Estimation of the equivalent radar reflectivity factor from measured snow size spectra

In this paper, a method for the estimation of radar reflectivity from measu red snow particle size distributions is presented based on earlier works of Marshall and Gunn and of Smith. During two snowfalls, the method was appli ed to estimate the equivalent reflectivity factor from measured snow size d istributions obtained by the Particle Size and Velocity (PARSIVEL) optical disdrometer. The results are compared with the data of conventional C-band Doppler radar. Here, two snowfalls are presented as case studies. In additi on, a comparison during one rainfall is included, which shows good agreemen t between the two instruments. In the case of snow, the calculation of the equivalent reflectivity factor from the PARSIVEL data is based on a relatio n between the mass and the size of the snow particles. In this study, a mas s-size relation for graupel-like snow was used for all snowfalls. Because t his is a crude description of naturally occurring snow, which can be of any other type (e.g., dendrites), the differences with the radar-measured refl ectivities here are strongly dependent on the snow particle type. Neverthel ess, in one case, the snowfall was fairly homogeneous in time, space, and s now type, and so the agreement was reasonably good, with a relatively const ant underestimation (3-5 dB) of the radar data by PARSIVEL and a low varian ce of the differences. This underestimation could be due to non-graupel-lik e particles or the tendency of PARSIVEL to underestimate the reflectivities , as outlined in the text. The other snowfall was convective, with strong s patial and temporal variations in precipitation intensity and snow type. Th e instrument differences in this case ranged from -6 to 16 dB because of ch anging snow types, but both instruments showed the same qualitative variati ons. The agreement can be improved by an advanced signal processing of PARS IVEL in which the snow type is determined automatically and a proper mass-s ize relation is used.