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

Excessive precipitation over steep and high mountains (EPSM) is a well-known problem in GCMs and mesoscale models. This problem impairs simulation and data assimilation products. Among the possible causes investigated in this study, it was found that the most important one, by far, 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. Other possible causes investigated include 1) a poorly designed horizontal moisture flux in the terrain-following coordinates, 2) the conditions for cumulus convection being too easily satisfied at mountaintops. 3) conditional instability of the computational kind, and 4) the absence of blocked flow drag. They are all minor or inconsequential. The ventilation effects of the subgrid-scale heated-slope-induced vertical circulation (SHVC) have been parameterized by removing heat from the boundary layer and depositing it in the layers higher up when topographic variance exceeds a critical value. Test results using the NASA Goddard Earth Observing System GCM version 5 (GEOS-5) have shown that the EPSM problem is largely solved.