The next generation radiometer and radar systems to be flown on the Global Precipitation Measurement (GPM) mission's core satellite will be advanced over the corresponding instruments that were flown on the Tropical Rainfall Measuring Mission (TRMM) satellite. The GPM radiometer will provide increased spatial resolution and is expected to have greater frequency diversity, while the GPM radar system will include two radars at different frequencies, i.e., 13.6 GHz (Ku-band) and 35 GHz (Ka-band). These instrument upgrades can be used to surmount the problem of having to specify the drop size distribution (DSD) properties of the precipitation a priori, because at the two radar frequencies the associated size parameters of most meaningful rain hydrometeors across the rainrate spectrum are in the Mie scattering regime and thus produce substantive differential reflectivity signals.

The a priori specification of DSD properties has been a long-standing problem in remotely-sensed precipitation retrieval, whether from space or ground platforms, because the DSD spectrum can vary significantly over short space and time scales, the effect of which can be large on a retrieved solution. The advantage of measuring differential reflectivity is that it can be used to directly retrieve bulk properties of the DSD spectrum, which are the foremost "mass factors" in quantifying precipitation at its fundamental level.

This study first presents a theoretical framework of how to use differential reflectivity to derive the fundamental microphysical properties of precipitation. It then presents an application of the theory showing how a GPM-type retrieval solution improves both the accuracy and precision errors relative to a TRMM-type retrieval solution under a combined radiometer-radar retrieval method of solution that has been used during the TRMM research program.