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Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Further Reading: Chapter 06 of the text book
Outline
- stability and vertical motions
- five examples
- orographic precipitation
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Lecture 14: Stability
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Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
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Myneni
Lecture 14: Stability
Feb-21-07
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Introduction
Previously,
– We learned that when air is displaced vertically, it cools due to adiabatic processes
– If it continues to lift, we expect clouds to form as water vapor changes to liquid
Today,
– What produces lifting in the real environment?
Causes of Vertical motions
– Convection
– Orographic lifting
– Convergence of air at the surface
– Frontal Lifting
– Will discuss convection and orographic lifting now; will discuss convergence and frontal
lifting later
Convection:
– Process in which we find localized vertical motion due to instabilities in the
atmosphere
– Key criteria is stability
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
Feb-21-07
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Stability
Stable
•
•
Unstable
Stability:
– When something tends to return to where it started
Instability:
– When something tends to continue in the direction it is initially moved
• What makes air stable or unstable?
• Need to consider one more lapse rate
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Lapse Rates
cool
warm
warm
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•
•
•
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Lecture 14: Stability
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cool
cool
Environmental lapse rate:
– The actual (or measured) temperature change of the air with respect to altitude
Density
– The number (or equivalently, weight) of molecules in a given volume
– =1/T: Density is proportional to the inverse of temperature
– If parcel is cooler (more dense) than surrounding air it will sink
– If parcel is warmer (less dense) than surrounding air it will rise
From before, we know the dry adiabatic lapse rate (=10K/1km) and the moist adiabatic lapse
rate (~6K/1km)
Now lets see how we determine stability from these lapse rates
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
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Vertical Motions
• Test: What we want to test is if a hypothetical parcel of air will continue to move in
the direction it is pushed (unstable) or if it returns to where it started (stable)
• Because it is a “hypothetical” parcel, we know it will either follow the dry
adiabatic lapse rate or the moist adiabatic lapse rate depending upon whether it is
saturated or not
• Five examples to follow ….
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
Feb-21-07
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Example: #1
3 km
Dry Adiabatic Lapse Rate
Moist Adiabatic Lapse Rate
2 km
Height
1 km
Environmental Lapse Rate
270
280
290
Temperature
300
Absolutely Unstable
ge>Gd, Gm
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
Feb-21-07
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Example #1 Discussion
– We measure the environmental lapse rate
– Start with the dry adiabatic lapse rate which we know
– Steps:
• #1: raise the parcel to 1km and see if it is warmer or colder than the surrounding
atmosphere: warmer
• #2: Determine whether a warmer parcel will rise or sink: rise
• #3: Does the parcel continue in the direction that it started: yes -> the environment
is unstable
– Do the same procedure with a saturated parcel; it is also unstable
– Therefore the environment is “absolutely unstable”
– This is true if the environmental lapse rate is greater than both the dry adiabatic and
moist adiabatic lapse rates
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
Feb-21-07
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Example #2
3 km
2 km
Height
Environmental Lapse Rate
1 km
Dry Adiabatic Lapse Rate
270
280
290
300
Temperature
Absolutely Unstable
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
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Example #2 Discussion
– Now lets start with a parcel at 2km but with the same environmental lapse rate
– We measure the environmental lapse rate
– Steps:
• #1: lower the parcel to 1km and see if it is warmer or colder than the surrounding
atmosphere: cooler
• #2: Determine whether a warmer parcel will rise or sink: sink
• #3: Does the parcel continue in the direction that it started: yes -> the environment is
unstable
– “unstable” does not just refer to air rising; it also applies to air that is sinking
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
Feb-21-07
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Example #3
3 km
Moist Adiabatic Lapse Rate
2 km
Height
1 km
Environmental Lapse Rate
Dry Adiabatic Lapse Rate
270
280
290
Temperature
300
Absolutely Stable
ge<Gd, Gm
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
Feb-21-07
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Example #3 Discussion
– We measure the environmental lapse rate
– Start with the dry adiabatic lapse rate which we know
– Steps:
• #1: raise the parcel to 1km and see if it is warmer or colder than the surrounding
atmosphere: cooler
• #2: Determine whether a warmer parcel will rise or sink: sink
• #3: Does the parcel continue in the direction that it started: no -> the environment is
stable
– Do the same procedure with a saturated parcel; it is also stable
– Therefore the environment is “absolutely stable”
– This is true if the environmental lapse rate is less than both the dry adiabatic and moist
adiabatic lapse rates
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
Feb-21-07
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Example #4
3 km
Environmental Lapse Rate
Moist Adiabatic Lapse Rate
2 km
Height
1 km
Dry Adiabatic Lapse Rate
270
280
290
Temperature
300
Conditionally Stable
Gdge>Gm
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
Feb-21-07
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Example #4 Discussion
– We measure the environmental lapse rate
– Start with the dry adiabatic lapse rate which we know
– Steps:
• #1: raise the parcel to 1km and see if it is warmer or colder than the surrounding
atmosphere: cooler
• #2: Determine whether a warmer parcel will rise or sink: sink
• #3: Does the parcel continue in the direction that it started: no -> the environment
is stable
– Do the same procedure with a saturated parcel; it is unstable
– Therefore the environment is “conditionally stable”
– This is true if the environmental lapse rate is less than the dry adiabatic lapse rate but
greater than the moist adiabatic lapse rate
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
Feb-21-07
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Example #5
3 km
Environmental Lapse Rate
Moist Adiabatic Lapse Rate
2 km
Height
1 km
Dry Adiabatic Lapse Rate
270
280
290
300
Temperature
geGd > Gm
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
Feb-21-07
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Example #5 Discussion
– We measure the environmental lapse rate
– Start with the dry adiabatic lapse rate which we know
– Steps:
• #1: raise the parcel to 1km and see if it is warmer or colder than the surrounding
atmosphere: same
• #2: Determine whether a warmer parcel will rise or sink: neither
• #3: Does the parcel continue in the direction that it started: neither -> the
environment is neutral
– Do the same procedure with a saturated parcel; it is unstable
– Therefore the environment is “neutral” for a unsaturated parcel but is “unstable” for a
saturated parcel
– This is true if the environmental lapse rate is equal to one of the adiabatic lapse rates
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
Feb-21-07
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Real World-01
– We measure the environmental
lapse rate
– Start with the dry adiabatic lapse
rate which we know
– Steps:
• #1: raise the parcel to 1km and see
if it is warmer or colder than the
surrounding atmosphere: warmer
• #2: Determine whether a warmer
parcel will rise or sink: rise
• #3: Does the parcel continue in the direction that it started: yes -> the environment is unstable
–
–
–
–
At 1km, the air becomes saturated
Now our parcel must follow the moist adiabatic lapse rate
It is still unstable; in fact it is even more unstable than before
The deepest cloud development occurs under unstable conditions with warm, moist air
• Common to tropics
• Also common to the southern, central and eastern US in the summer
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
Feb-21-07
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Real World-02
Therefore, although convection might be occurring, we do not see it until we see the
formation of clouds at the lifting condensation level
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 14: Stability
Feb-21-07
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Orographic Lifting (The Movie)
– This occurs when air is forced to flow over a
mountain range
– Steps:
• Air starts to rise over the mountains and cools
• At 500m it reaches the lifting condensation
level, i.e. it becomes saturated
• As the air continues to rise, water must
condense out
• In addition, now it cools as the moist adiabatic
lapse rate because as moisture condenses,
latent heat is released and warms the parcel
• As the air descends on the other side, it warms with the dry adiabatic lapse rate, hence when it
reaches its starting elevation on the other side it is drier and warmer than when it started
– On the windward side, we find cool, moist air
– On the leeward side, we find warm, dry air
• This side is in the “rainshadow”