G109:
8. Atmospheric Stability
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8. A TMOSPHERIC S TABILITY
Reading Assignment:
• A&B:
Ch. 5
(p. 142-146)
•
Ch 6
(p. 160-168)
• LM:
Lab# 8
1. Lapse Rates
Lapse rate: - rate at which temperature decreases
with height.
- in [K / km] or [°C / km]
- a positive value indicates decrease of T
with height
• Troposphere - general decrease in T with height
• Three types of lapse rates:
a) ELR - Environmental Lapse Rate
b) DALR - Dry Adiabatic Lapse Rate
c) SALR - Saturated (wet) Adiabatic Lapse Rate
a) Environmental Lapse Rate
• Actual air T we measure: i.e. Observed air T at any
height
• Varies in space and with time
• Upper air T sounding
http://weather.unisys.com/upper_air/skew/skew_KILN.html
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b) Dry Adiabatic Lapse Rate
• Rate which a non-saturated air parcel cools as it rises
• Rate:
2500
Height (m)
2000
1500
1000
500
0
0
5
10
15
20
25
Temperature (C)
− 10 K − 1 K − 10 K
=
=
1000 m 100 m 1 km
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Why does this decrease in T occur?
• Adiabatic Process - a physical change of the state of
the air parcel that does not involve exchange of
energy with the air surrounding the air parcel.
• Pressure and density decreases with height
⇒ As air parcel rises - expands & cools
c) Saturated Adiabatic Lapse Rate
• Air is saturated - condensation occurs
⇒ Release of latent heat of vaporization (LV)
amount of energy (per mass) required to change
the phase of a substance from liquid to gaseous –
gas → liquid — energy release
⇒ Keeps the parcel warmer than it would
otherwise
⇒ Decrease in T with height is not as great
• Dependent on the amount of moisture in the
atmosphere
• More moisture in the atmosphere the greater will be
⇒ warmer the air will remain
the release of LV
• Range:
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to
More moisture
9 K km-1
Less moisture
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1000
Height (m)
800
600
400
200
DALR
SALR = 9 K/km
SALR = 4 K/km
0
10
12
14
16
18
20
Temperature (C)
• When air cools to the Tdew (saturation!), then convert
from DALR to SALR
• At base of clouds Tair = Tdew =Lifting condensation level
1200
Height (m)
1000
DALR
SALR
800
600
400
200
0
8
10
12
14
16
18
20
Temperature (C)
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2. Atmospheric Stability
• Talked about air parcels moving up & down in the
atmosphere but have not talked WHY
Stability:
Tendency of an air parcel to move vertically
following an initial dislocation (up or down)
Atmospheric stability is dependent on T and ρ
• The thermal stratification of the air: vertical temperature
profile, environmental lapse rate, ELR
• To determine stability, vertical motions of air parcels
are assumed to be adiabatic processes
Principle:
• Temperature differences between an air parcel and its
surrounding lead to density differences and thus to
buoyancy forces in upward (positive) or downward
(negative) direction.
Î
Air warmer than its surroundings will tend to rise
(because of its lower density)
Î
Air cooler than its surroundings will tend to sink
(because of its greater density)
Î
Air at the same temperature as its surroundings
will tend to remain at the same height (because
there is no density difference)
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In Practice:
• Can use the lapse rates to determine:
o Which direction the air will move?
o How far the air will move?
Method to determine stability:
• Compare the environmental air T (given by the ELR)
with the DALR or SALR
• Start at a given point on the ELR (T for given height)
• From that point, move upward, along the dry (or
saturated) adiabat (i.e., draw a line with the slope of
the DALR or the SALR)
• If T on the adiabat is less than ELR - STABLE
2500
Height (m)
2000
1500
1000
500
0
0
5
10
15
20
25
Temperature (K)
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• If T on the adiabat is greater than ELR - UNSTABLE
2500
Height (m)
2000
1500
1000
500
0
0
5
10
15
20
25
Temperature (K)
• If T on the adiabat is same than ELR - Neutral
2500
Height (m)
2000
1500
1000
500
0
0
5
10
15
20
25
Temperature (K)
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The atmosphere is considered neutral in this case.
There is no tendency to enhance or suppress vertical
motions.
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The atmosphere is now stable. It acts to resist vertical
motions, and restore displaced parcels of air to their
original positions.
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The air is unstable, and vertical motions are enhanced.
Rising motions are very likely in such an atmosphere.
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•
At a particular height temperature on the
ELR is Greater === ELR is less than the
DALP
•
Temperature on the ELR is Less ==== ELR is
greater than the DALP
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Conditionally unstable or conditional instability:
stable or unstable depends on whether the air is
saturated or dry
ELR is not normally just a simple line (see Lab 8)
•
Air pollution will get trapped under an inversion
o Inversion - T increases with height
o Lapse - T decrease with height
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If there are unstable conditions and the air rises
• Cools as it rises
• May cool down to the dew point T
• Condensation - clouds
Where air sinks - warms up
• T moves away from Tdew - so no clouds will form
• Clear air
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Problem:
An air parcel at the surface is not saturated and has a
Temperature of 30°C and Tdew=25°C. Assuming it moves
adiabatically and the SALR is -8.5 °C km-1 what will the
air T be at 3000 m?
4000
3500
Height (m)
3000
2500
2000
1500
1000
500
0
-5
0
5
10
15
20
25
30
35
Temperature (K)
Air parcel will be at saturation when it is cooled to ___
With the DALR: What Height ?_____
Then the rate at which it cools is SALR
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Factors which modify air's stability
Enhanced instability - make air more unstable
⇒ more mixing
1. Intense solar heating
• Warm air from below (free convection) accompanied
by updraft to form clouds.
• Summer day - cumulus clouds mid/late afternoon
2. Heating of an air mass from below as it traverses a
warm surface
3. Forceful lifting
a) Uplift over a mountain (orographic lifting): upward
displacement → adiabatic cooling
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b) Convergence e.g. low pressure system
Plan view
Side View
Convergence - air lifted
c) Frontal wedging - due to air ρ differences
i) Stable air lifted
ii)
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Unstable air lifted
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