A comparative study of rainfall retrievals based on specific differential phase shifts at X- and S-band radar frequencies

A comparative study of the use of X- and S-band polarimetric radars for rainfall parameter retrievals is presented. The main advantage of X-band polarimetric measurements is the availability of reliable specific differential phase shift estimates, K-DP, for lighter rainfalls when phase measurements at the S band are too noisy to produce usable K-DP. Theoretical modeling with experimental raindrop size distributions indicates that due to some non-Rayleigh resonant effects, K-DP values at a 3.2-cm wavelength (X band) are on average a factor of 3.7 greater than at 11 cm (S band), which is a somewhat larger difference than simple frequency scaling predicts. The non-Rayleigh effects also cause X- band horizontal polarization reflectivity, Z(eh), and differential reflectivity, Z(DR), to be larger than those at the S band. The differences between X- and S-band reflectivities can exceed measurement uncertainties for Z(eh) starting approximately at Z(eh) > 40 dBZ, and for Z(DR) when the mass-weighted drop diameter, D-m, exceeds about 2 mm. Simultaneous X- and S-band radar measurements of rainfall showed that consistent KDP estimates exceeding about 0.1 degrees km(-1) began to be possible at reflectivities greater than similar to 26-30 dBZ while at the S band such estimates can generally be made if Z(eh) > similar to 35-39 dBZ. Experimental radar data taken in light-to-moderate stratiform rainfalls with rain rates R in an interval from 2.5 to 15 mm h(-1) showed availability of the K-DP-based estimates of R for most of the data points at the X band while at the S band such estimates were available only for R greater than about 8-10 mm h(-1). After correcting X- band differential reflectivity measurements for differential attenuation, Z(DR) measurements at both radar frequency bands were in good agreement with each other for D-m < 2 mm, which approximately corresponds to Z(DR) approximate to 1.6 dB. The Z(DR)-based retrievals of characteristic raindrop sizes also agreed well with in situ disdrometer measurements.