EFFECTS OF NONUNIFORM BEAM FILLING ON RAINFALL RETRIEVAL FOR THE TRMMPRECIPITATION RADAR

The Tropical Rainfall Measuring Mission (TRMM) will carry the first sp aceborne radar for rainfall observation. Because the TRMM Precipitatio n Radar (PR) footprint size of 4.3 km is greater than the scale of som e convective rainfall events, there is concern that nonuniform filling of the PR antenna beam may bias the retrieved rain-rate profile. The authors investigate this effect theoretically and then observationally using data from the NASA Jet Propulsion Laboratory Airborne Rain Mapp ing Radar (ARMAR), acquired during Tropical Oceans Global Atmosphere C oupled Ocean-Atmosphere Response Experiment in early 1993. The authors ' observational approach is to simulate TRMM PR data using the ARMAR d ata and compare the radar observables and retrieved rain rate from the simulated PR data with those corresponding to the high-resolution rad ar measurements. The authors find that the path-integrated attenuation and the resulting path-averaged rain rate are underestimated. The ref lectivity and rain rate near the top of the rainfall column are overes timated. The near-surface reflectivity can be overestimated or underes timated, with a mean error very close to zero. The near-surface rain r ate, however, is usually underestimated, sometimes severely.