The Relationship between Winds, Surface Roughness, and Radar Backscatter at Low Incidence Angles from TRMM Precipitation Radar Measurements

Abstract

One year of collocated, rain-free normalized backscatter cross-section measurements from the Tropical Rainfall Mapping Mission (TRMM) precipitation radar (PR) and wind speed estimates from the TRMM Microwave Imager are used to construct fully empirical model functions relating cross section to wind speed for incidence angles from 0° (nadir) to 18°. With the exception of a ∼1.9-dB offset, the TRMM PR model function at nadir compares well with TOPEX and Geosat results. For the first time using spaceborne data, least squares fits of the TRMM PR model functions to the geometric optics scattering formulation allow direct solution for the magnitudes and wind speed dependencies of Ku-band effective nadir reflectivity and effective mean square slope [s(u)]. Effective reflectivity is found to decrease slightly with increasing wind speed above 3.5 m s–1. A logarithmic dependence of s(u) is found for wind speeds between 1 and 10 m s–1. Both linear and two-branch logarithmic dependencies (with a slope break at 10 m s–1) are excellent and statistically indistinguishable parameterizations for s(u) for wind speeds between 5 and 19 m s–1. Calculations using the Elfouhaily et al. wave model suggest that the effective Ku-band s(u) corresponds to a cutoff wavelength of 3 to 6 times the radar wavelength for wind speeds from 5 to 25 m s–1; at lower wind speeds, the cutoff wavelength increases rapidly consistent with earlier observations.