Estimation of the Equivalent Radar Reflectivity Factor from Measured Snow Size Spectra

In this paper, a method for the estimation of radar reflectivity from measured snow particle size distributions is presented based on earlier works of Marshall and Gunn and of Smith. During two snowfalls, the method was applied to estimate the equivalent reflectivity factor from measured snow size distributions 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 addition, a comparison during one rainfall is included, which shows good agreement between the two instruments. In the case of snow, the calculation of the equivalent reflectivity factor from the PARSIVEL data is based on a relation between the mass and the size of the snow particles. In this study, a mass–size relation for graupel-like snow was used for all snowfalls. Because this is a crude description of naturally occurring snow, which can be of any other type (e.g., dendrites), the differences with the radar-measured reflectivities here are strongly dependent on the snow particle type. Nevertheless, in one case, the snowfall was fairly homogeneous in time, space, and snow type, and so the agreement was reasonably good, with a relatively constant underestimation (3–5 dB) of the radar data by PARSIVEL and a low variance of the differences. This underestimation could be due to non-graupel-like particles or the tendency of PARSIVEL to underestimate the reflectivities, as outlined in the text. The other snowfall was convective, with strong spatial and temporal variations in precipitation intensity and snow type. The instrument differences in this case ranged from −6 to 16 dB because of changing snow types, but both instruments showed the same qualitative variations. The agreement can be improved by an advanced signal processing of PARSIVEL in which the snow type is determined automatically and a proper mass–size relation is used.