RAIN ESTIMATION FROM SATELLITES - EFFECT OF FINITE-FIELD OF VIEW

Nonuniform rain rates within a field of view (FOV) and a nonlinear rain rate-microwave temperature (R-T) relation lead to a bias in the estimation of areal average rain rate from spaceborne microwave measurements. This bias is estimated from rain rate data collected during GATE using an R-T relation which is derived from the model results of Wilheit et al. (1977). The bias is about 25% (30%) for a footprint size of a 8 km and increases to about 40% (45%) for a footprint size of 40 km for phase I (II) of GATE. In the large FOV limit of 280-km footprint size, the bias is 48% (50%). An experiment was performed in which the biases calculated from phase I were applied to phase II for different rain rate categories. The empirical correction works quite well. An approximate formula which takes account of the effect of spatial inhomogeneity within the FOV and a nonlinear R-T relation is derived. -from Authors