MET4300_SWX_LEC8

MET4300_SWX_LEC8 - MET 4300 Lecture 8 Atmospheric Stability...

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MET 4300 Lecture 8 Atmospheric Stability
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Stability Concept Stable: Ball returns to original position Neutral: Ball stays wherever it is placed Unstable: Displacement grows with time.
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Atmospheric Stability Air parcel: a distinct blob of air that we will imagine we can identify as it moves through the atmosphere Stable: If the parcel is displaced vertically, it will return to its original position Neutral: If the parcel is displaced vertically, it will remain in its new position Unstable: If the parcel is displaced vertically, it will accelerate away (upward or downward) from its original position
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Adiabatic Process: is one in which a parcel of air does not mix or exchange heat energy with its environment. Rising: expansion because of lower pressure cool Sinking: compression because of higher pressure warm
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Lapse Rate The rate at which temperature decreases with height in the atmosphere Standard atmosphere lapse rate = 6.5 o C km -1 Adiabatic Process: no heat added or removed Dry Adiabatic: 9.8 o C km -1 Cooling balances work done as air expnds Moist Adiabatic: ~6.5 o C km -1 Cooling and latent heat released by condensation balance pressure work
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Parcel Thermodynamics A Parcel is a mass of air that moves from one point to another, perhaps changing its properties in reaction to its surroundings Adiabatic Process : No heat added or removed, but temperature and/or water mixing ratio as the parcel Expands into lower pressure using internal energy to do work Is compressed by higher pressure converting work done by its surroundings into internal energy
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Dry Adiabatic Process Do Work Cool Warm Work Done An unsaturated parcel experiences an adiabatic pressure change Lower p Higher p
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Moist Adiabatic Process Warm Evaporate Water Work Done Do Work Cool Condense Water A saturated parcel experiences an adiabatic pressure change Lower p Higher p
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Two Different Lapse Rates
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Stability for These Lapse Rates Absolute Stability Absolute Instability
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Conditional Instability
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Note: The moist adiabatic lapse rate actually may vary from 4 to 10 o C km -1
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Some Representative Soundings Unstable Stable
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