Improvement of passive microwave rainfall retrievals over the mountain area in Japan

Rainfall maps have been produced using data from satellite-borne passive microwave radiometers (MWRs). Information obtained by the TRMM precipitation radar (PR) accelerated the development of rainfall retrieval from MWRs. One algorithm using precipitation-related variable models and retrieval methods based on TRMM observation studies is the Global Satellite Mapping of Precipitation (GSMaP) MWR algorithm (hereinafter referred to as GSMaP_MWR). Chief among many improvements in the GSMaP_MWR algorithm have been using of statistical precipitation profiles to calculate look-up tables (LUTs) relating to surface rainfall rates and brightness temperatures. A scattering signature from ice particles over the spectrum of higher frequencies is used for over-land rainfall estimation, because emission from land surfaces is warm and variable. Relationship between surface rainfall rates and a scattering signature from ice particles is indirect. Therefore, rainfall estimates over land are less reliable than those over ocean where an emission signature from raindrop over the spectrum of lower frequencies can be used. Underestimation of the GSMaP_MWR algorithm has been found for a case of orographic heavy rainfall over Japan (Kubota et al. 2009). Precipitation profiles for orographic heavy rainfall were characterized by a large increase in rain rates below the freezing level, because orographic lifting of wet air can supply a large amount of condensates, which create rainfall at low-levels. Thus, MWR scattering associated with ice particles above the freezing was relatively weak, and retrieval from scattering signatures could be regarded as small. In this study, we intend to improve GSMaP_MWR estimates over the mountain area in Japan. Shallow precipitation profiles observed by TRMM PR for orographic heavy rainfall are used to calculate LUT. The LUT are applied for rain retrievals over areas with topographic forced upward and low-level moisture convergence, which are estimated from the Japan Meteorological Agency (JMA) global analysis (GANAL) and terrain heights. Revised estimates from TRMM microwave imager (TMI) showed a better agreement with TRMM PR and ground-based measurements.