocities associated with meteoric dust (Summers and
Siskind, 1999).
Would this plot also apply to other dust, like volcanic dust lofted by gravitophotophoresis (different composition, perhaps not)? See
http://geo.arc.nasa.gov/sgp/aero/aerocloud2.html
Ice in polar mesospheric clouds may also provide for recombination of water from
hydrogen and oxygen that reaches the ice particle surface or diffuses into the ice lattice.
[See Thomas 1991, p567b]
O + H2 → H2O + 4.8eV
surface (meteor dust) [Summers & Siskind, p1838]
O + (H2O) s → OH + OH
meteor smoke surface [Thomas 1991,567b
attributed to Kopp, 1990]
H2 + Os → (H2O) s (speculative)
ice particle surface [See Thomas 1991, p567b]
The turbulent mixing acts to homogenize the air, but processes like chemistry and
photochemistry alter the composition. Compositional features (like the ozone layer) form
where rates of production and inflow of gas reactants are comparable to rates of depletion
and outflow.
b. Methane & water
A very important reaction that adds to the water vapor concentrations in the
stratosphere and mesosphere is the oxidation of methane: Methane oxidizes in the
31
atmosphere producing water vapor and heat and carbon dioxide, with the exothermic
chemical equations (reference):
These are the methane combustion reactions from http://en.wikipedia.org/wiki/Methane.
I naively thought combustion and oxidation the same—maybe just related.
CH4 + O2 → CO + H2 + H2O
2 H2 + O2 → 2 H2O
2 CO + O2 → 2 CO2
CH4(g) + 2 O2(g) → CO2 (g) + 2 H2O(l) + 890 kJ/mol
“A key reaction which initiates the methane oxidation process in the stratosphere is
CH4 + OH → CH3 + H2O
(kCH4 + OH)” [Summers et al, 1997a , p3516]
“The oxidation of one methane molecule will in general produce slightly less than two
water molecules [LeTexier et al., 1988]. The conversion efficiency to H2O depends on
location and details of the complete methane oxidation process. Many of the steps in the
oxidation process remain unverified in the stratosphere, in part due to lack of
simultaneous high-quality measurements of both CH4, H2O, H2, and important
intermediaries as, for example, formaldehyde.” [Summers et al, 1997a , p3517]
The general circulation is upward in the tropics, so the methane ‘burns hottest’ in
the tropical stratosphere. Where heat energy is available endothermic reactions can also
proceed. Methane is generated naturally by vegetative decomposition and cattle digestion
and is present in concentrations of about 2 ppmv in ‘clean dry air’ at sea level (Houghton
p 226). In comparison, a humidity of 50% at sea level and 20 or 25 °C (68 or 77°F)
corresponds to about 10,000 ppmv water vapor.
Thanks—10,000 is such a nice number too.
Most water vapor is trapped in the troposphere, and only a small portion passes
into the stratosphere, about 3 ppmv. (Other atmospheric gases and aerosols pass through
the tropopause too, upwards or downwards as they are driven, but gases behave
differently—especially in terms of phase change.) A majority of methane, about 1.6
ppmv, passes the tropopause into the stratosphere. Approximately 1.6 ppmv of methane
rises through the cold tropopause as gas (methane’s boiling temperature is 112 K, much
lower than the troposphere’s temperature. Methane’s mass is about 16 g/mol, which
makes it lighter than water vapor at 18 g/mol.
c. The photochemistry of ozone
A photochemical reaction is triggered by, or its rate of reaction is increased by the
absorption of light by the reactants, specifically light of wavelengths characteristic to the
reactant’s absorption/emission spectrum. A particularly important reaction is the
photolysis of ozone which absorbs biologically harmful ultraviolet rays (reference
32
Perhaps the outstanding feature is the relationship between the absorption
spectrum of ozone and the protection of living systems from the full intensity of solar
ultraviolet radiation. The macromolecules, such as proteins and nucleic acids, that are
characteristic of living cells, are damaged by radiation of wavelength shorter than about
290 nm. Major components of the atmosphere, especially O2, filter out solar ultraviolet
with wavelengths < 230 nm; at that wavelength, only about 1 part in 1016 of the intensity
of an overhead sun would be transmitted through the molecular oxygen, But at
wavelengths longer than ~230 nm, the only species in the atmosphere capable of
attenuating the Sun’s radiation is ozone. Ozone has an unusually strong absorption just at
the critical wavelengths 230-290 nm), so that it is an effective filter in spite of its
relatively small concentration. Or example, at =250 nm, less than 1 part in 1030 of the
incident (overhead) solar radiation penetrates the ozone layer.” [Wayne, p. 10-11]).
When a particular wavelength is absorbed from a beam of light, that wavelength
is attenuated: if the beam encounters enough of a particular absorbent gas along its path,
that wavelength can be filtered out completely. In that case, the gas has acted to shield
parts of the atmosphere farther along the beam path. A sunbeam that comes from directly
overhead passes through a minimum thickness of atmosphere before reaching the earth’s
surface. A sunbeam that passes through the atmosphere at an angle is attenuated more
when it reaches the earth’s surface and any sunbathers on the surface—that is why it is
important to be mindful of sun exposure more so during midday hours than early morning
or late afternoon.
The rate of the photochemical process depends upon the amount of the
photosensitive gas, i.e. the composition of the atmosphere, and it depends upon the
distribution because of the shielding effect. Of course, it affects the amount and
distribution of that gas and contributes to the heating in atmosphere. The end result in the
earth’s atmosphere is an ozone layer with maximum concentration centered at an
approximately 20-30 km altitude. Other gases undergo photochemical reactions too, and,
may form layers in the atmosphere at other altitudes depending on their concentrations
and distributions.
Molecular photolysis is not the only interaction between light and the atmosphere.
Photons of the appropriate wavelengths can free electrons from a gas (photoionization) or
aerosol particles (photoelectric effect), affecting (gas and gas-aerosol) reaction rates and
aerosol coagulation (clumping) rates. Additionally, some gases and aerosols can scatter
light and some gases can absorb and re-emit photons without reacting chemically or
being ionized. Of course, clouds reflect a lot of light back to space as well as scattering
and diffusing it.
The unseen ozone layer is probably the most commonly known compositional
feature of the atmosphere. It is especially important because it represents a protection for
us on the surface of the earth against biologically harmful ultraviolet rays from the sun.
The rays are absorbed in photolysis reactions involved in the destruction of ozone and of
diatomic oxygen. Light is written into the equation as a photon of energy h :
33
Oxygen photolysis O2 + h → O + O,
 ≤ 242 nm
[Isaksen et al, p 4356]
“virtually all ground state O atoms will regenerate ozone:
O + O2 + M → O3 + M,
M is N2, O2 ” [Wayne, p214]
Ozone photolysis O3 + h → O*(1D) + O2*(1g),
 ≤ 310 nm
[Isaksen et al, p 4357]
(The asterisks on O and O2 indicate energetic excitation and the state is given in
parentheses.) Because nitrogen and oxygen are so abundant, ozone regeneration
proceeds rapidly. The limiting factor is the amount of light at the reaction wavelength(s),
so the reactions occur less and less at lower altitudes as those wavelengths of light are
depleted. The energy absorbed is converted into kinetic or heat energy or may be reemitted as photons of lower energy (when the products de-excite).
“Perhaps the outstanding feature [of ozone] is the relationship between the
absorption spectrum of ozone and the protection of living systems from the full
intensity of solar ultraviolet radiation [which are vulnerable to] wavelengths shorter
than about 290 nm. Major components of the atmosphere, especially O2, filter out
solar ultraviolet with wavelengths < 230 nm; at that wavelength, only about 1 part
in 1016 of the incident (overhead) solar radiation penetrates the ozone layer.”
[Wayne, p10,11]
The ozone layer conforms to the spheroid shape of the earth’s surface simply
because of the air density profile under gravity. Because the atmosphere is denser at
lower altitudes, so the absorption profile is biased towards lower altitudes. The layer has
a meridional maximum that follows the heat equator over the course of the year where
ozone exceeds 10 ppm at about 30-35 km altitude. Concentrations decrease poleward
due to insolation angle and above and below due to the absorption and filtering effects on
the incoming sunlight. Half-maximum (or more) concentrations extend from about 25-45
km altitude encompassing much of the stratospheric layer through the mid-latitudes and
tropics (about 11/12 of a sphere) (reference
area: dA
= r 2 sindd 
zone: dAzone = 2 r 2 sind
cap: Acap = 2 r 2 sind , angles measured from the pole.
= 2 r 2 ( (– cos – (–cos 0) )
= 2 r 2 ( cos 0 – cos )
= 2 r 2 ( 1 – cos 23.4°)
= 2 r 2 ( 1 – 0.9178 )
= 2 r 2 ( 0.0822 )
 2 r 2 ( 1/12 ),
approximate 1/12  0.0833  0.0822
 ( 1/24 ) 4 r2,
34
or each polar cap is approximately 1/24 of a whole sphere. Both caps together are about
1
/12 , so the remainder is about 11/12 .
Using ( cos (90°-23.4°) – cos 90° ), each tropic band gives 0.3971 . Using (cos 23.4° –
cos (90°-23.4°) ) for a mid latitudes band yields ( 0.9178 – 0.3971 ) = 0.5206 each, or
roughly half a sphere for the pair. The sum for one hemisphere is 0.0822 + 0.3971 +
0.5206 = 1.
).
The effects of solar flux changes on ozone chemistry are manifested through a
chain of chemical perturbations, in particular reactions R1 and 4. which increase ozone
abundances in the upper stratosphere.
R1:
R4:
O2 + h → O + O,
N2O + h → N2 + O,
 ≤ 242 nm
 ≤ 210 nm
(Isaksen et al, p 4357; reference
“The effect of solar flux changes through the main chemical perturbations, reactions
R1 and R4, is to increase ozone abundances in the upper stratosphere at solar maximum.” [Isaksen et al, p 4357]
looking for a second reference…
[Allen, 1984, p. 4843] has a rate constant for R1 (also labels it as R1) and a reference
and puts the wavelength in the range 177.5 nm ≤  ≤ 256 nm
It has many other reactions that produce or consume ‘Active-O’ and ‘Active-H’ and the
wavelengths and rate constants, including the water analog to Isaksen’s R4:
(R6) H2O + h → H2 + O(1D),
 ≤ 121.57 nm (Lyman-
.
I guess I could go back and fill in a lot of the ’s and the reaction rates too.
). All atmospheric variables being equal, increasing oxygen photolysis increases ozone
production. At solar max there is more ozone causing less water and more hydroxide. At
solar min (like during SOFIE 2007-2008) then the same effect would allow more water to
remain. Direct water photolysis will do the same.
These reactions involving ozone and oxygen compounds are affected by the
presence and reactions of other compounds as well, as seen previously with chlorine.
When water is present, oxygen atoms which could form ozone can also with the water to
form hydroxyl radicals instead:
O*(1D) + H2O → OH + OH
[Wayne, p214].
35
This particular effect on ozone concentrations is small in comparison since the
atmospheric concentration water is so much smaller than that of diatomic oxygen.
Hydroxyl also reacts with products of some reactions that consume ozone (e.g. NO2,
HO2, Cl), thus it is able to favor those reactions and further reduce ozone indirectly. The
radicals OH, NO, and Cl all can affect ozone in ways like this, they all can be stored in
reservoir molecules as reaction products, and photolysis greatly enhances the breakdown
of the reservoir molecules to free them. [Wayne, See the chemical cycle diagrams on p.
136, 137.]
This is not to say that there are no other chemical or photochemical reactions that
would enhance ozone production—electrical discharge phenomena like lightning can
produce ozone too. There are a multitude of chemical and photochemical reactions (and
physical processes) that continue throughout the atmosphere nudging equilibrium one
way or the other (references
[Allen, 1984, p. 4843;
Garcia 1989, p.14612 has a section on effects of solar flux variation;
Rodger, 2008 concludes TLEs are not significant to the mesosphere’s neutral
chemistry… actually ‘finds little evidence’ is the word choice.
I didn’t want to exclude possible explanations, but it seems the electrical
phenomena are covered.
). Obviously, the equilibrium of the atmosphere generally has been providing a
beneficial environment for us to enjoy.
d. Surface Chemistry on ice
Aerosols are particles of liquid or solid in the air (aero) that are suspended ‘in
solution’ (sol). They originate from a variety of sources, including salts evaporated from
sea spray, wind-lofted dessert dust, smoke condensed from vaporized meteors, and
volcanic ash (and some products of chemical reactions in the atmosphere, like the nitric
acid trihydrate mentioned in the polar stratospheric clouds section below). If there is
enough of an aerosol present in large enough particles, it absorbs, reflects, and scatters
light enough to become visible—clouds are a familiar example of aerosol droplets of
water or ice. Aerosol particles can clump together, settling in the atmosphere as heavier
particles overcome weaker air currents, and finally falling out when they grow too heavy.
(Aerosol particle types, sizes, and distributions also contribute to the state of the
atmosphere.)
Cloud ice particles provide an aerosol surface (and a volume) that enhances
reaction between atmospheric compounds: the cloud particle interacts and provides
another pathway for transitions otherwise constrained and limited or even forbidden by
conservation laws for the same reactions between the reactants alone (reference
36
“In the gas phase, reaction (1) is thermodynamically accessible with an
exothermicity of 4.8eV, and even though it is spin-forbidden, such reactions can occur at
rapid rates on surfaces, e.g. the reaction between N and NO to form NeO [Halpern and
Rosner, 1982]. The surface acts as a catalytic agent and both removes the spin
restrictions and stabilizes the reaction product. Incomplete energy accomodation is also
known to occur in surface reactions where for highly exothermic reactions the energy
release can appear as translational energy sufficient to desorb the product molecule
before surface accomodation can occur.” [Summers & Siskind 1999]
“Liberation of the active chlorine from the reservoirs [HCl, ClONO2] is normally rather
slow. But it turns out that the two reservoir molecules can react together in the presence
of ice particles, such as those that make up the polar stratospheric clouds
ClONO2 + HCl → Cl2 + HNO3. The outcome is that molecular chlorine is
released as a gas, and the nitric acid remains in the ice particles, which can ultimately
transport water and nitric acid out of the vortex.” [Wayne, p. 188]
). They also sequester products which are highly soluble, such as nitrates
(reference Wayne, p. 188). This is important because there is an unnatural abundance of
chlorine in the atmosphere that originates from pollutants like chlorofluorocarbons and
cycles into reservoir molecules like HCl and ClONO3:
Cl + CH4
ClO + NO2
→
→
HCl + CH3,
ClONO3.
[Wayne, p. 188]
Atomic chlorine and other free radical compounds (like OH– and NO–, which are at more
natural levels) can destroy ozone molecules catalytically, being recycled in the process
and freed again to destroy more. Nitrate can bind with atomic chlorine, but the nitrate
uptake by the clouds leaves the chlorine free. Chlorine levels accumulate over the winter
and the rapid increase in light in the September spring activates the chlorine.
ClONO2 + HCl
Cl2 + h
→
→
Cl2 + HNO3 ,
Cl + Cl.
Cl + O3
→
ClO + O2.
[Wayne, p. 188-189]
Catalytic chlorine reactions reduce ozone levels by half over a few weeks [Wayne, 185],
much faster than photolysis can replenish the ozone.
2(
ClO + ClO + M
(ClO)2 + h
ClOO + M
Cl + O3
→
→
→
→
(ClO)2 + M,
Cl + ClOO,
Cl + O2 + M,
ClO + O2 .
)
Net
2 O3 + h
→
3 O2
[Wayne, p. 189]
37
(where M is a variable that represents a compatible material/molecule.)
The annually appearing ‘ozone hole’ has well-defined edges until the vortex
breaks up and air circulation dissipates the feature (reference
Sorry, ‘dissipates’ was a bad word choice. …and circulation brings some air and
ozone from lower latitudes into the polar region and … reduces the depletion…
is the hole persistent?
“Simultaneous analyses of CO2 and CFCs suggest that the ‘age of air’ in the
vortex may be as much as five years. … The rather shallow hole found in 1988 is a result
of just such meteorological factors. It most certainly does not mean that the problem has
gone away, and , indeed, the depletion in September 1989 was as great as that in 1987.
The variability of the depth of the hole from year to year illustrates just how sensitive the
phenomenon is to changes in dynamical activity, which are superposed on a general
deepening trend resulting from the increased atmospheric burden of the
chlorofluorocarbons. ” [Wayne , p. 191]
I think I associated too closely the temporary appearance of the vortex with the ozone
hole. Now with the reference to ‘five years’ for the age of air in the hole, the hole seems
to not go away completely, and I’m not sure that the southern vortex goes away, though
the northern one does … http://research.iarc.uaf.edu/IPY-CTSM/strato_tropo.php
“[about the Antarctic]… Later in spring, when the vortex breaks up, more extensive
lateral mixing takes place. Export of ozone-poor air from polar regions to middle
latitudes might lead to a dilution of ozone that could persist for up to a year because of
the relatively slow photochemical replacement of ozone, and transport of processed air
rich in active chlorine could enhance the effect. If the deficit lasts from one spring to
another, there could be a cumulative, permanent depletion of ozone.” [Wayne, p. 193]
Thank goodness for Brewer-Dobson circulation… the tropopause is lower at the poles (I
think due to ozone distribution) [NASA SEES plot]… Brewer-Dobson circulation was to
explain total column ozone being more at polar latitudes than at tropical latitudes where
there is maximum sunshine [Houghton, p. 68-70] (except maybe at the summer pole
when it is continuously lit over 24-hour daylight days, if talking about daily total
sunshine)… The plot of total column ozone did show more at the north pole in spring
than at the south pole in spring [plot on Houghton, p. 69]… summer upwelling [Thomas,
1991] will probably send ozone up into the mesosphere and then equatorward in the
mesospheric circulation cell [Houghton]… So, is the ozone hole more of a hole, or more
of a souffle’ that has risen and become less dense’—is the animation at
http://svs.gsfc.nasa.gov/vis/a000000/a003000/a003067/index.html a total column ozone
picture, or only for a layer in a certain altitude range. It would be nice to be able to see
the rest of the globe in the animation and a full year cycle—wait, there’s HALOE.
). The mesospheric circulation cell slows, and, with the passage of the sun into
the other hemisphere and the change of seasons, it begins the reverse cycle with the poles
exchanging roles. The pole cap regions (within the Arctic and Antarctic circles) together
38
comprise only 1/12 of the earth’s surface, and the remainder of the area covers the tropics
(about 5/12) and the mid-latitude bands (about half).
A multitude of chemical and photochemical reactions proceed throughout the
atmosphere even though they do not have such strong accompanying visible signals as
the high-altitude PSCs and PMCs that signal water variation over the polar regions.
Compositional features arise where there are regions of similar composition which
undergo similar processes and that are surrounded by or near some kind of difference in
composition that helps to distinguish the feature—otherwise the atmosphere would be
uniform and featureless.
Figure 10. Temperature profile with circulation: northern summer/southern winter
[Reference: NASA. Studying Earth's Environment From Space. June 2000. (Aug 2008)
http://www.ccpo.odu.edu/SEES/index.html
Reactions that occur in the atmosphere depend upon what compounds are present
and what promoting factors (e.g. sunlight energy, aerosol surface) are available. Of
course, reactions yield products, and the presence of those compounds can be taken as
supporting evidence for particular reactions.
e. PSCs in Winter: polar stratospheric clouds
In the dark of the winter pole, the lower stratosphere holds its coldest
temperatures. Air flowing downward from the mesosphere and upper stratosphere
encounters the cold and the higher pressure, both of which can (and often do) promote the
condensation of water vapor into ice or liquid. Polar stratospheric clouds form in mixes
of water ice or liquid with nitric acid, and sometimes sulfuric acid, depending upon
conditions (reference
Wayne, p. 187 has the nitric acid in PSCs. Later I saw sulphuric acid in the types
of PSCs on Wikipedia: http://en.wikipedia.org/wiki/Polar_stratospheric_clouds —it
39
makes sense if there are sulphur compounds as from volcanoes… okay, Wayne, p. 194
says, “The understanding that heterogeneous chemistry is central to Antarctic ozone
depletion has brought a heightened awareness of the importance of aerosol processes in
stratospheric chemistry as well as physics. Recent laboratory experiments have shown
that reactions can proceed on sulphuric acid particles that are similar to those taking place
on the surfaces of PSCs. Sulphuric acid aerosol is widely distributed in the stratosphere,
and might thus be responsible for ozone depletion in non-polar regions. Such activity
would obviously be enhanced in periods following volcanic eruptions that deposit
sulphate aerosol in the stratosphere. Stratospheric ozone trends in middle-to-high
latitudes in winter are hard to explain by gas-phase mechanisms (section 4.6), but could
be accounted for if the sulphate aerosol injected into the stratosphere by the El Chicho’n
explostion (see section 1.3) was responsible for heterogeneous loss. …This circumstantial
evidence is at least suggestive of chemical processing on aerosol surfaces.” [Wayne, p.
194]
).
The stratosphere is colder than ‘normal’ because it can radiate heat. Because it is
dark, it does not get heat from sunshine. Downwelling air should be warmed by
compression (see Figure 10). Increased pressure should increase the partial pressure of
water in a parcel, so that should make it promote condensation—but condensation should
warm the air.
Wayne [p184] likens the cold core of the southern winter vortex to a reaction
vessel that almost isolates the air inside from the rest of the atmosphere. Photochemistry
diminishes in the cold darkness of the polar winter vortex, and chemical reactions which
do occur can progress further towards depletion of reactants. Distinct compositional
features develop: for example, ozone levels over the Antarctic are reduced in the south
polar vortex and replenished when mixing resumes, seasonally forming and refilling an
ozone ‘hole’. The vortex edge wanders, subject to the same disturbances that cause
Rossby waves.
Stratospheric aircraft measurements of 1987 confirmed anomalous chemistry
within the southern polar vortex region (reference
[This paragraph is rephrased from Wayne, p. 185-187]
). Late August measurements at 18 km altitude showed normal concentrations of ozone,
but a sharp tenfold increase in the ClO (chlorine oxide) concentration crossing poleward
into the vortex over a few hundred kilometers around 65°S. (Atmospheric chlorine
originates mainly from human activities.) Mid-September measurements showed that the
ozone concentration outside the vortex was a little higher than before, but the
concentration inside the vortex was half the value outside. Other measurements showed
that the vortex air depleted in water vapor and oxides of nitrogen, which occurs through
formation of clouds of water and nitric acid mixes. [Wayne, p185-187]
“The dehydration and denitrification is explained by the condensation of water and the
conversion of the oxides of nitrogen to nitric acid in the polar stratospheric clouds. The
40
clouds are made up of particles of two sizes, the smaller of which (about 1m diameter)
condense at a temperature of about -80° C, and the larger (about 10 m diameter)
condense at about -85° C. The smaller sized cloud particles are, in fact, composed of
nitric acid trihydrate, but the larger ones are made up of water-ice, with substantial
amounts of nitric acid dissolved in them and are large enough to sink slowly through the
atmosphere, and thus remove both water and active nitrogen compounds. It is this
removal of the oxides of nitrogen which leads to the anomalous chlorine chemistry. Only
the core of the vortex is cold enough for the larger particles to form, so we see again that
it is the combination of low temperature and special dynamics in the atmosphere that set
up the conditions needed for perturbed chemistry.” [Wayne 1991, p 187]
“Reactions on PSC particles activate chlorine to forms that are capable of
photochemical ozone destruction, and sequester nitrogen oxides (NOx) that would
otherwise deactivate the chlorine3,4 … We find that these PSCs were not composed of
nitric acid trihydrate but instead had a more complex composition, perhaps that of a
ternary solution. Because cloud formation is sensitive to their composition, this finding
will alter our understanding of the locations and conditions in which PSCs form. In
addition, the extent of ozone loss depends on the ability of the PSCs to remove NOx
permanently through sedimentation. The sedimentation rates depend on PSC particle size
which in turn is controlled by the composition and formation mechanism14”[ Owen B.
Toon & Margaret A. Tolbert Spectroscopic evidence against nitric acid trihydrate in
polar stratospheric clouds. [Nature 375, 218 - 221 (18 May 1995); doi:10.1038/375218a
Toon & Tolbert, 1995]
f. PMCs in Summer: polar mesospheric clouds and noctilucent clouds
Polar mesospheric clouds (PMCs) are the highest detected cloud formations,
sometimes appearing as noctilucent clouds. They are so high in the mesosphere, about 83
km altitude, that in high latitude regions (over 50N) during summer, observers can see
sunlight reflected down from them in the local night sky. The PMCs are made of water
ice [Hervig 2001 First confirmations] and appear seasonally extending over the summer
polar caps to about 50 N in a thin layer, 1-3 km thick, appearing about 5 wks before
solstice to about 7 wks after (reference [Thomas, 1991, p. 553b]).
“SME satellite measurements indicate that PMC are ubiquitous within the heart of the
season although always spatially patchy. PMC approach 100% probability of occurrence
near the pole. … The striking difference between PMC and NLC is that during the cloud
season, PMC occurs every day (on every orbit of the SME spacecraft) , whereas even for
conditions of optimum viewing, NLC may be completely absent for many consecutive
days” [Thomas, 1991 p558b-559a]
Though sunshine is continual over the summer pole between equinoxes and for
about seven midsummer weeks the daily insolation exceeds that of any other location on
earth, the summer polar mesosphere is cooled to the lowest temperature of any part of the
atmosphere.
41
Gravity waves propagating into the atmosphere from disturbances in the
troposphere alter the changes in the upper stratosphere circulation in summer (from
westerly to easterly) alter its impedance to gravity waves and thus the spectrum of gravity
waves that reach and pass it. This also changes the amount of energy that the waves
carry and where the waves break and release that energy. Cooling is caused by the
upwelling and adiabatic expansion of air induced by seasonal (summer) circulation
changes in the middle atmosphere (reference [Thomas 1991]
“As described by Garcia and Solomon [1985], the year-round persistence of
westerlies in the lower stratosphere is important, since the combination of westerlies at all
heights below the mesopause in winter means that the majority of gravity waves with
phase speeds less than about 20 m s–1 will pass unimpeded to the mesosphere. Their
large amplitudes will cause these waves to "break" in the vicinity of 75 km, where their
effects are rather minor because of the comparatively high air density In contrast, during
summer the existence of both easterly and westerly winds causes a much narrower
portion of the wave spectrum to penetrate to the mesosphere. These fast moving waves
will have a much smaller amplitude and will therefore break at higher altitudes, near or
above the mesopause.” [Thomas 1991]
).
The upper mesosphere is cold in summer and warm in winter, and this is
important to the theory of ice formation in the mesosphere and hence the appearance of
PMCs. Gravity waves and planetary waves originating from the troposphere reach up to
and dissipate energy into the stratosphere and mesosphere, providing momentum drag on
the circulation there [Summers, Vertical couplings [Summers, 1999] I may have to
recheck 1999 references because Summers & Siskind was also in 1999]]
Figure 11. SOFIE temperature profiles averaged over 5/24-9/8/2007
“In contrast, during summer the existence of both easterly and westerly winds
causes a much narrower portion of the wave spectrum to penetrate to the mesosphere.
These fast moving waves will have a much smaller amplitude and will therefore break
42
at higher altitudes, near or above the mesopause. The annual oscillation of the highlatitude zonal wind in the mesosphere and upper stratosphere reaches its peak
(easterly) value about 2 to 3 weeks following summer solstice. This coincidence with
the PMC maximum (see Figure 2) illustrates how the underlying winds may be the
overall controlling factor for the development of the cold mesopause and the
seasonal variation of PMC/NLC [Schröder, 1974].”
Polar Mesospheric Clouds form over the summer pole which is the coldest part of
earth in the summer polar mesosphere in full sun.
“The sharp lower boundary is now explained by ice particle sublimation. It now appears
that cosmic dust (probably in the form of coagulated "smoke" particles produced from
meteor ablation) is important as sites for ice nucleation despite it being optically
unobservable [Hunten et al., 1980; Clancy and Rusch, 1989]. This reliance on an
unobserved nucleating agent remains one of the most uncomfortable aspects of the
theory. The possibility that large water cluster ions may also act as ice-forming embryos
will be discussed later.” [Thomas, 1991, p554a ]
If the upwelling at the summer pole draws air through the tropopause, then most
likely water vapor is supplied at 3 ppmv (if the 3 ppmv continues into the polar latitudes,
which don’t show in the HALOE plots presented later). This would increase the amount
of water in the atmosphere above the tropopause. Likewise, the downwelling should
send water back down towards the tropopause and decrease the amount of water over the
winter pole.
4. Satellite Observations of Water Vapor, Ozone, and Methane.
The atmosphere can be sampled directly (in situ) by airplane, balloon, and devices
such as falling spheres launched and then released from some height, but these methods
return data from a small portion of the atmosphere , e.g. the path of the detector, for each
instance of effort. Remote measurements using spectroscopy on reflected light or light
transmitted and filtered through the atmosphere provide composition data for each path of
the light beam used, which usually can be redirected relatively easily to check another
part of the atmosphere and repeated at subsequent times with minimal additional effort.
Two remote measurement techniques are lidar (light detection and ranging), of
which a typical example would be to measure the reflected signals from a ground-based
upward directed laser beam (or perhaps airplane-based equipment), and satellite remote
sensing, which can be designed to detect wavelengths reflected and/or transmitted
through the atmosphere to spectroscopic equipment based on a moving satellite platform.
A laser source can be pulsed and the time profile of the return signal used to determine
distance to the samples. Using sunlight that passes through the atmosphere (either
directly to the satellite detector or reflected from the earth’s surface or aerosols towards
the detector) does not provide sample distance: instead the comparison of the filtered
beam with the unfiltered solar spectrum shows the extended distribution of wavelengths
that have been absorbed somewhere along the path.
43
In a process called solar occultation, a detector measures sunlight that has been
filtered through the atmosphere (reference
I guess this sounds like we can do solar occultation in the laboratory.
[Russell et al, 1993, p. 10778]
rewrite: In a process called solar occultation, a detector measures sunlight that
has been filtered through the atmosphere: the detector is embodied in a satellite platform
and makes measurements as the satellite proceeds through the sunrise or sunset portions
of its orbit.
). When the detector is embodied in a satellite platform, it can make the
measurements as the satellite proceeds through the sunrise or sunset portions of its orbit.
Depending on the relative position of the sun, the ‘straight’ sunbeams pass through upper
layers of the atmosphere on entry and exit, and pass through a lower altitude where the
sunbeam is tangent to the earth. Beams tangent at high altitudes can be used to determine
atmospheric composition at high altitudes: since beams that pass closer to the earth’s
surface have to penetrate the higher-altitude parts of the atmosphere too, the high-beam
information can be used to ‘subtract out’ effects of the high-altitude portions of lower
beams, effectively providing a way to better determine absorption (and therefore the
composition) at lower altitudes.
The HALOE (Halogen Occultation Experiment) instrument used this method and
was able to obtain a calibration spectrum by detecting sunlight directly, either before the
sun set into the atmosphere or after the sun rose beyond the atmosphere at the appropriate
part of its orbit. HALOE used the principle of solar occultation, which is the
measurement of sunlight that reaches the detector after passage through the atmosphere.
As the satellite orbits, it ‘sees’ multiple sunrises and sunsets during each 24-hour day. At
equinox the beam follows a parallel of latitude, and at solstice the beam crosses parallels
of latitude at an angle about the same as the earth’s tilt relative to the sun, but the latitude
span is between entry and exit is limited and small due to the earth’s and atmosphere’s
curvature. The occultation latitude changes little from day to day, but varies according to
the earth’s relative tilt (e.g. the earth’s position in orbit around the sun) and the satellite’s
orbital tilt and precession. Of course, consecutive sunrises and sunsets generally alternate
between north and south latitudes, and it is possible for certain orbits to be entirely in
sunlight for days at a time at certain orientations during the year.
The beam path enters the atmosphere at high altitudes, crosses longitudes into
lower altitudes over the curved surface of the earth, and leaves again at high altitudes to
reach the satellite. Portions of the solar spectrum are absorbed by the atmosphere all
along the beam path with the noise of reflection or scattering away or into the detected
beam, thus the detected irradiances are the result of effects all along the beam path
combined together without information of where certain attenuations occurred. However,
the satellite makes multiple measurements across the sun’s disk, which changes elevation
as the satellite orbits through the region sunrise or sunset, and measurements where
44
beams cut tangentially through only higher altitudes of curved sections of atmosphere can
be ‘deducted’ from those which pass through lower parts of the atmosphere. The satellite
can recalibrate at each occultation by measuring the unfiltered sunlight when the satellitesun line is clear of the earth’s atmosphere.
5. Thesis Plan
The overall goal of this thesis is to characterize the middle atmospheric water
vapor distribution in order to understand the atmosphere’s water budget. In particular,
analysis of satellite observations of middle atmosphere water reveal a variety of layered
water features in the atmosphere that must be associated with source regions. In this
thesis we will quantify the layered water vapor distribution observed in the low latitude
mesosphere (Summers et al., Science, 277, 1967, 1997a; Summers and Siskind, GRL, 26,
1837, 1999). We will also detail its connection with layer(s) of elevated water vapor
observed at high polar latitudes (Summers et al., GRL, 28, 3601, 2001). The former are
arguably a consequence of heterogeneous chemistry, whereas the later may be due to
both water ondensation/sublimation physics associated with polar mesospheric clouds
(Stevens et al., GRL, 28, 4449, 2001; Hervig et al., GRL, 28, 971, 2001) and
heterogeneous chemistry. This research will use both UARS HALOE water vapor
observations (1991-present) and recent SOFIE (1997 to present) to characterize the
mesospheric water vapor layer and its annual variation.
The specific steps that have been undertaken in this research are:
(1) HALOE observations (1991-2005) were used to characterize the mesospheric
water vapor layer and its annual variation.
(2) The HALOE climatology was used as a means to develop expectations for water
vapor variations that would be observed with the AIM SOFIE experiment
launched in April, 2007.
(3) Detailed comparisons between the HALOE climatology and SOFIE 2007-2008
observations have revealed both expected and unexpected behavior of middle
atmospheric water vapor in the arctic summer mesosphere.
(4) The HALOE/SOFIE comparisons have been analyzed to explore the possibility
that solar cycle effects may cause long-term variations in mesospheric water
vapor.
45
Figure 12. Comparions between HALOE and SOFIE 2007 monthly mean water vapor
(left) N Jun-Aug (right) S Oct-Dec.
Figure 12 shows an example of the HALOE/SOFIE comparisons that we will
present in Chapter 3 of this thesis. The HALOE-SOFIE 2007 water vapor abundances
are really close together for May-Aug for the northern hemisphere 55-90 km altitude
range. Major changes in the vertical variation of water vapor are seen throughout the
PMC season, i.e., summer months June through August. These differences are
independent of whether HALOE sunrise or sunset data or AM/PM data are used.
Though HALOE data covers the majority of the globe’s latitude zones, it is sparse
in the sampling in time, so that deducing time-dependent variations at a given latitude are
difficult to determine from such a comparison. For any particular year, HALOE’s return
to a given zone often took around 40 or more days, and adding samplings from
consecutive years only closes the gaps a little.
As we will show in Chapter 4, for those few latitudes and years where we have
substantial overlap between HALOE monthly average observations and SOFIE
observations at the same latitude and month, there appears to be a weak correlation with
solar cycle. A solar cycle influence is not surprising. Solar heating of the upper
atmosphere causes the atmosphere to “breath” upward and downward during a solar cycle
as seen in Figure 13.
46
Figure 14 ( taken from reference[Brasseur & Solomon, 1986, p.
126]…)
In addition SOFIE has a relatively narrow band of latitude coverage and began its
mission near solar min. A satellite that provides data in a latitude zone frequently enough
and for a long enough duration, perhaps six or seven years from solar min to solar max,
may be able to provide a set of sample data where the atmosphere’s response to the solar
cycle is more apparent. Even so, mixed in will be the atmosphere’s response to
influences like volcanic eruptions and changes in industrial inputs.
The atmosphere is a complex system and its dynamics, chemistry, and physical
phenomena are intricately interrelated. The time scale of the atmosphere's response to
changes in inputs of material or energy is long enough that multi-year study is important,
particularly because vertical circulation in the stratosphere takes years. For example,
Volcanic ash from Mt Pinatubo in 1991 was injected into the stratosphere and so could
affect the atmosphere for many years. The somewhat regular variance of the energy
input from the sun during its 11 (22)-year solar cycle also acts on a multi-year timescale.
Meteoric dust seems like a more steady input of material into the atmosphere. It should
be somewhat periodic at least as the Earth’s orbit takes it through the Leonid, Perseid,
Taurid or other meteor debris zones, but otherwise rather constant (reference
This was written just recently. I think the thought occurred while I was looking through
the references while making the list. It was probably inspired by seeing this:
“The number densities of meteoric smoke is obviously very dependant on the amount of
meteoric material that is deposited in the atmosphere. For the studies above we have used
Hughes’s (1978) estimate of 44 tonnes per day .However, this number is not well known
and measurements range from ~5 to 110 tonnes per day (Mathews et al., 2001; Love and
Brownlee, 1993). Panel d of Fig. 2 and the black dashed-dotted line in Fig. 3 shows the
47
smoke distributions obtained with a meteoric influx of 100 tonnes per year. As expected”
[Megner, 2008, p. 46-47)]
Certain meteor showers are named according to the constellation where the meteors
appear in the sky. I tried recalling the names of the meteor showers I had heard about. I
have a mental image now of regions of potential meteors around the earth’s orbit, moving
too. If the earth moves into and out of meteor-dense regions, the influx should vary, like
Megner and Mathews noted.
).
Atmospheric dust (e.g. dust recondensed from evaporates of meteors) is thought
to provide a pathway for the recombination of the products of water photolysis back into
water molecules. Considerations of the effects of the dust might account for the
difference between standard models and layering of water vapor seen by HALOE. But in
the absence of validated models, the SOFIE measurements have provided an alternate
dataset for comparison and extending the time period of mesospheric water observations.
The satellite measurements still can provide information (14 years HALOE + SOFIE)
for other researchers. It provides a reference climatology (averaged over HALOE's 14
years) which can be taken as a standard even if it is neither a normal nor ideal standard.
48