The combination of a passive microwave based satellite rainfall estimation algorithm with an IR based algorithm

Abstract

Recently, rainfall derived from low earth orbiting satellite passive microwave sensor (PMW) retrievals and geostationary satellite window channel InFrared (IR) data have been combined in a unique manner in order to develop the CPC morphing (CMORPH) technique (Joyce et al. 2004) in which the IR is used only as a means to spatially and temporally transport the rainfall features. Half-hourly analyses of CMORPH at a grid resolution of 8 km (at the equator) have been produced operationally since November 22, 2002. Validation of the CMORPH analyses indicate that the method is consistently better than blended IR-PMW rainfall estimation techniques that use IR-derived estimates of rainfall when PMW data are not available (Joyce et al. 2004). Furthermore, CMORPH estimates perform better than mere composites of PMW precipitation analyses and sometimes perform better than radar.

There are several CMORPH related issues that CPC continues to investigate. The possibility of extending the CMORPH analyses back in time – perhaps back to the early to mid 1990’s, is desirable. One of the limiting factors is the availability of sufficiently dense PMW coverage progressing backwards from current time. A shortcoming of the CMORPH method even with the present PMW sensor equipped satellite constellation (at the time of this writing) is that when precipitation forms and dissipates over a region between overpasses by PMW instrumentation it will not be detected. Other situations include when IR derived PMW rainfall propagation vectors used in CMORPH are not correct or when the morphing of both the forward/backward in time propagated PMW rainfall does not match the actual building and decaying processes of the actual rainfall complexes. A method is presented in which rainfall derived from a geostationary satellite IR-based PMW/IR combined sensor type algorithm called IR frequency (IRFREQ) is used to supplement the PMW-based CMORPH estimates for these situations to create the CMORPH IR precipitation estimation (CMORPH-IR) algorithm.