Improvement of the Orographic/Nonorographic Rainfall Classification Scheme with Moist Frouid Number in the GSMaP Algorithm

Abstract:

The orographic/nonorogrpahic rainfall classification scheme has been implemented to the latest standard version (V6) of Global Satellite Mapping of Precipitation (GSMaP) algorithm for passive microwave radiometers. The scheme consists of orographically forced upward motion (w) and moisture flux convergence (Q) given at surface. The scheme improves rainfall estimation over the entire Asian region, particularly over the Asian region dominating shallow orographic rainfall. However, overestimation and false-positive of orographic rainfall remain. This is because the orographic rainfall conditions has moderate thresholds for global application. In order to improve the orographic rainfall detection, the moist Frouid number (Fw) with w and Q are examined to the scheme.
The GSMaP rainfall estimates with the scheme in V6 (GSMaP1) and revised version using Fw (GSMaP2) are compared with the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data for the case of Typhoon Morakot (2009) caused heavy orographic rainfall over Taiwan. GSMaP1 detects false-positive orographic rainfall in the western part of Taiwan. This is because strong wind such as Typhoon makes upward motion even in moderate slope areas. On the contrary, GSMaP2 improves the area of orographic rainfall since the scheme is switched off for Fw greater than or equal to 1.0 corresponding to the low altitude. A verification score and zonal mean rainfall amounts obtained from the TRMM Microwave Imager are better agreement with those from the PR over Asian region.