Constraints on the Profiles of Total Water PDF in AGCMs from AIRS and a High-Resolution Model

Atmospheric general circulation model (AGCM) cloud parameterizations generally include an assumption aboutthesubgrid-scaleprobabilitydistributionfunction(PDF)oftotalwateranditsverticalprofile.Inthepresent study, the Atmospheric Infrared Sounder (AIRS) monthly-mean cloud amount and relative humidity fields are used to compute a proxy for the second moment of an AGCM total water PDF called the ‘‘RH01 diagnostic,’’ whichistheAIRSmeanrelativehumidityforcloudfractionsof0.1orless.Thedependenceofthesecondmoment on horizontal grid resolution is analyzed using results from a high-resolution global model simulation. The AIRS-derived RH01 diagnostic is generally larger near the surface than aloft, indicating a narrower PDF near the surface, and varies with the type of underlying surface. High-resolution model results show that the verticalstructureofprofilesofthe AGCMPDFsecond momentis unchanged asthegrid resolutionchangesfrom 200to100to50 km,andthatthesecond-momentprofilesshifttowardhighervalueswithdecreasinggridspacing. Several Goddard Earth Observing System, version 5 (GEOS-5), AGCM simulations were performed with several choices for the profile of the PDF second moment. The resulting cloud and relative humidity fields were shown to be quite sensitive to the prescribed profile, and the use of a profile based on the AIRS-derived proxyresultsinimprovementsrelativetoobservationalestimates.TheAIRS-guidedtotalwaterPDFprofiles, including their dependence on underlying surface type and on horizontal resolution, have been implemented in the version of the GEOS-5 AGCM used for publicly released simulations.

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