Asymmetry and Anisotropy of Microwave Backscatter at Low Incidence Angles

This paper presents the first results from a study of upwind/downwind asymmetry (UDA) and upwind/crosswind anisotropy (UCA) of the Ku-band radar backscatter at low incidence angles (0 ^° to 18^°) from satellite observations. Incidence angle, wind speed, and sea state dependence were a particular focus. Data were obtained from the Ku-band HH-polarized precipitation radar data in the Tropical Rainfall Mapping Mission colocated with measurements from buoys. It is shown that there is a statistically significant directional signal in the UDA and UCA data when the radar incidence angle is greater than 5^°. In particular, a higher radar backscatter was observed from the downwind as opposed to the upwind look direction. This is contrary to scatterometer measurements at moderate incidence angles. As a whole, the main trends for these negative asymmetries increase the absolute magnitudes with increasing wind speed. This UDA is explained by the use of non-Gaussian statistics of the sea surface slope. By contrast, there is a change in behavior of the UCA of microwave backscatter at a critical wind speed of about 6 to 8 m/s. The magnitude of UCA apparently decreases with increasing wind speed below this wind speed range, and then it increases for higher wind speeds. Sensitivities to both incidence angle and sea state are also analyzed.