Prospects for Measuring Rainfall Using Propagation Differential Phase in X- and Ka-Radar Bands

Abstract

Model calculations and measurements of the specific propagation and backscatter differential phase shifts (KDP and δo, respectively) in rain are discussed for X- (? ? 3 cm) and Ka-band (? ? 0.8 cm) radar wavelengths. The details of the drop size distribution have only a small effect on the relationships between KDP and rainfall rate R. These relationships, however, are subject to significant variations due to the assumed model of the drop aspect ratio as a function of their size. The backscatter differential phase shift at X band for rain rates of less than about 15 mm h?1 is generally small and should not pose a serious problem when estimating KDP from the total phase difference at range intervals of several kilometers. The main advantage of using X-band wavelengths compared to S-band (? ? 10?11 cm) wavelengths is an increase in KDP by a factor of about 3 for the same rainfall rate. The relative contribution of the backscatter differential phase to the total phase difference at Ka band is significantly larger than at X band. This makes propagation and backscatter phase shift contributions comparable for most practical cases and poses difficulties in estimating rainfall rate from Ka-band measurements of the differential phase. Experimental studies of rain using X-band differential phase measurements were conducted near Boulder, Colorado, in a stratiform, intermittent rain with a rate averaging about 4?5 mm h?1. The differential phase shift approach proved to be effective for such modest rains, and finer spatial resolutions were possible in comparison to those achieved with similar measurements at longer wavelengths. A KDP?R relation derived for the mean drop aspect ratio (R = 20.5K0.80DP) provided a satisfactory agreement between rain accumulations derived from radar measurements of the differential phase and data from several nearby high-resolution surface rain gauges. For two rainfall events, radar estimates based on the assumed mean drop aspect ratio were, on average, quite close to the gauge measurements with about 38% relative standard deviation of radar data from the gauge data.