Estimation of raindrop size distribution from polarimetric radar measurements at C-band

Many studies have proposed methods to estimate the raindrop size distribution (DSD) parameters from polarimetric radar data as a part of rain attenuation correction and/or rainfall rate estimation algorithms, in which a modified gamma distribution model has been often used to characterize the natural variation of DSD. The parameters that determine the modified gamma DSD are a shape parameter μ, median volume diameter D₀ or slope parameter Λ, and a number concentration N₀ or its normalized version N_W. While D₀ (or Λ) and N_W can be retrieved relatively straightforward from the polarimetric radar measurements, estimation of shape parameter is not an easy task. Instead, empirical relations including μ-Λ and/or μ-D₀ relations derived from surface measurements of DSD are widely used to estimate μ implicitly.

   Adachi et al. (2015) proposed a method to estimate the three parameters of the DSD from polarimetric radar data without any assumptions of relationship among the parameters. In that method, they assumed a constant shape parameter in a range profile. However, this assumption may not be satisfied if the radar is sampling mixed convective/stratiform echoes that simultaneously exist in a single profile. Theoretically, on the other hand, a shape parameter can be estimated from a correlation coefficient ρ_HV at each range gate (e.g., Thurai et al. 2008). However, to estimate shape parameters with a method of this kind, it is necessary to obtain a correlation coefficient with quite high accuracy, for which a very long sampling time is needed to apply it to radars to satisfy this condition for most radar measurements. The MRI C-band polarimetric radar is equipped with solid-state transmitters, which enable the radar to make observations of correlation coefficient with high accuracy in a relatively short time, especially in high-SNR regions. Thus, we have developed an algorithm to estimate a shape parameter at each range gate both from correlation coefficient and differential reflectivity along with D₀ and N_W, and compared it with surface measurements.