Course: Introduction to Atmospheric sciencesATOC210 by GyuWon LEE

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Chapter 10 Air stability and Cloud development
Reading materials Chap 7 of the text book
Atmospheric stability
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
2
Questions 1) Why does air rise on some
occasions and not others? 2) Why do the size
and shape of clouds vary so much?
Objectives 1) Understand the concept of the
airs stability. 2) Understand the
relationship between the airs stability and
cloud development.
Approach 1) General concept Definition of the
air stability 2) Types of stability. 3)
Controls What factors control the air
stability? 4) Application Relation between air
stability and clouds
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
3
General Concept
Stable equilibrium After being lifted or
lowered, air tends to return to its
original position. Unstable equilibrium air
moves farther away from its original position.
(Air favors vertical air currents).
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
4
General Concept
Cooling or heating by dry/moist adiabatic process

• Recall pressure decreases with height!!
• Thus, when air parcel is lifted, it expands and
  cools, while a sinking parcel is compressed and
  warms. When there is no interchange of heat with
  its surroundings
• (Dry/moist) Adiabatic process
• The temperature of a lifted air parcel is
  greater than it surrounding, air parcel rises
  continuously (unstable). Otherwise, air parcel
  goes back to its original position. (stable)

Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
5
General Concept
Thus, the decrease of surrounding temperature
(environmental lapse rate) should be compared
with the change of temperature by (dry/moist)
adiabatic process.
Temp. of environment
Lifted air parcel
4km 18 C
0 C
When the air in the parcel is unsaturated, dry
adiabatic rate ( 10 C /1km). When saturated,
moist adiabatic rate. This rate is smaller than
dry adiabatic rate because of release of latent
heat by condensation. (Here, we assume moist
adiabatic rate 6 C /1km).
2km 22 C
10 C
1km 26 C
20 C
Sfc 30 C
30 C
Environmental lapse rate 4 C /1km
adiabatic rate 10 C /1km
lt

• Stable!!
• Lifted air parcel goes back to
• its original position

Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
6
Types of stability
Absolute stable the environmental lapse rate is
less than the moist adiabatic rate
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
7
Types of stability
Absolute unstable the environmental lapse rate
is greater than the dry adiabatic rate
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
8
Types of stability
Conditional unstable the environmental lapse
rate is between the moist adiabatic rate
and the dry adiabatic rate
The rising, unsaturated air at each level is
colder than its surroundings and saturated air is
warmer than its surrounding. Thus, the atmosphere
is stable if the rising air is unsaturated and
unstable if the rising air is saturated.
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
9
Types of stability
Summary
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
10
What factors control the air stability?
The atmosphere stabilizes as the air aloft warms
and the surface cools, and destabilizes as the
air aloft cools and the surface air warms.
Causes of instability 1) cooling of the air
aloft - winds brings in colder air (cold
advection) - clouds (or the air) emitting
infrared radiation to space (radiational
cooling) 2) warming of the surface air -
daytime solar heating of the surface.
- an influx of warm air brought in by the
wind (warm advection) - air
moving over a warm surface. 3) mixing or
lifting of a layer of air. Causes of stable
condition - nighttime radiational cooling of the
surface. - cold advection at the
surface - air moving over a cold
surface - sinking of an entire layer
of air.
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
11
What factors control the air stability?
The atmosphere becomes more stable when an entire
layer of air sinks and more unstable when an
entire layer of air is lifted.
As a layer of air sinks, it will warm by
adiabatic compression and becomes compressed by
the weight of the atmosphere and shrinks
vertically. Then, the upper part of the layer
sinks farther and hence warms more than the
bottom. In the figure, the top of the layer is
actually warmer than the bottom ? (subsidence)
inversion absolutely stable
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
12
What factors control the air stability?
lifting of a layer of air
Suppose the bottom of the layer is saturated
while the air at the top is unsaturated. If the
layer is forced to rise, the upper portion of the
layer cools at the dry adiabatic rate, while the
air near the bottom cools more slowly at the
moist adiabatic rate. Convective (potential)
instability
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
13
What factors control the air stability?
Mixing of a layer of air
Suppose the air in the layer is mixed by external
forces. Air is cooled adiabatically as it is
brought up from below and heated adiabatically as
it is mixed downward.
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
14
What factors control the air stability?
External forces to lift the air
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
15
Air stability vs. cloud development
Vertical extension of clouds
Animation
(Ref) Dew point temperature decreases 2
C/1000m. When the air saturates and condensation
continues to occur, the dew point temperature
decreases at the moist adiabatic rate.
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
16
Air stability vs. cloud development
Vertical extension of clouds
Three different environmental lapse rates
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE
17
Air stability vs. cloud development
Orographic uplift, cloud development, and the
formation of a rain shadow
Rocky Mountains
moist pacific flow
Chinook (Foehn) winds
Course Introduction to Atmospheric
sciences(ATOC210) by GyuWon LEE