Comparison of Space-borne and Ground-based Radar Measurements from the Tropical Rainfall Measuring Mission

The focus of this study is to compare space-borne radar observations with ground-based radar measurements. In November 1997, NASA launched the Tropical Rainfall Measuring Mission (TRMM) satellite. The instrument payload includes the first-ever Precipitation Radar (PR). NASA has made an enormous effort to develop ground validation (GV) sites for TRMM. A goal of the GV sites is to provide high-quality ground measurements for comparison and validation of the space-borne measurements. One GV site is located on the island of Guam and represents the climatic rainfall regime of the northern portion of the western tropical Pacific Ocean. Data collected by the Guam (PGUA) WSR-88D radar and a network of rain gauges are compared with the TRMM satellite observations.

A total of 237 TRMM PR overpasses have been identified where coincident WSR-88D and rain gauge data were collected during the period of December 1997 - August 2000. Comparisons of the area-averaged echo statistics between the space-borne and ground-based measurements show that the PR underestimates the conditional and unconditional rainfall, convective fraction of echo, and vertical depth of convection over Guam. This study attempts to provide insights to the reasons for the systematic differences. Using data from the 237 cases, detailed inter-comparisons will be made with co-located echoes. Also, radar observations will be simulated using a radar beam propagation model to better understand how different viewing angles, beam geometries, and other differences in radar characteristics effect the measurements of precipitation. The estimated errors due to these differences will be compared to the actual observations.