1Lecture Ch. 12a• Review of simplified climate model• Current research– Aerosols, precipitation, and evaporation• Course evaluationsCurry and Webster, Ch. 12For Tuesday: Read Ch. 13For Dec 6: Review Past Homework, Quizzes, Reading, MidtermSimplified Climate Model• Atmosphere described as one layer– Albedo αp~0.31: reflectance by surface, clouds,aerosols, gases– Shortwave flux absorbed at surface• Earth behaves as a black body– Temperature Te: equivalent black-body temperature ofearth– Longwave flux emitted from surfaceCurry and Webster, Ch. 12 pp. 331-337; also Liou, 1992FS=0.25*S0(1- αp)FL=σTe4FLEmitted from spheresurface 4πr2FSIncident onprojected disc πr2FS = FLSimplified Climate Model• Incoming shortwave = Outgoing longwave• Energy absorbed = Energy emittedFS = 0.25*S0(1- αp)FL = σTe4Solar Constant• Luminosity of the sun• Irradiance at earth S0 = L0/(4πd2) = 1.4x103 W/m2€ d = 1.5 ×1011m (p.437)€ Area = 4πd 2L0 ~ 3.9x1026 W (p. 331)Simplified Climate Model• At thermal equilibrium (why?)• Observed surface temperature T = 288K• What’s missing?FS = FL0.25*S0(1- αp) = σTe4Te = [0.25*S0(1- αp)/σ]0.25Te ~ 255KSensitivity to Albedo• What if albedo changes?• 1% decrease in albedo warms temperature 1K• 1% increase in albedo cools temperature 1KTe = [0.25*S0(1- αp)/σ]0.25αp=0.31, Te ~ 255Kαp=0.30, Te ~ ?2Add an Atmosphere!• Atmosphere is transparent to non-reflected portionof the solar beam• Atmosphere in radiative equilibrium with surface• Atmosphere absorbs all the IR emissionFsurfFSTOA: FS = Fatm0.25*S0(1- αp) = σTatm4 Tatm = 255KFatmFatmAtmos: Fsurf = 2Fatm σTsurf4 = 2σTatm4 Tsurf = 303KWhat’s wrong?• With no atmosphere, Tsurf = 255K• With “atmosphere”, Tsurf = 303K• From observations, Tsurf = 288K• Real atmosphere:– Not perfectly transparent to incoming solar– Not perfectly opaque to infrared– Not in pure radiative equilibrium with surface• Three assumptions were wrong!Ramanathan et al., 2001Current Research: Radiation• Large surface cooling: -20 W m-2– Reduces evaporation, short-circuitingprecipitationErlick, Ramaswamy, and Russell, 2005Current Research: Global Models• Aerosolimpacts onrain are notlocalChange in JJA meanprecipitation (mm day-1) between the 6-yearperturbation and the 12-year control.“…aerosols counteractwarming by an uncertain,but potentially large,amount…Strong aerosolcooling would imply thatfuture global warming mayproceed at or even above theupper extreme of the rangeprojected by the
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