Outline Further Reading: Chapter 06 of the text book - stability and vertical motions - five examples - orographic precipitation Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (1 of 18)

Introduction Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (2 of 18) 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

Stability Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (3 of 18) StableUnstable 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

Lapse Rates Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (4 of 18) 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

Vertical Motions Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (5 of 18) 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 ….

km 2 km 280 Temperature Height 3 km 270 Dry Adiabatic Lapse Rate Moist Adiabatic Lapse Rate Environmental Lapse Rate Absolutely Unstable  e >  d,  m Example: #1 Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (6 of 18)

Example #1 Discussion Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (7 of 18) –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 (8 of 18) Example # km 2 km 280 Temperature Height 3 km 270 Dry Adiabatic Lapse Rate Environmental Lapse Rate Absolutely Unstable

Example #2 Discussion Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (9 of 18) –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 (10 of 18) Example # km 2 km 280 Temperature Height 3 km 270 Dry Adiabatic Lapse Rate Moist Adiabatic Lapse Rate Environmental Lapse Rate Absolutely Stable  e <  d,  m

Example #3 Discussion Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (11 of 18) –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 (12 of 18) Example # km 2 km 280 Temperature Height 3 km 270 Dry Adiabatic Lapse Rate Moist Adiabatic Lapse Rate Environmental Lapse Rate Conditionally Stable  d  e >  m

Example #4 Discussion Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (13 of 18) –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 (14 of 18) Example # km 2 km 280 Temperature Height 3 km 270 Dry Adiabatic Lapse Rate Moist Adiabatic Lapse Rate Environmental Lapse Rate  e  d  >  m

Example #5 Discussion Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (15 of 18) –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

Real World-01 Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (16 of 18) –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

Real World-02 Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (17 of 18) Therefore, although convection might be occurring, we do not see it until we see the formation of clouds at the lifting condensation level

Orographic Lifting (The Movie)(The Movie) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 14: Stability Feb (18 of 18) –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”