A Surface Reference Technique for Airborne Doppler Radar Measurements in Hurricanes

The operational algorithm for rainfall retrieval from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar data requires a measurement of the path-integrated attenuation (PIA) as a constraint. This constraint is derived via the surface reference technique, whereby a measurement of the ocean surface in a raining area is compared with the ocean surface backscatter in a neighboring clear area. This method assumes that the surface backscatter difference is due only to the presence of rain, although variation in surface winds could also cause differences in the reference and rain measurements. An alternative surface reference method is to use a measurement of the surface winds and a backscatter model to predict the rain-free, or reference, cross section. Such an approach is developed here for airborne Doppler radar measurements in hurricanes. This approach provides an independent measurement of the reference backscatter, which is compared with the standard clear-air reference. The mean difference between the standard and Doppler-derived PIA is less than or equal to 1 dB; the rms difference is in the range 0.9–2.6 dB. In deriving the model function for backscatter estimation from wind measurements, the authors also find that the dependence of ocean backscatter on wind appears to saturate at high wind speeds at 25Њ incidence.