The Correction of Excessive Precipitation over Steep and High Mountains in a GCM

Winston Chao


Excessive precipitation over steep and high mountains (EPSM) is a well-known problem in GCMs--including EMC's GFS--and mesoscale models. This problem impairs simulation and data assimilation products. The main cause of EPSM is a missing upward transport of heat out of the boundary layer due to the vertical circulations forced by the daytime upslope winds, which are forced by heated boundary layer on the subgrid-scale slopes. These upslope winds are associated with large subgrid-scale topographic variation, which is found over steep and high mountains. Without such subgrid-scale heat ventilation, the resolvable-scale upslope flow in the boundary layer generated by surface sensible heat flux along the mountain slopes is excessive. Such an excessive resolvable-scale upslope flow combined with the high moisture content in the boundary layer results in excessive moisture transport toward mountaintops, which in turn gives rise to EPSM. The solution to the EPSM problem is to parameterize the ventilation effects of the subgrid-scale heated-slope-induced vertical circulation (SHVC). The speaker has designed an SHVC parameterization scheme. Test results using the NASA Goddard Earth Observing System GCM version 5 (GEOS-5) have shown that the EPSM problem is largely solved.

A fundamental contribution of this work is its opening up a new area in the GCM modeling research--the SHVC parameterization.

The paper corresponding to this talk has appeared in the May 2012 issue of the Journal of Atmospheric Sciences, p1547-1561.