Use of the TRMM-PR for Estimating the TMI Beam Filling Correction

In addition to its obvious use for measurement of rainfall, the Precipitation Radar (PR) is also quite useful for the determination of precipitation physics for the improvement of passive microwave algorithms The knowledge obtained from the radar can then be applied to the interpretation of the TRMM Microwave Imager (TMI) over its wider swath, and for that matter, for the interpretation of microwave radiometers flying on other satellites including those from before the TRMM launch and those in orbit after the eventual demise of TRMM. An example of this application was given by Wilheit et al. (2007) who used radar data to infer drop size distributions for inclusion in radiometric algorithms. Here we provide another example; we have used the three dimensional rainfall structure information derived from PR for better understanding of passive microwave retrievals and their uncertainties. It is convenient to consider this structural information in two parts. First in the vertical dimension, we are interested in the surface rainfall, but the radiometric signal is, to first approximation, an integral over the entire profile and therefore provides information on the average rain rate over the liquid portion of the rain column. Thus, there is some error in expressing the radiometer-based rain rate in terms of a surface rain rate. This source of error would remain even in the limit of very high spatial resolution observations. We will leave this error for a future analysis. Second, we can consider the (quasi-horizontal) rain fall structure perpendicular to the view direction. In Fig. 1 the solid line shows the relationship between the brightness temperature and rain rate for the 10 GHz, vertically polarized channel for a 4 km freezing level over ocean as computed with a plane-parallel model. One can see that the relationship is quite nonCorresponding author: Thomas T. Wilheit, Department of Atmospheric Science Texas A&M University, TAMU 3150, College Station, TX 77843-3150, USA E-mail: [email protected] ©2009, Meteorological Society of Japan Abstract