Spatial variability in modulations of radar backscatter from the ocean

Spatial variations in the time-average level of microwave backscatter from the ocean surface are often observable and are related to variations in surface roughness. Spatial variability is also observable in the modulations of radar backscatter. The authors report observations of such variability in measurements obtained with the FOPAIR imaging radar. This radar provides image sequences of the zeroth and first moments of the Doppler spectrum. Spectral analysis performed on each image pixel´s time-series yields a set of backscatter and velocity variance images, with each image tuned to a particular frequency. The authors present open ocean data obtained from the R/P FLIP during the 1995 Marine Boundary Layer Experiment during the early development of a storm. At low (swell) frequencies, the authors observe spatial variations in modulation with a mottled, somewhat random structure. The spatial variation of the structure seen at this frequency is finer than the wavelength of the corresponding long waves. At higher frequencies near and beyond the wind-wave peak, linear structures roughly aligned with the wind direction are evident in several cases. The crosswind spatial frequency of these structures was observed to correlate with the temporal frequency. These patterns were most observable early in the development of the storm and appear to be a consequence of wave-wave interference under gradually turning wind conditions