The Effects of Rain on Topex Radar Altimeter Data

Abstract

Rain has long been categorized as a contaminant of altimeter data, but little has been done previously to ascertain the magnitude and frequency of its effect or its geographical distribution. Proceeding from recent analysis of ERS-1 data, and the insight gained, this paper addresses all these issues, highlighting the particular advantages of Topex's dual-frequency capability. The primary effect of rain is on the observed backscatter values σ0 through attenuation of the radar signal. An empirical relationship is derived linking the observed σ0 values at C band and Ku band. Through a series of case studies and global analyses, prominent departures from this relationship are shown to be caused by regions of significant precipitation. As it is predominantly the Ku-band returns that are affected, C-band σ0,s are likely to be of great use for the determination of wind speeds in regions or events of intense rain. The different attenuation rates give dual-frequency altimetry the potential to yield information on rain distribution and intensity that complement that achievable by infrared sensors alone.