Correcting Propagation Effects in C-Band Polarimetric Radar Observations of Tropical Convection Using Differential Propagation Phase

Abstract

A propagation correction algorithm utilizing the differential propagation phase (ϕ_dp) was developed and tested on C-band polarimetric radar observations of tropical convection obtained during the Maritime Continent Thunderstorm Experiment. An empirical procedure was refined to estimate the mean coefficient of proportionality a (b) in the linear relationship between ϕ_dp and the horizontal (differential) attenuation throughout each radar volume. The empirical estimates of these coefficients were a factor of 1.5–2 times larger than predicted by prior scattering simulations. This discrepancy was attributed to the routine presence of large drops [e.g., differential reflectivity Z_dr ≥ 3 dB] within the tropical convection that were not included in prior theoretical studies. Scattering simulations demonstrated that the coefficients a and b are nearly constant for small to moderate sized drops (e.g., 0.5 ≤ Z_dr ≤ 2 dB; 1 ≤ diameter D₀ < 2.5 mm) but actually increase with the differential reflectivity for drop size distributions characterized by Z_dr > 2 dB. As a result, large drops 1) bias the mean coefficients upward and 2) increase the standard error associated with the mean empirical coefficients down range of convective cores that contain large drops. To reduce this error, the authors implemented a “large drop correction” that utilizes enhanced coefficients a* and b* in large drop cores. Validation of the propagation correction algorithm was accomplished with cumulative rain gauge data and internal consistency among the polarimetric variables. The bias and standard error of the cumulative radar rainfall estimator R(Z_h) [R(K_dp,Z_dr)], where Z_h is horizontal reflectivity and K_dp is specific differential phase, were substantially reduced after the application of the attenuation (differential attenuation) correction procedure utilizing ϕ_dp. Similarly, scatterplots of uncorrected Z_h (Z_dr) versus K_dp substantially underestimated theoretical expectations. After application of the propagation correction algorithm, the bias present in observations of both Z_h(K_dp) and Z_dr(K_dp) was removed and the standard errors relative to scattering simulation results were significantly reduced.