Sampling errors of SSM/I and TRMM rainfall averages: Comparison with errorestimates from surface data and a simple model

Quantitative use of satellite-derived maps of monthly rainfall requires som e measure of the accuracy of the satellite estimates. The rainfall estimate for a given map grid box is subject to both remote sensing error and, espe cially in the case of low-orbiting satellites, sampling error due to the li mited number of observations of the grid box provided by the satellite. A s imple model of rain behavior predicts that rms random error in grid-box ave rages should depend in a simple way on the local average rain rate, and the predicted behavior has been seen in simulations using surface rain gauge a nd radar data. This relationship is examined using Special Sensor Microwave Imager (SSM/I) satellite data obtained over the western equatorial Pacific during the Tropical Ocean and Global Atmosphere Coupled Ocean-Atmosphere R esponse Experiment. Rms error inferred directly from SSM/I rainfall estimat es is found to be larger than was predicted from surface data and to depend less on local rain rate than was predicted. Preliminary examination of Tro pical Rainfall Measuring Mission (TRMM) microwave estimates shows better ag reement with surface data. A simple method of estimating rms error in satel lite rainfall estimates is suggested, based on quantities that can be compu ted directly from the satellite data.