Analysis of C and Ku band ocean backscatter measurements under low-wind conditions

Airborne ocean backscatter measurements at C and Ku band wavelengths obtain ed in low to moderate-wind conditions are presented. The differences betwee n the low-wind backscatter data and the CMOD4 and SASS-II models are report ed. The measurements show that the upwind/crosswind backscatter ratio is gr eater than predicted. These large upwind/crosswind backscatter ratios are a ttributed to a rapid decrease in the crosswind backscatter at low winds. Qu alitative agreement with the composite surface model proposed by Donelan an d Pierson suggests the rapid decrease in the crosswind backscatter may be c aused by viscous dampening of the Bragg-resonant capillary-gravity waves. W e show that for larger antenna footprints typical of satellite-based scatte rometers, the variability in the observed wind field smooths out the backsc atter response such that the rapid decrease in the crosswind direction is n ot observed.