Measurements of the Effect of Rain-Induced Sea Surface Roughness on the QuikSCAT Scatterometer Radar Cross Section

Radar measurements of the sea surface, with satellite scatterometers that operate at K-u-band, are affected by the presence of rain through modification of the sea surface roughness by rain impacts. This is in addition to wind driven roughness, atmospheric scattering, and attenuation that affect the measured normalized radar cross section (NRCS). This paper presents a case study of the increase of the total radar cross section, averaged across surface illuminated areas (individual footprints) of the SeaWinds scatterometer (on QuikSCAT) caused by rain striking the sea surface. This effort combines satellite-based K-u-band data with high-resolution 3-D volumetric rain measurements, from simultaneous collocated Next Generation Weather Radar data. The results to be presented were acquired during a significant rain event in the Gulf of Mexico, to the east of Corpus Christi, and just south of Houston, TX, in May 2005. The results of this paper show dependence on wind speed, rainrate, and polarization. They agree with numerous surface-based studies (single point measurements), using ocean platforms and wind-wave tanks, whose data were collected under similar conditions. For example, at rainrates less than 10 mm/hr, the relative change in surface roughness is seen to decrease as the wind magnitude increases from 5 to 7 m/s. Another consistent observation is that the vertical polarization NRCS shows less sensitivity to rainrate than does horizontal.