Beamwidth Effects on Z-R Relations and Area-integrated Rainfall

Abstract

The effective radar reflectivity Ze, measured by a radar is the convolution of the actual distribution of reflectivity with the beam radiation pattern. Because of the nonlinearity between Z and nun rate R, Ze gives a biased estimator of R whenever the reflectivity field is nonuniform. In the presence of sharp horizontal reflectivity gradients, the measured pattern of Ze, extends beyond the actual precipitation boundaries to produce false precipitation echoes. When integrated across the radar image of the storm, the false echo areas contribute to the sum to produce overestimates of the areal rainfall. As the range or beamwidth increases the ratio of measured to actual rainfall increases. Beyond some range, the normal decrease of reflectivity with height dominates and the measured rainfall underestimates the actual amount. The net effect is a Ze-R relationship that may differ largely from that which would be obtained from consideration of the drop-size distribution alone. The range dependence is also altered significantly. The. results are especially important for quantitative measurement of convective rainfall over large areas.