Combined Use of the Radar and Radiometer of TRMM to Estimate the Influence of Drop Size Distribution on Rain Retrievals

Nicolas Viltard

Universities Space Research Association, and NASA Goddard Space Flight Center, Greenbelt, Maryland

Christian Kummerow

Laboratory for Atmospheres, Goddard Space Flight Center, Greenbelt, Maryland

William S. Olson

Joint Center for Earth Systems Technology, University of Maryland, Baltimore County, Baltimore, Maryland

Ye Hong

Caelum Research Corporation, and NASA Goddard Space Flight Center, Greenbelt, Maryland


A combination of passive microwave and radar observations from the Tropical Rainfall Measuring Mission (TRMM) satellite is used to investigate the consistency between the two sensors. Rather than relying on some absolute “truth” to verify retrievals, this paper focuses on one assumption—namely, the drop size distribution (DSD)—and how different DSDs lead to improved or reduced 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 the 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 conclusions, it does demonstrate that rainfall validation need not be confined to surface rainfall measurements, which are only loosely related to the volumetric observations made by most sensors. As mean size distributions of raindrops are measured in the TRMM field experiments by disdrometers, profilers, multiparameter radars, and direct aircraft observations, the technique presented in this paper can be applied on a storm-by-storm basis, and conclusions can be verified directly.