THE EFFECTS OF RAIN ON TOPEX RADAR ALTIMETER DATA

Rain has long been categorized as a contaminant of altimeter data, but little has been done previously to ascertain the magnitude and freque ncy of its effect or its geographical distribution. Proceeding from re cent analysis of ERS-1 data, and the insight gained, this paper addres ses all these issues, highlighting the particular advantages of Topex' s dual-frequency capability. The primary effect of rain is on the obse rved backscatter values sigma(0) through attenuation of the radar sign al. An empirical relationship is derived linking the observed sigma(0) values at C band and K-u band. Through a series of case studies and g lobal analyses, prominent departures from this relationship are shown to be caused by regions of significant precipitation. As it is predomi nantly the K-u-band returns that are affected, C band sigma(0,)s are l ikely to be of great use for the determination of wind speeds in regio ns or events of intense rain. The different attenuation rates give dua l-frequency altimetry the potential to yield information on rain distr ibution and intensity that complement that achievable by infrared sens ors alone.