Systematic Differences between TRMM 3G68 PR and TMI Rainfall Estimates and the Possible Association with Life Cycle of Convection

Abstract

Characteristics of systematic differences between TRMM Microwave Imager (TMI) and Precipitation Radar (PR) rain estimates derived from TRMM-3G68 version 5 datasets are analyzed and the possible factors contributing to these differences are discussed. This study utilizes conditional rain estimates which represent the average rain rate of rain pixels over a domain instead of unconditional rain (which is the domain averaged rain) used in most of the previous studies. It is found that TMI rainfall estimates are larger than PR estimates over most of the equatorial oceans. The distribution of rainfall differences with respect to their convective percentages brings out their characteristics under different convective/stratiform regimes. Different life stages of tropical convection are identified utilizing TMI and PR convective percentages, rain cover, Visible Infrared Scanner (VIRS) brightness temperature and the difference between TMI and PR rain estimates. Our result suggests that TMI and PR rainfall differences are associated with these life stages of organized convection. At low TMI convective percentages and high PR convective percentages, representing the formative stage of convection, PR rain estimates are found to be larger than TMI. But, significantly large differences in rain estimates, with overestimation of rain by TMI, occur when PR convective percentage is < 60% and TMI convective percentage is > 50% which is identified as mature to decaying stages of convection. During these stages, due to the presence of overshooting tall clouds, the time lag between the occurrence of maximum rain and the highest cloud top height, the increased back radiation from ice-scattering near the cloud top level, and the melting layer effect result in large overestimation of rain by TMI.