Combined use of the radar and radiometer of TRMM to estimate the influenceof drop size distribution on rain retrievals

A combination of passive microwave and radar observations from the Tropical Rainfall Measuring Mission (TRMM) satellite is used to investigate the con sistency between the two sensors. Rather than relying on some absolute "tru th'' to verify retrievals, this paper focuses on one assumption-namely, the drop size distribution (DSD)-and how different DSDs lead to improved or re duced consistency. Results from a case in the central Pacific suggest that a crude consistency may be achieved if a different drop size is used for th e radiometer and the radar. In this particular case, a Marshall-Palmer or a gamma distribution with the shape parameters properly set leads to similar results. Although this study offers no independent validation of its concl usions, it does demonstrate that rainfall validation need not be confined t o surface rainfall measurements, which are only loosely related to the volu metric observations made by most sensors. As mean size distributions of rai ndrops are measured in the TRMM field experiments by disdrometers, profiler s, multiparameter radars, and direct aircraft observations, the technique p resented in this paper can be applied on a storm-by-storm basis, and conclu sions can be verified directly.