Mapping Bragg Scatter with a Polarimetric WSR-88D

Abstract

Using a polarimetric Weather Surveillance Radar-1988 Doppler (WSR-88D) radar to distinguish Bragg scatterers from insects and birds in an optically clear atmosphere has the potential to provide information on convective boundary layer depth. Measured median differential reflectivities ZDR of Bragg scatterers lie between −0.08 and 0.06 dB, which supports the hypothesis that the intrinsic ZDR of Bragg scatters is 0 dB. Thus, the intrinsic 0 dB of Bragg scatter can be used for verifying of ZDR radar calibration. Measured copolar correlation coefficients ρhv have distributions peaked at about 0.998–1.0. If insects and birds are spatially separated from Bragg scatterers, the dual-polarization capability of the WSR-88D allows distinguishing echoes from these two types of scatterers since ZDR from biota is significantly larger than 0 dB. In mixtures of Bragg and biota scatter, polarimetric spectral analysis shows differences in portions of the H and V spectra where birds and insects could be contaminating echoes from Bragg scatterers. The values of ZDR ≈ 0 and ρhv ≈ 1 that characterize Bragg scatterers allow coherent summation of signals from the H and V receiver channels and restoration of measurement capability lost as a result of splitting power into H and V channels. Further enhancements to data collection and signal processing allow power measurement, with a standard deviation of about 1 dB, of weak echoes from Bragg scatterers having equivalent reflectivity factors of about −28 dBZ at distance of 10 km from the radar. This level of reflectivity corresponds to a refractive index structure parameter of about 4 × 10−15 m−2/3, a typical magnitude found in maritime air.