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

Thomas T. Wilheit

Department of Atmospheric Science, Texas A & M University, College Station, Texas, USA

Christian D. Kummerow

Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA

ABSTRACT

The unresolved rainfall structure within the Field of View (FOV) of a microwave radiometer, such as TRMM Microwave Imager (TMI), causes errors in the rainfall estimation due to the nonlinear relationship between the rain rate and brightness temperature. We have used rainfall structure data from the TRMM Precipitation Radar (PR) to estimate the bias caused by this structure, the random (FOV-to-FOV) error, and a rough estimate of the regional/seasonal scale uncertainty in the bias. We have expressed this bias in terms of a rainfall variance, V, over the FOV in the form V = a <r> b where <r> is the FOV average rain rate in mm/h. The data best fit by a = 0.144 mm2 h-2 and b = 2.46 at the pre-boost 19.35 GHz resolution. The beam filling error is very small relative to the rain rate at small rain rates but increases with increasing rain rate.