Physical Validation of Trmm Rainfall Products Using a Radiative Transfer Model

The Tropical Rainfall Measuring Mission (TRMM) satellite has been in operation for more than 7 years and providing distribution of rainfall throughout the Tropics using microwave observation from the Precipitation Radar (PR) and the TRMM Microwave Imager (TMI). PR, the first space-borne precipitation radar, provides height information based upon the time delay of the precipitation-backscattered return power, and has enabled us to directly obtain vertical profiles of precipitation over the global Tropics (Okamoto 2003). On the other hand, the TMI measures radiances that are the end product of the integrated effects of electromagnetic absorption/emission and scattering through a precipitating cloud along the sensor viewpath. Although differences in global averaged rainfall between the two sensors have been reducing from Version 4 to Version 5, a comparisons of tropical mean rainfall time series indicates that there are still large discrepancies in the 1998 El Nino-Southern Oscillation (ENSO) event (Robertson et al., 2003). Several improvements have been implemented to create the most recent algorithm, the TRMM Version 6 algorithms. Preliminary indications are that the overall bias is being drastically reduced in version 6 of the products, but certain evidences to support the claim that the TRMM version-6 rain estimates are better than version-5 estimates has not yet be obtained. The most common method of satelliteretrieved rain estimates is to directly compare to ground validation measurements (ground truth) derived from rain gauge networks, ground weather radar, or a combination of the two. However, it has pr oven to be extremely difficult to make accurate ground-based measurement of precipitation. Moreover, the sparse nature of ground based validation campaigns does not provide sufficient information about changing cloud characteristics leading to algorithm errors. In this paper, we validate TRMM version-5 and -6 rain estimates using a radiative transfer model. Comparisons between TMI-observed brightness temperatures and those simulated from PR2A25 and ∗Corresponding author address: Dr. Shoichi Shige, Department of Space Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka, 599-8531 Japan; e-mail: [email protected] TMI2A12 rain profiles are performed for ITCZ rain systems during the 1998 El Nino event.