Revised ocean backscatter models at C and Ku band under high-wind conditions

A series of airborne scatterometer experiments designed to collect C and Ku band ocean backscatter data in regions of high ocean surface winds has rec ently been completed. More than 100 hours of data were collected using the University of Massachusetts C and Ku band scatterometers, CSCAT and KUSCAT. These instruments measure the full azimuthal normalized radar cross sectio n (NRCS) of a common surface area of the ocean simultaneously at four incid ence angles. Our results demonstrate limitations of the current empirical m odels, C band geophysical model function 4 (CMOD4), SeaSat scatterometer 2 (SASS 2), and NASA scatterometer 1 (NSCAT) 1, that relate ocean backscatter to the near-surface wind at high wind speeds. The discussion focuses on wi nds in excess of 15 m s(-1) in clear atmospheric conditions. The scatterome ter data are collocated with measurements from ocean data buoys and Global Positioning System dropsondes, and a Fourier analysis is performed as a fun ction of wind regime. A three-term Fourier series is fit to the backscatter data, and a revised set of coefficients is tabulated. These revised models , CMOD4HW and KUSCAT 1, are the basis for a discussion of the NRCS at high wind speeds. Our scatterometer data show a clear overprediction of the deri ved NRCS response to high winds based on the CMOD4, SASS 2, and NSCAT 1 mod els. Furthermore, saturation of the NRCS response begins to occur above 15 m s(-1). Sensitivity of the upwind and crosswind response is discussed with implications toward high wind speed retrieval.