Theoretical Analysis of Possible Improvements in Precipitation Retrieval With TRMM-Type Satellite Carrying 2-Frequency Precipitation Radar

During the post-2000 EOS era, there is now a strong possibility that the U.S., Japan, and possibly additional international partners will fly a 9-unit satellite constellation for measurement of 3-hourly global precipitation. The experiment is planned to commence in the 2006 time frame. The current acronym for this mission is GPM, signifying Global Precipitation Mission.

The GPM satellite fleet in nominal orbit configuration would consist of one TRMM-like reference satellite carrying a passive microwave radiometer, similar to the TRMM Microwave Imager (TMI), plus a dual frequency (14-35 GHz) incoherent precipitation radar (d-PR), a step beyond the single frequency (14 GHz) precipitation radar (PR) used on TRMM. This reference satellite would be launched into a prograde orbit at roughly a 75 degree inclination. The 8 drone satellites would likely be a combination of microsatellites carrying a lightweight/low power 3-frequency passive radiometer and satellites already in space (or planned for space flight) carrying multispectral passive microwave radiometers (e.g., SSM/I and ADEOS). The drone satellites are expected to be in sun synchronous orbit, with 8 such platforms providing continuous 3-hour global coverage. The main concept behind the fleet configuration is that the reference satellite would provide high quality near-time calibration for converting passive microwave measurements from the drones to rainrates, virtually devoid of systematic errors between the various satellite measuring systems.

This study examines the potential improvements to precipitation retrieval afforded by the addition of a dual-frequency radar on the reference satellite, particularly the use of differential reflectivity to directly retrieve specific statistical moments of the underlying drop distribution function (DSD). The analysis also examines the value of knowing the polarimetric signal and the Doppler line of sight velocity (the latter being directly related to precipitation fall speeds), as such features are part of the long term strategy to improve precipitation measurement from space. The study focuses on the potential improvements in monitoring global precipitation through physically-based retrieval using a radiometer and the d-PR, based on radiative transfer modeling.