Evaluation and improvement of AMSU precipitation retrievals

The Advanced Microwave Sounding Unit-B ( AMSU-B) high frequency channels ( 89 and 150-GHz) provide the ability to detect the scattering associated with precipitation sized ice particles and, indirectly with precipitation rate. Despite the fact that AMSU ( a ``sounder'') wasn't designed for rainfall retrieval, many studies show that it is possible to use this information for precipitation retrieval and therefore, many researchers use them in blending techniques to estimate rainfall at global scale. The main advantage of this approach is the availability of three NOAA POES satellites spaced approximately 4 h apart with a spatial resolution of 16 km at nadir and a wider swath than SSMI ( 2200 km), offers an excellent opportunity to reduce the errors associated with the inadequate sample of the diurnal cycle; while the weakness of this technique are related with its present inability to retrieve rain that has little or no ice; and with the cross-scan characteristics of the instrument ( different footprints for different local zenithal angles). This situation tends to generate, in the first case, much less rain coverage over ocean than other algorithms based on emission techniques and a shift in the frequency distribution for low zenithal angles in the second case. An improvement to the current scattering based algorithm that uses cloud liquid water content ( over ocean only) and the local zenithal angle will be developed and tested in this paper. In particular, the application of the proposed correction scheme produces a more realistic rainfall amount distribution function ( especially over the tropics) and a light increase of rainfall occurrence in midlatitudes. The improvement is demonstrated by both global and regional studies over land and ocean.