Effects of radar beam shielding on rainfall estimation for polarimetric C-band radar

Contrary to radar reflectivity, phase measurements are not affected by partial shielding of the transmitted radar beam. Radar reflectivity (Z_h), differential reflectivity (Z_dr), and specific differential phase (K_dp) measured from the operational, polarimetric C-band weather radar located in Trappes, France, were used to examine the effects of radar beam shielding on reflectivity- and K_dp-based rainfall estimation. The objective of this study is to investigate the degree of immunity of K_dp-based rainfall estimates to beam shielding for C-band radar data during four typical rain events encountered in Europe. The rain events include a cold frontal rain bands with average rainfall rates of ~7 mm h^-1 in winter, and one event with average values of ~17 mm h^-1 in summer. Also two summertime events were part of the analysis with stratiform precipitation and partially embedded convection with average rainfall rates of 5 – 11 mm h^-1. The investigation also examined the sensitivity of beam shielding effects on rainfall rate estimation to i) axis-ratio parameterization and drop-size distribution, ii) methods used to smooth profiles of differential propagation phase (φ_dp) and estimate K_dp, and iii) event-to-event variability.

The large effects of beam shielding on rainfall accumulation were observed for algorithms using Z_h with differences up to ~2 dB (40 %) compared to a K_dp-based algorithm over a power loss range of 0 – 8 dB. This analysis reveals that Z_dr and K_dp are not affected by partial beam shielding. Standard reflectivity corrections based on the degree of beam shielding would have overestimated rainfall rates by up to 1.5 dB for less then 40 % beam shielding and up to 3 dB for beam shielding less than 75 %. This mismatch between the results and the reflectivity correction is primarily related to the accuracy with which beam shielding maps can be derived. While topography is typically known with a high spatial resolution, urban obstacles and trees within less than 5 km range from the radar are difficult to resolve with beam shielding maps. Although rainfall estimates were sensitive to drop-size distribution and axis-ratio parameterization, differences between Z_h- and K_dp-based rainfall rates increased independently from those parameters with amount of shielding. Different approaches to smoothing φ_dp profiles and estimating K_dp were examined and showed little impact on results.