Download Atmospheric Moisture and Stability

Atmospheric Moisture
and Stability
Lecture 7
October 21, 2009
Homework #2 Comments
• Winds are nearly geostrophic in the upper levels of
the atmosphere because there is less friction.
Homework #2 Comments
Fastest wind
at point b
because PGF
is largest
(isobars
closely
spaced
together)
Fr
CF
PGF
Wind
• Friction in the opposite direction of wind
• CF always at 90° perpendicular to wind
• Wind flows across isobars in lower levels
(not in geostrophic balance)
Review from last week
• Energy is the ability or capacity to do work
on some form of matter
• Kinetic energy – the energy an object
possesses as a result of its motion
KE = ½ mv2
• 1st Law of Thermodynamics – Energy
cannot be created or destroyed.
– Energy lost during one process must equal
the energy gained during another
Review
• Therefore the 1st law states that heat is really
energy in the process of being transferred
from a high temperature object to a lower
temperature object.
• Heat can be transferred by:
– Conduction: transfer of heat from molecule to molecule
within a substance by direct contact
– Convection: the transfer of heat by the mass movement
of a fluid in the vertical direction (up and down)
– Advection: the transfer of heat in the horizontal direction
– Radiation: the transfer of heat through electromagnetic
wave energy
Moving on to moisture and stability…
Water is responsible for many of
Earth’s natural processes
http://www.srh.weather.gov/jetstream/atmos/hydro.htm
Water can exist in all three phases in
our atmosphere
• What atmospheric variable do we use to
quantify the amount of water in any given
volume of air at one time?
• Answer: Moisture
Ways to measure the moisture
content of the atmosphere
• Absolute Humidity- The ratio of the mass of
water vapor to the volume occupied by a mixture
of water vapor and dry air.
• Specific Humidity- The mass of water vapor /
unit mass of air, including the water vapor.
• Mixing Ratio- ratio of the mass of water vapor
per kg of dry air
• Saturation Mixing Ratio- mass of water vapor
when a parcel is saturated / mass of dry air in
the parcel.
• Vapor Pressure- Pressure of water vapor
constituent of the atmosphere.
• Saturation Vapor Pressure- The pressure of
water vapor constituent when the atmosphere is
saturated.
The variables we will refer to most
• Mixing Ratioratio of the mass of water vapor per kg of dry air
(does not change with temperature)
• Relative HumidityVapor Pressure/ Saturation vapor pressure
• Dew Point TemperatureThe temperature to which a given air parcel must be
cooled at constant pressure and constant water vapor
content in order for saturation to occur.
There are only TWO ways
to saturate the air
(or increase the relative humidity)
1. Add more water vapor to the air
2. Cool the air until its temperature is
closer to the dew point temperature
Remember the water vapor molecules are
moving faster in warm air and less likely to
stick together and condense. If air cools to
the dew point temperature, there is
saturation.
Moisture
• An air parcel with a large moisture content
has the potential for that parcel to produce
a great amount of precipitation.
- Air with a mixing ratio of 13 g/kg will likely
rain a greater amount of water than air
with a mixing ratio of 6 g/kg.
Two parcels of air:
PARCEL 1: Temperature = 31 oF, Dew point = 28 oF
PARCEL 2: Temperature = 89 oF, Dew point = 43 oF
Parcel 2 contains more water vapor than Parcel 1, because
its dew point is higher.
Parcel 1 has a higher relative humidity, because it
wouldn’t take much cooling for the temperature to equal
the dew point! Thus, Parcel 1 is more likely to become
saturated. But if it happened that both parcels became
saturated then Parcel 2 would have the potential for more
precipitation.
RH is not simply equal to the dew point divided by the
temperature but is a good representation.
Moisture and the Diurnal Temperature Cycle
• Review: Water has a high heat
capacity (it takes lots of energy to
change its temperature)
• As a result, a city with a dry
climate (like Sacramento, CA) will
have a very large diurnal (daily)
temperature cycle
• A city with high water vapor
concentration (like Key West, FL)
will have a small diurnal cycle
Late July averages:
Sacramento: ~94/60
Key West: ~90/79
The other key component to the
hydrologic cycle- Stability
• What is stability?
• Stability refers to a condition of equilibrium
• If we apply some perturbation to a system, how
will that system be affected?
– Stable: System returns to original state
– Unstable: System continues to move away from
original state
– Neutral: System remains steady after perturbed
Stability Example
Stable: Marble returns to its original position
Unstable: Marble rapidly moves away from initial position
Stability
How does a bowl and marble relate to the atmosphere??
• When
the atmosphere is stable, a parcel of air that is
lifted will want to return back to its original position:
http://www.chitambo.com/clouds/cloudshtml/humilis.html
Stability Cont.
• When the atmosphere is unstable (with respect to a
lifted parcel of air), a parcel will want to continue to rise
if lifted:
http://www.physicalgeography.net/fundamentals/images/cumulonimbus.jpg
What do we mean by an air parcel?
– Imaginary small body of air a few meters wide
• Can expand and contract freely
• Does not break apart
• Only considered with adiabatic processes - External
air and heat cannot mix with the air inside the parcel
• Space occupied by air molecules inside parcel defines
the air density
• Average speed of molecules directly related to air
temperature
• Molecules colliding against parcel walls define the air
pressure inside
Buoyancy and Stability
• Imagine a parcel at some pressure level that is held
constant, density remains the same so the only other
variable that is changing is temperature. (REMEMBER:
the Ideal Gas Law)
• So if ρparcel < ρenv. Parcel is positively buoyant
• In terms of temperature that would mean:
T of parcel > T of environment – buoyant! (unstable)
T of parcel < T of environment – sink! (stable)
T of parcel = T of environment – stays put (neutral)
Atmospheric Stability
This is all well and good but what about day to day
applications?
Review: Atmospheric Soundings
• Vertical “profiles” of the atmosphere are taken at 0000 UTC (7
AM CDT) and 1200 UTC (7 PM CDT) at ~80 stations across the
country, and many more around the world. Sometimes also
launched at other times when there is weather of interest in the area.
• Weather balloons rise to over 50,000 feet and take measurements
of several meteorological variables using a “radiosonde.”
• Temperature
• Dew point temperature
• Wind
- Direction and Speed
• Pressure
http://www2.ljworld.com/photos/2006/may/24/98598/
Vertical Profile of Atmospheric
Temperature allows us to assess
stability of the atmosphere
Lapse Rates
•Lapse Rate: The rate at which temperature
decreases with height (Remember the inherent
negative wording to it)
•Environmental Lapse Rate: Lapse rates
associated with an observed atmospheric sounding
(negative for an inversion layer)
•Parcel Lapse Rate: Lapse rate of a parcel of air as
it rises or falls (either saturated or not)
•Moist Adiabatic Lapse Rate (MALR): Saturated air
parcel
•Dry Adiabatic Lapse Rate (DALR): Dry air parcel
DALR
• Air in parcel must be unsaturated (Relative
Humidity < 100%)
• Rate of adiabatic heating or cooling =
~10°C for every 1000 meter (1 kilometer)
change in elevation
– Parcel temperature decreases by about 10° if
parcel is raised by 1km, and increases about
10° if it is lowered by 1km
MALR
• As rising air cools, its RH increases
because the temperature approaches the
dew point temperature, Td
• If T = Td at some elevation, the air in the
parcel will be saturated (RH = 100%)
• If parcel is raised further, condensation will
occur and the temperature of the parcel
will cool at the rate of about 6.5°C per 1km
in the mid-latitudes
DALR vs. MALR
• The MALR is
less than the
DALR because
of latent heating
– As water vapor
condenses into
liquid water for a
saturated parcel,
LH is released,
lessening the
adiabatic
cooling
Remember no heat exchanged with environment
DALR vs. MALR
Absolute Stability
• The atmosphere is
absolutely stable
when the
environmental lapse
rate (ELR) is less than
the MALR
ELR < MALR <DALR
– A saturated OR
unsaturated parcel
will be cooler than
the surrounding
environment and
will sink, if raised
Absolute Stability
• Inversion layers are
always absolutely
stable
– Temperature
increases with
height
– Warm air above
cold air = very
stable
Absolute Instability
• The atmosphere is
absolutely unstable
when the ELR is
greater than the DALR
ELR > DALR > MALR
– An unsaturated OR
saturated parcel will
always be warmer
than the
surrounding
environment and
will continue to
ascend, if raised
Conditional Instability
• The atmosphere is
conditionally
unstable when the
ELR is greater than the
MALR but less than the
DALR
MALR < ELR < DALR
– An unsaturated
parcel will be cooler
and will sink, if
raised
– A saturated parcel
will be warmer and
will continue to
ascend, if raised
Conditional Instability
• Example: parcel at
surface
– T(p) = 30°C, Td(p) =
14°C (unsaturated)
– ELR = 8°C/km for first
8km
• Parcel is forced
upward, following
DALR
• Parcel saturated at
2km, begins to rise at
MALR
• At 4km, T(p) =
T(e)…this is the level
of free convection
(LFC)
Conditional Instability
• Example continued…
– Now, parcel will rise
on its own because
T(p) > T(e) after 4km
– The parcel will freely
rise until T(p) = T(e),
again
• This is the
equilibrium level (EL)
• In this case, this
point is reached at
9km
– Thus, parcel is stable
from 0 – 4km and
unstable from 4 – 9km
EL
LCL
Rising Air
• Consider an air parcel
rising through the
atmosphere
– The parcel expands as it
rises
– The expansion, or work
done on the parcel causes
the temperature to
decrease
• As the parcel rises,
humidity increases and
reaches 100%, leading to
the formation of cloud
droplets by condensation
Rising Air
• If the cloud is sufficiently
deep or long lived,
precipitation develops.
• The upward motions
generating clouds and
precipitation can be
produced by:
– Convection in unstable air
– Convergence of air near a
cloud base
– Lifting of air by fronts
– Lifting over elevated
topography
Lifting by Convection
• As the earth is heated by
the sun, thermals
(bubbles of hot air) rise
upward from the surface
• The thermal cools as it
rises, losing some of its
buoyancy (its ability to
rise)
• The vertical extent of the
cloud is largely
determined by the
stability of the
environment
Lifting by Convection
• A deep stable layer
restricts continued
vertical growth
• A deep unstable layer
will likely lead to
development of rainproducing clouds
• These clouds are
more vertically
developed than
clouds developed by
convergence lifting
Lifting by Convergence
• Convergence exists
when there is a
horizontal net inflow
into a region
• When air converges
along the surface, it is
forced to rise
Lifting by Convergence
• Large scale convergence can lift air
hundreds of kilometers across
• Vertical motions associated with
convergence are generally much weaker
than ones due to convection
• Generally, clouds developed by
convergence are less vertically developed
Lifting due to Topography
• This type of lifting occurs
when air is confronted by
a sudden increase in the
vertical topography of the
Earth
– When air comes across a
mountain, it is lifted up and
over, cooling as it is rising
• The type of cloud formed
is dependent upon the
moisture content and
stability of the air
Lifting due to Topography
Lifting Along Frontal Boundaries
• Will discuss origin more in detail
next week as we begin to discuss
cyclones and fronts