Prospects for measuring rainfall using propagation differential phase in X- and K-a-radar bands

Model calculations and measurements of the specific propagation and backsca tter differential phase shifts. (K-DP and delta(o), respectively) in rain a re discussed for X- (lambda similar to 3 cm) and K-a-band (lambda similar t o 0.8 cm) radar wavelengths. The details of the drop size distribution have only a small effect on the relationships between K-DP 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. Th e backscatter differential phase Shift at X hand for-rain rates of less tha n about 15 mm h(-1) is generally small and should not pose a serious proble m when estimating K-DP from the total phase difference at range intervals o f several kilometers. The main advantage of using X-band wavelengths compar ed to S-band (lambda similar to 10-11 cm) wavelengths is an increase in K-D P by a factor of about 3 for the same rainfall rate. The relative contribut ion of the backscatter differential phase to the total phase difference at K-a 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 K-a-band measureme nts of the differential phase. Experimental studies of rain using X-band differential phase measurements w ere conducted near Boulder, Colorado, in a stratiform, intermittent rain wi th a rate averaging about 4-5 mm h(-1). The differential phase shift approa ch proved to be effective for such modest rains, and finer spatial resoluti ons were possible in comparison to those achieved with similar measurements at longer wavelengths. A K-DP-R relation derived for the mean drop aspect ratio (R = 20.5K(DP)(0.80)) provided a satisfactory,agreement between rain accumulations derived from radar measurements of the differential phase and data from several nea;by high-resolution surface rain gauges. For two rain fall events, radar estimates based on the assumed mean drop aspect ratio we re, on average, quite close to the gauge measurements with about 38% relati ve standard deviation of radar data from the gauge data.