Editorial Type:
Article
Article Type:
Research Article

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

Andrea Molod Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, Maryland

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Print Publication:
01 Dec 2012
DOI:
https://doi.org/10.1175/JCLI-D-11-00412.1
Page(s):
8341–8352
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Abstract

Atmospheric general circulation model (AGCM) cloud parameterizations generally include an assumption about the subgrid-scale probability distribution function (PDF) of total water and its vertical profile. In the present 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,” which is the AIRS mean relative humidity for cloud fractions of 0.1 or less. The dependence of the second moment 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 vertical structure of profiles of the AGCM PDF second moment is unchanged as the grid resolution changes from 200 to 100 to 50 km, and that the second-moment profiles shift toward higher values with decreasing grid spacing.

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 proxy results in improvements relative to observational estimates. The AIRS-guided total water PDF profiles, 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.

Additional affilliation: Earth System Science Interdisciplinary Center, University of Maryland, College Park, College Park, Maryland.

Corresponding author address: Andrea Molod, Earth System Science Interdisciplinary Center, University of Maryland, College Park, 5825 University Research Court, Suite 4001, College Park, MD 20740. E-mail: andrea.m.molod@nasa.gov

Abstract

Atmospheric general circulation model (AGCM) cloud parameterizations generally include an assumption about the subgrid-scale probability distribution function (PDF) of total water and its vertical profile. In the present 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,” which is the AIRS mean relative humidity for cloud fractions of 0.1 or less. The dependence of the second moment 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 vertical structure of profiles of the AGCM PDF second moment is unchanged as the grid resolution changes from 200 to 100 to 50 km, and that the second-moment profiles shift toward higher values with decreasing grid spacing.

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 proxy results in improvements relative to observational estimates. The AIRS-guided total water PDF profiles, 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.

Additional affilliation: Earth System Science Interdisciplinary Center, University of Maryland, College Park, College Park, Maryland.

Corresponding author address: Andrea Molod, Earth System Science Interdisciplinary Center, University of Maryland, College Park, 5825 University Research Court, Suite 4001, College Park, MD 20740. E-mail: andrea.m.molod@nasa.gov
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