G109: 8. Atmospheric Stability 1 LapseStability_CB_L 10/17/08 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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8. ATMOSPHERIC STABILITY 1. Lapse Ratesgeog109/topics/08_stability/LapseStability_web.pdf · 10/17/2008 · G109: 8. Atmospheric Stability 1 ... DALR - Dry Adiabatic Lapse Rate c)
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b) Dry Adiabatic Lapse Rate • Rate which a non-saturated air parcel cools as it rises • Rate:
kmK
mK
mK
110
1001
100010 −
=−
=−
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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 the release of LV ⇒ warmer the air will remain
• Range: 4 to 9 K km-1 More moisture Less moisture
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• When air cools to the Tdew (saturation!), then convert
from DALR to SALR
• At base of clouds Tair = Tdew =Lifting condensation level
0
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Hei
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DALR SALR = 9 K/km SALR = 4 K/km
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ght (
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Temperature (C)
DALR
SALR
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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.
Air warmer than its surroundings will tend to rise (because of its lower density)
Air cooler than its surroundings will tend to sink (because of its greater density)
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
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Temperature (K)
Hei
ght (
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• If T on the adiabat is greater than ELR - UNSTABLE
• If T on the adiabat is same than ELR - Neutral
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Temperature (K)
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Temperature (K)
Hei
ght (
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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?
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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-5 0 5 10 15 20 25 30 35
Temperature (K)
Hei
ght (
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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