Download Orbit of a Geostationary Satellite

Volcanological Meteorology
Some background on more specific
meteorological topics which we will
discuss later in the class
With materials from American Meteorological Society and
various web sources
Lecture #8 Ashfall Grad Course Fall 2009
ATMOSPHERIC SCIENCE
Academic Press, Second Edition
By John Wallace, University of Washington, Seattle,
U.S.A.
Peter Hobbs, University of Washington, Seattle,
U.S.A.
Contents
1.Introduction and Overview
2.The Earth System
3. Atmospheric Thermodynamics
4. Radiative Transfer
5. Atmospheric Chemistry
6. Cloud Microphysics
7. Atmospheric Dynamics
8. Weather Systems
9. The Atmospheric Boundary Layer
10. Climate Dynamics
Air
Temperature
and Altitudinal
Relationships in
the
Atmosphere
© American Meteorological Society
3
• Troposphere
– Lowest layer
– Where almost all weather occurs
– Temperature decreases with
altitude
• Generally, but with frequent
exceptions (e.g., inversion,
isothermal layer)
• Average temperature drop is
6.5 °C/1000 m (3.5 °F/1000
ft)
– ~ 6 km (3.7 mi) thick at the
poles, ~20 km (12 mi) thick at
the equator
– Upper boundary/transition zone
to next layer is called the
tropopause
4
• Stratosphere
– Goes from troposphere up to ~ 50
km (30 mi)
– In lower stratosphere, temperature
is constant
• This is an isothermal condition
– Above 20 km (12 mi), temperature
increases with increasing altitude
• Stratosphere is warmed by the
energy released by ozone
absorbing UV radiation
– Stable conditions are ideal for jet
aircraft travel, but can cause
trapping of pollutants (e.g. from
volcanic eruptions) in lower
stratosphere
– Upper boundary/transition zone to
next layer is called the stratopause
5
Average Air
Pressure
Variation with
Altitude
Expressed in mb
6
Air Pressure Measurement
• A mercury thermometer employs air
pressure to support a column of
mercury in a tube
• Air pressure at sea level will support
the mercury to a height of 760 mm
(29.92 in.)
• Height of the mercury column
changes with air pressure
• Adjustments required for:
– The expansion and contraction of
mercury with temperature
– Gravity variations with latitude and
altitude
7
Air Pressure
Measurement
• An aneroid barometer is less
precise, but more portable than a
mercury barometer
• It has a chamber with a partial
vacuum
• Changes in air pressure collapse or
expand the chamber
• This moves a pointer on a scale
calibrated equivalent to mm or in.
of mercury
• New ones are piezoelectric –
depend on the effect of air
pressure on a crystalline substance
• Home-use aneroid barometers
often have a fair, changeable, and
stormy scale
– These should not be taken literally
8
Air Pressure Measurement
• Forecasting uses air
pressure and
tendency values
– changes over time
• Barometers may
keep a record of air
pressure
– These are called
barographs
9
• Units of length
– Millimeters or inches
Air Pressure Units
• Inches typical for TV
• Units of pressure
– Pascal – worldwide standard
• Metric scale
• Sea-level pressure =
– 101,325 pascals (Pa)
– 1013.25 hectopascals (hPa)
– 101.325 kilopascals (kPa)
– Bars – U.S.
• A bar is 29.53 inches of mercury
• A millibar (mb) is the standard used on weather maps, meaning 1/1000 of
a bar
– Usual worldwide range is 970 – 1040 mb
– Lowest ever recorded - 870 mb (Typhoon Tip in 1979)
– Highest ever recorded – 1083.8 mb (Agata, Siberia)
10
Variations in Air Pressure w/Altitude
• Overlying air compresses the atmosphere
– the greatest pressure is at the lowest elevations
• Gas molecules are closely spaced at the surface
• Spacing increases with altitude
• At 18 km (11 mi), air density is only 10% of that at sea level
• Because air is compressible, the drop in pressure with altitude is
greater in the lower troposphere
– Then it becomes more gradual aloft
• Vertical profiles of average air pressure and temperature are
based on the standard atmosphere – state of atmosphere
averaged for all latitudes and seasons
• Even though density and pressure drop with altitude, it is not
possible to pinpoint a specific altitude at which the atmosphere
ends
– ½ the atmosphere’s mass is below 5500 m (18,000 ft)
– 99% of the mass is below 32 km (20 mi)
– Denver, CO average air pressure is 83% of Boston, MA
11
Horizontal Variations in Air Pressure
• Horizontal variations are much
more important to weather
forecasters than vertical
differences
– In fact, local pressures at
elevations are adjusted to
equivalent sea-level values
– This shows variations of
pressure in the horizontal
plane
– This is mapped by connecting
points of equal equivalent sealevel pressure, producing
isobars
12
Horizontal Variations in Air Pressure
• Horizontal changes in pressure can be accompanied by
significant changes in weather
• In middle latitudes, a continuous procession of
different air masses brings changes in pressure and
weather
– Temperature has a much more pronounced affect on air
pressure than humidity
• In general, the weather becomes stormy when air
pressure falls but clears or remains fair when air
pressure rises
13
Horizontal Variations in Air Pressure
• Influence of temperature and humidity
– Rising air temperature = rise in the average kinetic
energy of the individual molecules
• In a closed container, heated air exerts more pressure on
the sides
– Density in a closed container does not change
– No air has been added or removed
• The atmosphere is not like a closed container
– Heating the atmosphere causes the molecules to space
themselves farther apart
– This is due to increased kinetic energy
– Molecules placed farther apart have a lower mass per unit
volume, or density
– The heated air is less dense, and lighter.
14
Horizontal Variations in Air Pressure
• Influence of temperature and humidity, continued
– Air pressure drops more rapidly with altitude in a column of
cold air
• Cold air is denser, has less kinetic energy, so the molecules are closer
together
– 500 mb surfaces represent where half of the atmosphere is
above and half below by mass
• This surface is at a lower altitude in cold air vs. in warm air
– Increasing humidity decreases air density
• The greater the concentration of water vapor, the less dense is the air
due to Avogadro’s Law
• We often refer to muggy air as heavy air, but the opposite is true
– Muggy air only weighs heavily on our personal comfort factor
15
Air pressure varies continuously
16
Horizontal Variations in Air Pressure
• Influence of temperature and humidity,
continued
– Cold, dry air masses are the densest
• These generally produce higher surface pressures
– Warm, dry air masses generally exert higher
pressure than warm, humid air masses
– These pressure differences create horizontal
pressure gradients
• Pressure gradients cause cold or warm air advection
– Air mass modifications can also produce changes in
surface pressures
– Conclusion: local conditions and air mass advection
can influence air pressure
17
Horizontal Variations in Air Pressure
• Influence of diverging and converging winds
– Diverging = winds blowing away from a column of air
– Converging = winds blowing towards a column of air
– Diverging/converging caused by :
• Horizontal winds blowing toward or away from some
location (this chapter)
• Wind speed changes in a downstream direction (Chapter
8)
18
Horizontal Variations in Air Pressure
19
Highs and Lows
• Isobars are drawn on a map as previously discussed
– U.S. convention – these are drawn at 4-mb intervals (e.g., 996
mb, 1000 mb, 1004 mb)
• A High is an area where pressure is relatively high
compared to the surrounding air
• A Low is an area where pressure is relatively low
compared to the surrounding air
• Highs are usually fair weather systems
• Lows are usually stormy weather systems
– Rising air is necessary for precipitation formation
– Lows are rising columns of air. Highs are sinking columns of
air.
20
Orbit of a
Polar-Orbiting
Satellite
http://coastwatch.glerl.noaa.gov/modis/modis.cgi/modis?region=s&page=1
21
Geostationary Satellites for Meteorology (and Volcanology!)
• http://www.ssec.wisc.edu/data/geo/
• http://www.rap.ucar.edu/weather/satellite/
• http://www.ssec.wisc.edu/data/volcano.html
Weather Satellite Imagery
 Visible
– Black and white
photograph of
the planet
– Only available
during daylight
hours
– Highly
reflective
surfaces appear
bright white
and less
reflective
surfaces are
darker
23
Weather Satellite Imagery
• Infrared
– Available
anytime, not
just during
daylight
– Provides
temperature
comparison of
features within
image
– Whiter = colder
• Higher cloud
tops appear
whiter,
because they
are colder
24
Weather Satellite Imagery
• Water vapor
imagery
– Enables
tracking of
plumes of
moisture
– Shades of
white =
increasing
moisture
– Upper-level
clouds appear
milky to bright
white
25
Remote Sensing
• Measurement of
environmental
conditions by
processing signals that
are either emitted by an
object or reflected back
to a signal source
– Radar
– Satellites
26
Weather Radar
 Complements
satellite
surveillance
 Doppler radar
detects
movement
– Excellent tool
to forecast
tornadoes
27
Combination of Images
 Composite of
IR and Radar
28
What do clouds tell us?
• Clouds just don’t happen - there’s always a
reason
• A particular cloud’s shape and location
depend on (and can therefore tell us about):
- the movement of the air
- amount of water vapor in air
- stability (flat clouds = stable air while puffy
clouds = unstable air)
Types of Clouds
• Sorted by height
– High (> 7km) Cirrus (ci), Cirrostratus (cs),
Cirrocumulus (cc)
– Middle (2-7km) Altostratus (as), Altocumulus (ac)
– Low (< 2km) stratus (st), cumulus (cu), stratocumulus (sc)
Types of Clouds
• Sorted by structure
– Cirrus – Composed of Ice, ‘wispy’
– Cumulus – Detached elements ‘puffy’, ‘cottonlike’
– Stratus – Uniform Layer, ‘sheetlike’
– Nimbo – Precipitation producing
• Nimbostratus
• Cumulonimbus
Cirrus Clouds
http://seaborg.nmu.edu/clouds/types.html
Cumulus Clouds
Stratus Clouds
Cumulonimbus
What is this?
http://www.rap.ucar.edu/weather/satellite/
What this is
Snow
Snow
Cirrus
Stratus
Cirrus
Stratus
Snow is reflective, but not too cold. Stratus in Mississippi is the same, and the
wispy but cold values in Ontario are cirrus
http://www.rap.ucar.edu/weather/satellite/
What is this?
What this is
Cumulus
Cumulo
Nimbus
Cirrus
Cumulus
Cumulo
Nimbus
Cirrus
The low (warm) and scattered clouds in Georgia are cumulus. The scattered by
higher clouds in central Florida are cumulonuimbus. The wispy clouds southwest
of Florida are cirrus clouds
Upper Air Observations
• Radiosondes
– Invented in late 1920s
– Transmits altitude readings (soundings) of:
• Temperature
■
Dewpoint
■
Air pressure
– Data is received immediately
• No need to recover equipment
– Rawinsonde
• A radiosonde that is tracked by direction-finding antennas
• Provides data on wind direction and speed
• Dropwindsonde is not launched with a balloon
» It is dropped from an aircraft on a parachute
– These devices are launched simultaneously worldwide
• Launched at 0000Z and 1200Z
• Only 20% of the devices are recovered
40
Graphic measurements
of air temperature, pressure,
& dew point up to about
30,000 m (100,000 ft)
A radiosonde
Launching a
radiosonde
41
Upper Air Data
• http://weather.uwyo.edu/upperair/sounding.html
• http://www.rap.ucar.edu/weather/upper/
• http://weather.unisys.com/upper_air/
Heat Imbalance: Atmosphere vs. Earth’s
Surface
43
Heat Imbalance: Atmosphere vs. Earth’s
Surface
44
Latent Heating
Latent heat of vaporization
• Some of the absorbed
solar radiation is used to
vaporize water at Earth’s
surface
• This energy is released to
the atmosphere when
clouds form
• Large amounts of heat
are needed for phase
changes of water
compared to other
substances
45
Latent heat of fusion
Bowen Ratio
• Describes how the energy
received at the Earth’s
surface is partitioned
between sensible heating
and latent heating
• Bowen ratio = [(sensible
heating)/(latent heating)]
• At the global scale, this is
[(7 units)/(23 units)] = 0.3
46
Sensible Heating
• Heat transfer via conduction and convection can
be sensed by temperature change (sensible
heating) and measured by a thermometer
• Sensible heating in the form of convectional
uplifts can combine with latent heating through
condensation to channel heat from Earth’s
surface into the troposphere
– This produces cumulus clouds
– If it continues vertically in the atmosphere,
cumulonimbus clouds may form
47
Phase Changes of Water
• Water absorbs or
releases heat upon
phase changes
– This is called latent heat
• Latent heating
– This is the movement of
heat from one location to
another due to phase
changes of water
• Example – evaporation of
water, movement of vapor
by winds, condensation
elsewhere
48