Assessing Snowfall Rates from X-Band Radar Reflectivity Measurements

Abstract

Realistic aggregate snowflake models and experimental snowflake size distribution parameters are used to derive X-band power-law relations between the equivalent radar reflectivity factor Ze and the liquid equivalent snowfall precipitation rate S (Ze = ASB). There is significant variability in coefficients of these relations caused by uncertainties in the snowflake bulk densities (as defined by the mass?size relationships), fall velocities, and particle size distribution parameters. The variability in snowflake parameters results in differing Ze?S relations that provide more than a factor of 2 difference in precipitation rate and liquid equivalent accumulation estimates for typical reflectivity values observed in snowfall (?20?30 dBZ). Characteristic values of the exponent B in the derived for dry snowfall relations were generally in the range 1.3?1.55 (when Ze is in mm6 m?3 and S is in mm h?1). The coefficient A exhibited stronger variability and varied in the range from about 30 (for aircraft-based size distributions and smaller density particles) to about 140 (for surface-based size distributions). The non-Rayleigh scattering effects at X band result in diminishing of both A and B, as compared to the relations for longer wavelength radars. The snowflake shape and orientation also influences its backscatter properties, but to a lesser extent compared to the particle bulk density. The derived relations were primarily obtained for snowfall consisting of dry aggregate snowflakes. They were applied to the X-band radar measurements during observations of wintertime storms. For approximately collocated measurements, the in situ estimates of snowfall accumulations were generally within the range of radar-derived values when the coefficient A was around 100?120.