Global Energy and Water Budgets

Transcription

1 Global Energy and Water Budgets

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6 Pressure (hpa) 100 Pure radiative equilibrium Dry adiabatic adjustment 20 Altitude (km) 6.5 C/km adjustment Temperature (K)

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8 Tropopause

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11 Cloud Radiative Effects

12 Global Physical Climatology by Dennis Hartmann Clouds and Earth s Temperature! All the previous results were for clear skies, but clouds have Cloud a substantial Radiative effect Effects on the TOA energy balance.

13 0: solar constant (incoming solar flux, 1367 W m 2 ). p: planetary albedo (0.30). ( ): outgoing longwave (IR) radiation at TOA (234 W m 2 ).

14 Heuristic Model of Cloud Radiative Effect (CRE)! TOA Energy Balance a.k.a. Cloud Forcing R TOA = S 0 4 (1! " p )! F # ($)!R TOA = R cloudy " R clear =!Q abs "!F # ($)! Cloud Radiative Effect Add Clouds, what changes?!q abs = S 0 4 (1" # cloudy ) " S 0 4 (1" # clear ) = S 0 4 (# cloudy " # clear ) = " S 0 4!# p

15 Heuristic Model of Cloud Radiative Effect (CRE) a.k.a. Cloud Forcing Cloud Radiative Effect Add Clouds, what changes?!r TOA = R cloudy " R clear =!Q abs "!F # ($) Shortwave bit Longwave bit!q abs = S 0 4 (1" # cloudy ) " S 0 4 (1" # clear ) = S 0 4 (# cloudy " # clear ) = " S 0 4!# p!f " (#) = F " cloudy (#) $ F " (#) clear

16 Heuristic Model of Cloud Radiative Effect (CRE) Longwave bit a.k.a. Cloud Forcing!F " (#) = F " cloudy (#) $ F " (#) clear Expand using grey absorption integral equations T {z ct,#} & T {z s,#}!f " (#) = $T 4 zct T {z ct,#}% $T 4 s T {z s,#}% $ T ( z ') 4 dt { z ',#} Assume cloud top is above most of water vapor, then OLR is emission from top of cloud T {z ct,!}" 1.0!F " (#) = $T 4 zct % $T 4 s T {z s,#}% $ T ( z ') 4 dt { z ',#}!F " (#) = $T zct & 1 T {z s,#} 4 % F " clear (#)

17 Heuristic Model of Cloud Radiative Effect (CRE) a.k.a. Cloud Forcing Putting the pieces together, becomes!r TOA = R cloudy " R clear =!Q abs "!F # ($)!R = " S 0 TOA 4!# p + F $ (%) " &T 4 clear zct The solar and longwave parts tend to be of opposite sign and we can calculate the cloud top temperature at which they will exactly cancel. ') T zct =!(S / 4)"# + F $ (%) 0 p clear ( *) & + ), -) 1/4

18 50 ISCCP CLOUD CLASSIFICATION 180 CLOUD TOP PRESSURE (MB) CIRRUS ALTOCUMULUS CIRROSTRATUS ALTOSTRATUS DEEP CONVECTION NIMBOSTRATUS HIGH MIDDLE 800 CUMULUS STRATOCUMULUS STRATUS LOW CLOUD OPTICAL THICKNESS

19 Cloud Radiative Forcing A_i = cloud amount for cloud type i A = total cloud amount A = A_i I OvcCRF_i = ovc CRF for cloud type i CRF_i = avg CRF for cloud type i CRF = total avg CRF CRF_i = A_i * OvcCRF_i CRF = A_i CRF_i = OvcCRF_i I

20 A_i * OvcCRF_i = CRF_i from Hartmann, Moy, and Fu 2001

21 There is good agreement between modeled radiative fluxes (using observed cloud type frequencices) and observed (ERBE) radiative fluxes. TABLE 1. ERBE and modeled radiation balance components for the west Pacific convective region 0 15N, E (Wm 2 ). Longwave Shortwave Net radiation ERBE Model ERBE Model ERBE Model Average sky Clear sky Cloud forcing from Hartmann, Moy, and Fu 2001

22 Observed Cloud Fractions! High Clouds (p<440mb) Max High Cloud in tropical rain areas

23 Observed Cloud Fractions! Low Clouds (p > 680mb) Max low cloud subtropical stratus

24 Net Radiation Annual Mean

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26 Observed Cloud Radiative Effects in Wm -2 from CERES Jun-Aug

27 Observed Cloud Radiative Effects in Wm -2 from CERES Jun-Aug

28 Observed Cloud Radiative Effects in Wm -2 from CERES July-Aug