X-Band Polarimetric Radar Measurements of Rainfall

Abstract

A combined polarimetric estimator for rainfall rate (R) retrievals from polarimetric radar measurements at X band is proposed. This estimator uses the horizontal polarization radar reflectivity Ze, differential reflectivity ZDR, and specific differential phase shift KDP, and it intrinsically accounts for changes in how drop oblateness increases with size. Because this estimator uses power measurements (i.e., Ze and ZDR), a procedure for correcting these measurements for effects of partial attenuation and differential attenuation using the differential phase measurement is suggested. An altitude correction for estimates of rainfall rates is also suggested. The proposed combined polarimetric estimator that uses KDP, ZDR, and Ze, an estimator that uses KDP alone for equilibrium drop shapes, and different Ze–R relations were applied to the 15 rain events observed with the NOAA X-band transportable polarimetric radar during the eight-week field campaign at the NASA Wallops Island facility in Virginia. The observed rains ranged from very light stratiform events to very heavy convective ones with cells producing rainfall rates in excess of 100 mm h−1. The three different ground validation sites were equipped with high-resolution (0.01 in.) tipping-bucket rain gauges. One of these sites also was equipped with disdrometers. In terms of the relative standard deviation, the combined polarimetric estimator provided the best overall agreement with gauge data (22%), closely followed by a case-tuned Ze–R relation (23%) that was determined for each observational case from drop size distributions (DSD) measured in situ by a disdrometer and was available only a posteriori. The use of the KDP-only estimator and a mean Ze–R relation resulted in 30% and 32% relative standard deviations, correspondingly. The combined polarimetric estimator, the KDP-only estimator, and the case-tuned Ze–R relation estimator provided about a 6%–9% negative bias in comparison with the gauge data; the mean Ze–R relation estimator provided a larger negative bias (18%).