Correction of Excessive Precipitation over Steep and High Mountains in a GCM: A Simple Method of Parameterizing the Thermal Effects of Subgrid Topographic Variation

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Thursday, 8 January 2015: 11:00 AM
121BC (Phoenix Convention Center - West and North Buildings)
Winston Chao, NASA, Greenbelt, MD

The excessive precipitation over steep and high mountains (EPSM) in GCMs and meso-scale models is due to a lack of parameterization of the thermal effects of the subgrid-scale topographic variation. These thermal effects drive subgrid-scale-heated-slope-induced vertical circulations (SHVC). SHVC provides a ventilation effect of removing heat from the boundary layer of solvable-scale mountain slopes and depositing it higher up. Lacking a SHVC parameterization is the cause of EPSM. The author has previously proposed a method of doing SHVC parameterization called here SHVC.1 (JAS, 2012). Though successful in avoiding EPSM, SHVC.1 has the drawback of suppressing convective-type of precipitation in regions where it is applied. In this presentation we propose an alternative method of doing SHVC parameterization, called SHVC.2. In SHVC.2 the potential temperature and mixing ratio at the cloud base level are increased when used as input to the cumulus parameterization scheme over mountainous regions. This allows cumulus parameterization to assume the function of SHVC parameterization. SHVC.2 has been tested in NASA/Goddard's GEOS-5 GCM. It achieves the primary goal of avoiding EPSM. It also avoids the suppression of convective-type precipitation in the regions where it is applied.