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-1. The data were best fit by a=0.144 mm2h-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.
