Development in rain altimetry from Seasat to Envisat and Jason
National Oceanography Centre, Southampton,
Southampton SO17 3SH,
The return signal received by an altimeter is the product of the radar's interaction with the Earth's surface, and its propagation through the atmosphere. Of the latter's effects, the most spatially variable is the attenuation by raindrops. Anomalies in altimetry data, affecting estimates of sea surface height, wave height and wind speed, were noted for Seasat. These were studied in much greater detail for ERS-1, using the waveform data to explain the effect upon the various parameters. The arrival of Topex, the first dual-frequency altimeter, enabled much more reliable detection of rain, allowing the development of a high-resolution altimeter rain flag from and the first quantitative estimates of rainfall from an altimeter. Jason and Envisat also have this dual-frequency capability, providing quantitative estimates of rainfall. Now, the latest challenges are in combining their high-resolution rain records with data from other sensors to give a wider picture of rain events.