THE EFFECTS OF RAIN ON ERS-1 RADAR ALTIMETER DATA

An investigation into a potentially important but little-studied effec t on altimeter data-rain contamination-has been carried out using ERS- 1. The method involves identifying large changes in the radar backscat ter coefficient and relating these to atmospheric liquid water estimat es obtained from an onboard microwave radiometer. The latter is found to provide a useful means of distinguishing between wind and rain even ts. In general, the backscatter coefficient is reduced most when the l iquid water content is high, and by an amount that is consistent with atmospheric attenuation at the radar frequency in use. However, some e xamples of enhanced backscatter were also observed indicating a possib le reduction in surface roughness by the impact of raindrops on the oc ean surface. Examination of return pulse shapes across significant rai n events reveals behavior consistent with published theoretical work a nd shows how rain may lead to loss-of-lock in extreme conditions. The results of this study have implications for improved data quality nagg ing procedures and correction of ERS-1 altimeter winds.