Effects of TRMM Boost on Oceanic Rainfall Estimates Based on Microwave Emission Brightness Temperature Histograms (METH)

Abstract

A physical?statistical algorithm for estimating space?time average oceanic rainfall has been applied to microwave measurements taken by the Special Sensor Microwave Imager (SSM/I) on board the Defense Meteorological Satellite Program (DMSP) satellites and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) on board the TRMM satellite. The algorithm is based on Microwave Emission Brightness Temperature Histograms (METH) and produces monthly rainfall over 5° ? 5° latitude?longitude boxes as a TRMM standard product (3A11). The TRMM satellite was boosted from an altitude of 350?402 km in August 2001 to extend its mission life. The orbit boost affected the orbital parameters, rain-rate?brightness temperature relations, and then rain-rate parameters. Using oceanic rain rates derived from SSM/I, the difference between 3A11 and SSM/I for the preboost and postboost periods was analyzed. The difference shows a significant jump from the preboost to the postboost data if no adjustments were made for the postboost TRMM data. The jumps in rain-rate parameters are attributed to the changes in earth?s incidence angle of TMI, affecting the brightness temperature in the TMI channels, the retrieved altitude of the freezing level, and the beam-filling correction factor. The changes in the brightness temperature (and freezing level) estimates and the beam-filling correction factor accounted for differences of approximately 4.9% and 1.5%, respectively. After the orbital and radiometric parameters are corrected for the boost, there is no detectable jump between the pre- and postboost 3A11 rain rates. The intersatellite calibration results demonstrate the robustness of the technique in producing a long record of climate-scale oceanic rainfall.