ESTIMATING 13.8-GHZ PATH-INTEGRATED ATTENUATION FROM 10.7-GHZ BRIGHTNESS TEMPERATURES FOR THE TRMM COMBINED PR-TMI PRECIPITATION ALGORITHM

This study presents research in support of the design and implementati on of a combined radar-radiometer algorithm to be used for precipitati on retrieval during the Tropical Rainfall Measuring Mission (TRMM). Th e combined algorithm approach is expected to overcome various difficul ties that arise with a radar-only approach, particularly related to es timates of path-integrated attenuation (PIA) along the TRMM radar beam . A technique is described for estimating PIA at the 13.8-GHz frequenc y of the TRMM precipitation radar (PR) from 10.7-GHz brightness temper ature T-B measurements obtained from the TRMM microwave imager. Becaus e the PR measures at an attenuating frequency, an independent estimate of PIA is used to constrain the solution to the radar equation, which incorporates effects of attenuation propagation along a radar beam. T hrough the use of variational or probabilistic techniques, the indepen dent PIA calculations provide a means to adjust for errors that accumu late in estimates of range-dependent rain rates at progressively incre asing range positions from radar reflectivity vectors. The accepted ra dar approach for obtaining PIA from ocean-viewing radar reflectivity m easurements is called the surface reference technique, a scheme based on the difference in ocean surface cross sections between cloud-free a nd raining radar pixels. This technique has encountered problems, whic h are discussed and analyzed with the aid of coordinated aircraft rada r (Airborne Rain Mapping Radar) and radiometer (Advanced Microwave Pre cipitation Radiometer) measurements obtained during the west Pacific T ropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Expe riment in 1993. The derived relationship expressing 13.8-GHz PIAs as a function of 10.7-GHz T-B's is based on statistical fitting of many th ousands of a radiative transfer (RTE) calculations in which the releva nt physical and radiative parameters affecting transmission, absorptio n, and scattering in a raining column and the associated emission-scat tering properties of the wind-roughened ocean surface are systematical ly varied over realistic range intervals. The results demonstrate that the T-B-PIA relationship is stable, with a dynamic range up to about 8 dB. The RTE calculations are used to examine the relative merits of different viewing configurations of the radar and radiometer, and the associated uncertainty variance as the viewing configuration changes, since PIA uncertainty is an important control factor in the prototype TRMM combined algorithm.