Comparisons between radar observations and radar parameters derived from disdrometer observations (assuming a gamma drop-size distribution and equilibrium axis ratios) revealed that the radar estimates of differential reflectivity and specific differential phase were significantly less than those determined from disdrometer observations. Comparisons improved slightly when the calculations were based on a mean canting angle of 0o and a standard deviation of 10o and improved markedly for an empirical axis ratio relation representing more spherical drops.
Rainfall rates were estimated with the WSR-88D default radar reflectivity (ZH) relation and with previously-published reflectivity-differential reflectivity (ZHZDR) and specific differential phase (KDP) relations developed from simulations with equilibrium axis ratios and widely accepted ranges in the DSD governing parameters (No, L, and µ). Results showed small overall bias for radar reflectivity (~10%), but there was an overestimate of ~50% for the ZHZDR measurement pair and an underestimate of 30% for KDP. The use of Florida-tuned relations for radar reflectivity and the reflectivity-differential reflectivity pair (assuming equilibrium axis ratios) reduced the biases to 3 and 8%, respectively. However, the bias for KDP was unchanged, confirming an insensitivity to the drop-size distribution governing parameters.
Further fine tuning of the Florida relations for drop canting and more spherical drop shapes readily accounts for the residual bias with ZHZDR estimators. However, a significant underestimate of rainfall (14%) remained in the KDP estimates even after allowing for more spherical drop shapes. This residual bias with KDP is attributed to the loss of signal in storms dominated by small drops and the growth of errors at weak signals in light rainfalls.
In summary, rainfall estimates with ZHZDR estimators had the highest correlation with rain gauge observations (0.92), the smallest range in bias factors from storm to storm (1.73), and the smallest root mean square error for unbiased estimators (6.3 mm).