Effects of rain on Ku-band backscatter from the ocean

Abstract

During the Kwajalein Experiment (KWAJEX) in July and August 1999,measurements of the normalized radar cross section of the ocean,so, were made at Ku-band with HH and VV polarizations from the R/VRonald H. Brown. Data were collected at a variety of incidence anglesduring periods of rainfall as well as during clear conditions. Duringthe experiment the rainfall rate ranged from 0 to 80 mm hr1. Coincidentwith the backscatter measurements, measurements of rain rate, windspeed, wind direction, and fluxes of heat and momentum were made.Since we were primarily interested in backscatter from the surface,we removed backscatter from the raindrops themselves for the so measurementsreported here. As a secondary result we show that the backscatterfrom the rain drops is a good indicator of rain rate. Most of thedata were collected with the ship stationary and the bow held intothe wind. Thus the azimuth angle between the antenna look directionand the direction from which the wind came was predominantly between0 and 90. Over this range, rain was found to increase so at incidenceangles of 30 to 75, to have little effect near 20, and to decreaseso very slightly between 14 and 16. At all incidence angles, nodiscernible dependence of so on wind speed was found during rainfallfor wind speeds below 10 m s1; within experimental error the levelof so depended only on rain rate. For the lower incidence angles,this dependence was very small while at the higher incidence angles,so depended on rain rate to a power that varied between about 0.5and 1.2, being somewhat higher at large incidence angles and higherfor HH polarization. The implication of these results is that rainproduces small-scale surface displacements (wavelengths shorter thanabout 3 cm) that roughen the ocean surface much more than the windfor wind speeds below 10 m s1. The results also imply that whenrain fills the entire scatterometer footprint on the surface andthe wind speeds are low to moderate, scatterometry at Ku band isimpossible.