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Understanding Stability in Natural Environments

Learn about the stability and vertical motions in the atmosphere, including convection, orographic lifting, convergence of air, and frontal lifting. Explore the criteria for stability and instability and understand how lapse rates determine stability.

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Understanding Stability in Natural Environments

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  1. Myneni Lecture 14: Stability Feb-25-05 (1 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Further Reading: Chapter 06 of the text book Outline - stability and vertical motions - five examples - orographic precipitation

  2. Myneni Lecture 14: Stability Feb-25-05 (2 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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? Introduction • 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

  3. Myneni Lecture 14: Stability Feb-25-05 (3 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Stability: When something tends to return to where it started Instability: When something tends to continue in the direction it is initially moved Stability Stable Unstable • What makes air stable or unstable? • Need to consider one more lapse rate

  4. Myneni Lecture 14: Stability Feb-25-05 (4 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Lapse Rates • Environmental lapse rate: • The actual (or measured) temperature change of the air with respect to altitude • Density • The number (or 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

  5. Myneni Lecture 14: Stability Feb-25-05 (5 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Key: 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 …. Vertical Motions

  6. Myneni Lecture 14: Stability Feb-25-05 (6 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Example: #1 3 km Dry Adiabatic Lapse Rate 2 km Moist Adiabatic Lapse Rate Height 1 km Environmental Lapse Rate 270 280 290 300 Temperature Absolutely Unstable ge>Gd, Gm

  7. Myneni Lecture 14: Stability Feb-25-05 (7 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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 Example #1 Discussion

  8. Myneni Lecture 14: Stability Feb-25-05 (8 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Example #2 3 km 2 km Height Environmental Lapse Rate 1 km Dry Adiabatic Lapse Rate 270 280 290 300 Temperature Absolutely Unstable

  9. Myneni Lecture 14: Stability Feb-25-05 (9 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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 Example #2 Discussion

  10. Myneni Lecture 14: Stability Feb-25-05 (10 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Example #3 3 km 2 km Moist Adiabatic Lapse Rate Height 1 km Environmental Lapse Rate Dry Adiabatic Lapse Rate 270 280 290 300 Temperature Absolutely Stable e<Gd, Gm

  11. Myneni Lecture 14: Stability Feb-25-05 (11 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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 Example #3 Discussion

  12. Myneni Lecture 14: Stability Feb-25-05 (12 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Example #4 3 km Environmental Lapse Rate 2 km Moist Adiabatic Lapse Rate Height 1 km Dry Adiabatic Lapse Rate 270 280 290 300 Temperature Conditionally Stable Gdge>Gm

  13. Myneni Lecture 14: Stability Feb-25-05 (13 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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 Example #4 Discussion

  14. Myneni Lecture 14: Stability Feb-25-05 (14 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Example #5 3 km Environmental Lapse Rate 2 km Moist Adiabatic Lapse Rate Height 1 km Dry Adiabatic Lapse Rate 270 280 290 300 Temperature eGd>Gm

  15. Myneni Lecture 14: Stability Feb-25-05 (15 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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 Example #5 Discussion

  16. Myneni Lecture 14: Stability Feb-25-05 (16 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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

  17. Myneni Lecture 14: Stability Feb-25-05 (17 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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

  18. Myneni Lecture 14: Stability Feb-25-05 (18 of 18) Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University 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”

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