The NASA Scanning Radar Altimeter (SRA) sweeps a radar beam of 1 degree half-power width (two-way) across the aircraft ground track within ±22 degrees of nadir, simultaneously measuring the backscattered power at its 36 GHz (8.3 mm) operating frequency and the range to the sea surface at 64 points spaced across the swath at 0.7 degree incidence angle intervals. The ranges produce raster lines of sea surface topographic at a 10 Hz rate. The backscattered power measurements can be used to determine path integrated rain rate below the aircraft. At 36 GHz the attenuation in dB due to rain is approximately linearly related to rain rate, independent of drop size distribution. At 1.8 km height, the SRA signal suffers a 1 dB attenuation for each mm/hr of rain rate. If the sea surface radar backscatter coefficient does not change as one transitions into a region of rain, the loss of signal will determine the rain rate to an accuracy of a fraction of a mm/hr. Changes in the backscatter coefficient at nadir are small compared to the rain absorption in the hurricane high wind environment and can be differentiated from changes in rain absorption by examining the variation of backscattered power with incidence angle. The SRA rain rate estimates were well correlated with the rain rate estimates from the NOAA Step Frequency Microwave Radiometer (SFMR) during flights in Hurricane Humberto on 23 and 24 September 2001. At the 1.8 km height of the Humberto flights, the SRA signal margin was wiped out at a rain rate15 mm/hr and the entire SRA signal was lost. At a 1 km height the shorter path length would make the technique viable for rain rates up to 30 mm/hr.

E. J. Walsh presently on assignment at NOAA/Environmental Technology Laboratory, Boulder, CO 80305-3328