The impact of beam broadening on the quality of radar polarimetric data

The impact of beam broadening on the quality of radar polarimetric data in the presence of nonuniform beam filling (NBF) is examined both theoretically and experimentally. Cross-beam gradients of radar reflectivity Z, differential reflectivity Z(DR), and differential phase Phi(DP) within the radar resolution volume may produce significant biases of Z(DR), Phi(DP), and the cross-correlation coefficient p(hv.) These biases increase with range as a result of progressive broadening of the radar beam. They are also larger at shorter radar wavelengths and wider antenna beams. Simple analytical formulas are suggested for estimating the NBF-induced biases from the measured vertical and horizontal gradients of Z, Z(DR), and Phi(DP). Analysis of polarimetric data collected by the KOUN Weather Surveillance Radar-1988 Doppler (WSR-88D) demonstrates that frequently observed perturbations of the radial Phi(DP) profiles and radially oriented "valleys" of p(hv) depression can be qualitatively and quantitatively explained using the suggested NBF model.