The effect of rain on ASCAT observations of the sea surface radar cross section using simultaneous 3-d NEXRAD rain measurements

The ADVANCED Wind Scatterometer (ASCAT) instrument is now the principal resource for oceanographers and meteorologists needing timely observations of ocean vector winds across the globe, using an active microwave sensor. This is one of the instruments operated by the European Space Agency (ESA)/European Organization for the Exploitation of Meteorological Satellites (EUMETSAT), on the METOP-A satellite. This is a six-antenna beam radar which operates with vertical polarization at 5.3 Ghz. The retrieval of wind speed and direction for each surface cell area requires the accurate estimation of the radar cross section (RCS) from three antenna beams. The processed returned signals result in an operational product with 50 km resolution on a 25 km grid. When a rain event is being observed by ASCAT, these 50 km cells will contain a wide range of spatial variability of the precipitation. This circumstance creates a challenge for a quantitative assessment of the effect of the rain on the measured radar cross section as a function of surface wind speed, and for then utilizing this knowledge for more accurate wind speed and direction estimation.