MSU GEO 203 - Lecture #9: Atmospheric Stability

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GEO 203 Notes – 10/12/10 (Lecture #9: Atmospheric Stability)Air parcel: Balloon-like bubble of airBasic properties (temperature, moisture) are the same everywhere in the parcelIsolated from environment except that Ppar = Penv (pressure of parcel = pressure of environment)Environment: Atmosphere surrounding an air parcelTemperature, dew point, wind, etc. determined by large-scale motions of atmosphereCan be described by soundings (radiosondes) using weather balloonsSurrounds air parcels; is NOT affected by themRising air expands and cools; sinking air compresses and warms+height in atmosphere = -pressureAdiabatic process: if air parcel expands and cools or compresses and warms as it rises/sinks with no interchange of heat w/ environment(Diabatic process: heat exchange IS present)Dry adiabatic lapse rate: 9.8 °C (10 °C) per 1000 mRate of cooling/warming during adiabatic process if parcel is unsaturatedMoist adiabatic lapse rate: about 6 °C per 1 km (1000 m)Rate of cooling/warming during adiabatic process if parcel is saturatedLess than the dry adiabatic lapse rateCools/warms more slowly due to condensation from moist air releasing latent heatNOT a constant: depends on saturationRemember: air is saturated once its temperature is equal to or below its dew point Lifting Condensation Level: condensation occurs as air parcel rises up in atmosphere, reaching dew pointtemperatureUsed to determine cloud baseWhat determines whether an air parcel can go high enough to reach the lifting condensation level?Initial temperature of the air parcelMoisture content of the air parcelThe atmospheric stabilityIf an air parcel is displaced from its original height, it can:RETURN to original height (stable)ACCELERATE upward due to buoyancy (unstable)STAY where it was displaced (neutral)Stability: A property of the environment, not of the air parcelStable: displacement results in return to initial conditionUnstable: displacement results in further displacement from initial conditionAir parcel rises in atmosphere when densityair parcel < densitysurroundingsSince both parcel and environment at same height are at same pressure:When pparcel < penv, Tparcel > TenvWhen pparcel > penv, Tparcel < TenvTparcel > Tenv (unstable atmosphere)Tparcel < Tenv (stable)Tparcel = Tenv (neutral)Environmental lapse rate: rate at which temperature decreases with height as measured by weather balloonAtmosphere is absolutely stable when environmental lapse rate < moist adiabatic lapse rateAbsolutely stable: atmosphere is stable for both saturated and unsaturated air parcelsWhat conditions make atmosphere more stable?Warming aloft and cooling belowAir aloft may be warmed by warm air advection and subsidence warmingAir near surface may be cooled by:Nighttime radiational coolingCold air advectionAir moving over a cold surfaceWhen parcel is warmer than the environment, it will rise in the


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MSU GEO 203 - Lecture #9: Atmospheric Stability

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