The sensitivity of intraseasonal variability in the NCAR CCM3 to changes in convective parameterization

The National Center for Atmospheric Research (NCAR) Community Climate Model, version 3.6 (CCM3) simulation of tropical intraseasonal variability in zonal winds and precipitation can be improved by implementing the microphysics of cloud with relaxed Arakawa-Schubert (McRAS) convection scheme of Sud and Walker. The default CCM3 convection scheme of Zhang and McFarlane produces intraseasonal variability in both zonal winds and precipitation that is much lower than is observed. The convection scheme of Hack produces high tropical intraseasonal zonal wind variability but no coherent convective variability at intraseasonal timescales and low wavenumbers. The McRAS convection scheme produces realistic variability in tropical intraseasonal zonal winds and improved intraseasonal variability in tropical precipitation, although the variability in precipitation is somewhat less than is observed. Intraseasonal variability in CCM3 with the McRAS scheme is highly sensitive to the parameterization of convective precipitation evaporation in unsaturated environmental air and unsaturated downdrafts. Removing these effects greatly reduces intraseasonal variability in the model. Convective evaporation processes in McRAS affect intraseasonal variability mainly through their time-mean effects and not through their variations. Convective rain evaporation and unsaturated downdrafts improve the modeled specific humidity and temperature climates of the Tropics and increase convection on the equator. Intraseasonal variability in CCM3 with McRAS is not improved by increasing the boundary layer relative humidity threshold for initiation of convection, contrary to the results of Wang and Schlesinger. In fact, intraseasonal variability is reduced for higher thresholds. The largest intraseasonal moisture variations during a model Madden-Julian oscillation life cycle occur above the boundary layer, and humidity variations within the boundary layer are small.

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Copyright 2001 American Meteorological Society (AMS). Permission to use figures, tables, and brief excerpts from this work in scientific and educational works is hereby granted provided that the source is acknowledged. Any use of material in this work that is determined to be "fair use" under Section 107 of the U.S. Copyright Act or that satisfies the conditions specified in Section 108 of the U.S. Copyright Act (17 USC ยง108, as revised by P.L. 94-553) does not require the AMS's permission. Republication, systematic reproduction, posting in electronic form on servers, or other uses of this material, except as exempted by the above statement, requires written permission or a license form the AMS. Additional details are provided in the AMS Copyright Policy, available on the AMS Web site located at (http://www.ametsoc.org/AMS) or from the AMS at 617-227-2425 or copyright@ametsoc.org.
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Author Maloney, Eric
Hartmann, Dennis
Publisher UCAR/NCAR - Library
Publication Date 2001-05-01T00:00:00
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Topic Category geoscientificInformation
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Metadata Date 2023-08-18T18:21:07.177416
Metadata Record Identifier edu.ucar.opensky::articles:17590
Metadata Language eng; USA
Suggested Citation Maloney, Eric, Hartmann, Dennis. (2001). The sensitivity of intraseasonal variability in the NCAR CCM3 to changes in convective parameterization. UCAR/NCAR - Library. http://n2t.net/ark:/85065/d7m909z3. Accessed 23 June 2025.

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