Transcript Slide

Stability, Adiabatic Processes,
Precipitation
Stability of a parcel of air is tendency to
resist displacement (stay put!)
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surface
Air moves according to pressure gradient force (from High to Low )
So, why don’t air molecules escape into space?
Pressure gradient balanced by gravity.
“hydrostatic equilibrium” :
Upward pressure gradient is balanced by
downward force of gravity
Atmosphere (in general) is in hydrostatic
equilibrium.
• But…
• In any given place at any given time, there
are large bodies of air (parcels) that are
not in hydrostatic equilibrium with their
surroundings
If surrounding air is more dense / colder than
the parcel, parcel will rise.
warm
Colder / more dense
surrounding air
UNSTABLE
A parcel will rise until it gets to an altitude at
which surrounding air is at same density /
temperature
If surrounding air is less dense / warmer than the
parcel, parcel will sink.
Warmer / less dense air
cold
STABLE
Therefore,
stability is determined by temperature
difference between parcel and
surrounding air.
Temperature of surroundings decreases
with altitude at
AVERAGE environmental lapse rate:
(3.5°F / 1000 ft; 6.4°C / 1000 m)
What about parcel?
As parcel rises, it expands due to lower
atmospheric pressure of surrounding air;
as parcel expands, temperature drops.
As a parcel of air descends, it contracts due
to greater atmospheric pressure of
surrounding air; as parcel contracts,
temperature rises.
.
These changes in temperature:
– NOT due to heat exchange with surroundings,
– due to expanding and contracting.
called ADIABATIC
– (change in temperature with no gain or loss of
heat)
Dry Adiabatic Lapse Rate
Rate at which temperature drops in rising
body of unsaturated air:
5.5°F / 1000 ft.
OR
10°C / 1000 m.
Wet Adiabatic Lapse Rate
Rate at which temperature drops in
saturated rising air (temperature at dew
point):
3.3°F / 1000 ft. OR
6°C / 1000 m.
NOTE: this rate is lower than dry rate
WHY?
Mechanisms causing air parcels to
rise:
• Spontaneous:
– Convergence
– Convection
• Forced :
– Orographic Uplift
– Frontal Lifting
GOES
Cold front
Warm front
Lifting condensation level
Top & bottom elevations of clouds:
Bottom: lifting condensation level:
temperature of rising air reaches dew point
Top: Cloud rises until cloud temperature
reaches temperature of surroundings
(becomes stable).
Cloud thickness depends on thickness of
unstable layer.
Back to stability….
To test stability, push parcel up and see if it
continues to rise.
• Several possible results….
Example #1:
ELR = 11 degrees / 1000m
Parcel is saturated so use Wet AR: (What is it?)
6 degrees / 1000 m
Therefore, surroundings cool faster than
parcel. Parcel’s tendency will be to RISE.
UNSTABLE
(ELR>WAR)
Example #2:
ELR = 11 degrees / 1000m
Parcel is unsaturated so use Dry AR:
(What is it?)
10 degrees / 1000 m
Therefore, surroundings cool faster than
parcel. Parcel’s tendency will be to
RISE.
UNSTABLE
(ELR>DAR)
Example #3:
ELR = 4 degrees / 1000 m
Parcel is unsaturated so use Dry AR:
(What is it?)
10 degrees / 1000 m
Therefore, parcel cools faster than
surroundings. Parcel’s tendency will be to
STAY PUT.
STABLE
(ELR<DAR)
Example #4:
ELR = 4 degrees / 1000m
Parcel is saturated so use Wet AR: (What is it?)
6 degrees / 1000 m
Therefore, parcel cool faster than
surroundings. Parcel’s tendency will be to
STAY PUT.
STABLE
(ELR<WAR)
Example #5:
ELR = 8 degrees / 1000 m
If air is saturated,
use Wet AR.
If air is unsaturated,
use Dry AR.
Stable or unstable?
Stable or unstable?
unstable
stable
summary
If ELR > WAR , DAR : UNSTABLE
If ELR < WAR, DAR : STABLE
If WAR < ELR < DAR: CONDITIONALLY
STABLE (depends on saturation)
(Atmosphere is usually conditionally stable).
Sinking air warms at dry adiabatic rate.
PRECIPITATION
(rain , snow, sleet, freezing rain, hail)
• Large parcel of air rises and cools
– Adiabatic cooling
• Cools to dew point
• Cloud droplets grow large enough to fall
• Why do cloud droplets need to grow?
Processes by which cloud droplets/crystals
grow :
A. Bergeron Findeisen Process
Mid- and High latitudes: “Cold Clouds”
(tops colder than 0°C)
– Saturation vapor pressure is greater over
water than ice
•
It takes more water vapor to saturate air over
water than over ice
– ice crystals grow at the expense of water
droplets
Bergeron Findeisen process
B. Collision/Coalescence process
Low latitudes: “Warm Clouds”
(tops warmer than -15°C)
– Collision and coalescence: some big
drops form; they fall faster than small
drops and collect small drops
Precipitation Types
rain
Precipitation as liquid water
Height
Melting level
0°C
Temperature
virga
snow
Snowflake is several aggregated
crystals.
Height
Temperature
0°C
graupel
“riming”: shell forms around crystal as it
passes through tiny suspended water
drops: forms “graupel” (ice pellets)
Sleet
Frozen raindrops; raindrops that
freeze as they descend.
Height
Melting level
Temperature
0°C
Freezing rain
Supercooled droplets hit a surface
which is below freezing
Height
Melting level
Temperature
0°C