THE IMPACT OF SPATIAL-RESOLUTION ENHANCEMENT OF SSM I MICROWAVE BRIGHTNESS TEMPERATURES ON RAINFALL RETRIEVAL ALGORITHMS/

The impact of spatial resolution enhancement on estimates of tropical typhoon rainfall based on SSM/I (Special Sensor Microwave/Imager) meas urements is evaluated with six different microwave precipitation retri eval algorithms. Passive microwave estimates of rainfall are susceptib le to errors from nonhomogeneous beam filling. The SSM/I ground footpr ints for the 19-, 22-, and 37-GHz channels have considerable overlap, and thus deconvolution techniques can be applied to enhance spatial re solution of measurements at those frequencies. The authors utilize a B ackus-Gilbert matrix transform approach to accomplish the deconvolutio n so as to minimize noise amplification, as suggested by Stogryn. The deconvolution scheme is evaluated in terms of its impact on rain rates throughout the life cycles of seven tropical cyclones that occurred d uring the 1987 hurricane and typhoon season. The evaluation was perfor med on a single-frequency emission-based algorithm, a single-frequency scattering-based algorithm, two multiple-frequency statistical regres sion algorithms, and two physical inversion-based profile algorithms. While rainfall patterns detected by all algorithms were qualitatively enhanced by accentuating rainfall gradients and other smaller-scale fe atures, quantitative responses to the deconvolution process were quite different for each algorithm. Furthermore, each of the algorithms, wh ich uses its own distinct scientific approach, exhibits its own distin ct properties in retrieving the rainfall patterns and in recovering th e storm domain-averaged rain rates. The rain rates derived from the si ngle-frequency emission algorithm were consistently increased by appli cation of the deconvolution procedure. Time- and space-averaged rain r ates were elevated by approximately 5%-6% due to the nonlinear relatio nship of rain rate to brightness temperature. On the other hand, rain rates from the single-frequency scattering algorithm were consistently reduced, with the time-space-averaged reduction between 10% and 20%. This effect is not algorithm related but is due to alteration of noise properties of the two polarized 37-GHz channels introduced during the deconvolution process. The multiple-frequency algorithms have more co mplex responses to deconvolution. Although instantaneous rain rates ca n be changed quite significantly by these methods, differences between deconvolved and raw time-space-averaged rain rates are small compared to the single-channel algorithms-because the pixel-scale differences tend to be of a more random nature (positive and negative changes inst ead of consistent bias). However, it appears that the profile methods can undergo the greatest improvement to instantaneous rain rates after deconvolution is applied because they use perturbative inversion proc edures rather than fixed brightness temperature-rain rate relationship s.