The Precipitation Radar (PR) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite, the first space-borne radar designed to observe precipitation, is now into its sixth year of successful operation. With a range resolution of 250 m, a field-of-view (FOV) of 4.3 km, a cross-track scanning pattern with 49 FOVs, and a minimum detectable reflectivity of about 18 dBZ, the PR has sufficient sensitivity and vertical resolution to detect rainfall rates as low as ~0.5 mm/hr from shallow clouds over land and ocean. Standard TRMM PR products also provide information on the vertical and horizontal structure of precipitation echoes, with classifications into convective and stratiform categories. The TRMM orbit provides sampling from about 36N to 36S latitude throughout the diurnal cycle. Previous studies have indicated that several years of PR data are required to achieve uniform sampling of the diurnal cycle over a domain approximately 12 x 12 degrees in latitude and longitude.

This study exploits the above PR characteristics for an analysis of shallow rain over the oceans, especially that associated with precipitating marine stratocumulus and precipitating trade-wind cumulus. The TRMM PR monthly product 3A25 and the daily product 3G68 from the years 1998 to 2002 have been examined to determine the geographical patterns of shallow storm heights, the diurnal cycle of rainfall from shallow systems and the total rainfall associated with shallow systems. More limited examinations of the orbit-by-orbit 2A25 and 2A23 products have been used to examine shallow rain characteristics in the high resolution PR data. Because of instrument and algorithm limitations, several corrections specific to shallow systems have been developed in this study. These include geometric factors associated with the scan pattern, classification factors affecting the reflectivity-to-rainfall rate algorithm, impacts of the 18 dBZ sensitivity threshold and missing warm-rain occurrences in the 3A25 product.