Determination of Oceanic Rain Rate and Rain Cell Structure from Altimeter Waveform Data. Part I: Theory

Abstract

The predominant effect of rain on altimeter data is through the attenuation it causes of radar pulses propagating through it. Both the ERS-1 and TOPEX/Poseidon have recorded sharp decreases in the observed backscatter σ0, which have been attributed to rain events. However, the structure of rain cells and the spatial variation of rain rate can be much smaller than the altimeter footprint over which σ0 is calculated. Here an algorithm is derived to determine the rain rate and its spatial structure. This paper demonstrates that an unbiased estimator can be produced and shows the accuracy of its estimates through simulations of events of known magnitude and how the errors depend upon the accuracy of the assumptions used. Although only yielding information over a very narrow swath, rain structure and rain-rate information derived from altimetry could greatly extend records of precipitation over many areas of the oceans where little or no direct estimates exist. Routine processing of altimeter waveform data offers the prospect of a large dataset of high-resolution sections of precipitation patterns. This may be used to investigate the small-scale spatial structure of storms or in conjunction with other sensors to improve estimates of the global precipitation field. The application to real altimeter waveform data, and its validation against independent estimates of rain rate, is the subject of a forthcoming paper.