CumulonimbusClouds andConvectionStratus and MistCumulonimbus fromSpaceSurface WarmingSurface CoolingThick Low CloudThin High CloudClouds interact with both solarand terrestrial radiation.Thick clouds reflect sunlight welland so influence planetary albedoClouds are also fairly efficientabsorbers/emitters of terrestrialradiation and so contribute to thegreenhouse effectThin High Clouds22 deg haloMostly composed of ice crystals and becausethey are thin solar radiation passes through,yet infrared radiation is mostly absorbed.Contribute more to the greenhouse effect thenthe planetary albedo.Thick Low Clouds•Often composed of water(sometimes ice too)•Highly reflective so they increaseplanetary albedo•Good blackbody radiators• Their temperature is similar to thesurface temperatureBut still they usually their reduction in absorbedsolar is more than their enhancement of thegreenhouse effectStratus with vortices downwindof an islandGreenhouse Effect of CloudsTtop=250K390 W/m2221 W/m2Tsurface=288KCloud: Fout=sT4= 5.67e-8*2504= 221W/m2Surface: Fsurface=sT4= 5.67e-8*2884= 390 W/m2LW flux lost to space is 169 W/m2less due to cloud plus surfacereceives an additional 221 W/m2 ofradiation. Does this compensate forthe loss of absorbed solar flux? 221 W/m2What is assumed?•Cloud is a perfect blackbody•Cloud emits with cloud toptemperatureLocal Energy Balance Only Clouds in the Atmosphere!Outgoing LWFout= sT4= 5.67e-8*2504= 221 W/m2Net Incoming SW Fin= (S/4)(1-A)= 1370/4*0.8= 274 W/m2For a Thin High Cloud with Ttop=270K and A=0.2Fin-Fout = 53 W/m2Net incoming radiation is higher than outgoing so the planetIs not in balance. What happens? It warms till it is in balance,or the cloud goes away!Local Energy Balance Only Clouds in the Atmosphere!Outgoing LWFout=sT4= 5.67e-8*2804= 349 W/m2Net incoming SW Fin=(S/4)(1-A)= 1370/4*0.2= 137 W/m2For a Thick Low Cloud with Ttop=280K and A=0.8Fin-Fout = -212 W/m2Net Incoming radiation is lower than outgoing so the planetIs not in balance. What happens? It cools till it is in balance,or the cloud goes away!Thermal structure of the atmosphereConvection transports heat andmoisture in the atmosphereConvectionDry convection carries no condensate (but it does carry watervapor) while moist convection carries condensate (think clouds)Heating a gas (or fluid) from below results in rising plumes (thermals)of warmer air separated by sinking cool air.Thermals start out as “hot spots” near the ground. They rise untilthey run out of buoyancy (until their density is equal to the density ofthe air around them). Thermals expand and cool as they rise.If thermals rise high enough, the water vapor in them condenses anda cloud forms.Adiabatic approximation -To a goodapproximation thermals rise without losingheat to the surrounding air.Because atmospheric pressure decreaseswith height, the surrounding air pusheswith less force on thermals as they rise.This causes them to expand as they rise.Because they do work to increase theirvolume, they lose energy so they cool.Why do thermals expand and cool as they rise?Familiar examples of adiabatic expansion:Air escaping from an aerosol can or a tireClouds form when rising thermals cool enough so theirtemperature reaches the condensation (or saturation) point forwater vapor. Thermals may continue to rise, causing convectionin the clouds.Parcel of air - a volume of air with similar properties.All lifted air parcel (not just thermals) experiences adiabaticexpansion. Sinking air parcels experiences adiabatic compression.Sinking air parcels warm and therefore evaporate condensate if theyhave any. The condensate doesn’t last long. Clouds do not exist inareas with sinking air.Lapse Rate is the temperature change with height in the atmosphereIt describes the state of the atmosphereBut the atmosphere is neither perfectly unsaturatedor perfectly saturated. Also the real atmosphere isheated throughout by absorbing radiation as well asby convection. The real lapse varies between about4 and 9 C/km.The Dry Adiabatic Lapse Rate is the rate of coolingfor a rising air parcel that is unsaturated~ 10 C/kmThe atmosphere takes on the dry adiabatic lapserate if it has just experienced dry convectionAtmospheric Stability of a Dry AtmosphereThe atmosphere temperature profile is unstable if the rate thatthe temperature decreases with height is greater than the dryadiabatic lapse rate.When this occurs an air parcel that lifts spontaneously will finditself warmer than its new surroundings even though it cools as itlifts.The atmosphere never remains unstable for long.When the atmosphere is pushed towards an unstabletemperature profile, it often reacts violently with thunderstorms ortornadoesStable AirDefined by small temperature decrease with height Caused by weak surface warming Forced sinking in the down draft of convection, on thedownwind side of mountains, or due to large scaleatmospheric circulation Warm air transported over cold air near the surfaceUnstable AirDefined by large temperature decrease with height Caused by strong surface warming Forced lifting on the windward side of mountains or due tolarge scale atmosphere circulation Cold air transported above region with warm air near surfaceConvective clouds appear lumpy cumulusand cumulonimbus is the extremely tallversion.Large scale lifting, common in the PugetSound Convergence Zone, creates stratuscloudsCirrus clouds are the very high clouds thattypically arrive before a
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