Snow studies. Part II: Average relationship between mass of snowflakes and their terminal fall velocity

Abstract

This study uses a dataset of low-density snow aggregates measurements collected by a ground-based optical disdrometer that provides particle size and terminal fall speed for each size interval from which the velocity-size and area ratio-size relationships can be derived. From these relationships and relations between the Best and Reynolds numbers proposed in the literature, the mass power-law coefficients are obtained. Then, an approximate average relation between the coefficients in the experimentally determined velocity-size power law (with exponent fixed at 0.18) and the coefficients in the estimated mass power law (with exponent fixed at 2) is obtained. The validation of the retrieved relation is made by comparing, for each snowfall event, the time series of the reflectivity factor calculated from the derived mass-size relationship for a snowflake and from the size distribution measured by the optical disdrometer, with the reflectivity obtained from measurements. Using the measured snow size distribution and the retrieved mass-velocity relationship, a few useful relations between the bulk quantities of snow are derived. This study considers relations suitable for the microphysical modeling consistent with radar measurements of precipitating snow composed of unrimed or lightly rimed aggregate snowflakes. © 2010 American Meteorological Society.