Sampling Error Study for Rainfall Estimate by Satellite Using a Stochastic Model

Abstract

A parameter study of satellite orbits was performed to estimate sampling errors of area-time averaged rain rate due to temporal sampling by satellites. The sampling characteristics were investigated by accounting for varying visiting intervals and varying fractions of averaging area on each visit as a function of the latitude of the grid box for a range of satellite orbital parameters. The sampling errors were estimated by a simple model based on the first-order Markov process of the time series of area averaged rain rates. For a satellite of nominal TRMM orbit (30° inclination and 300 km altitude) carrying an ideal scanning microwave radiometer for direct precipitation measurements, sampling error would be about 8 to 12% of estimated monthly mean rain rates over a grid box of 5° × 5°. The effect of uneven sampling intervals with latitude tend to be offset by increasing sampling areas with latitude, therefore, the latitude dependence of sampling error was not important. Nomograms for sampling errors are presented for a range of orbital parameters centered at nominal TRMM orbit. An observation system based upon the low inclination satellite combined with a sunsynchronous satellite simultaneously would be especially promising for precipitation measurements from space. Sampling errors well below 10% can be achieved for this idealized system case for the monthly rain rate estimates for 5° × 5° boxes.