Assessing Snowfall Rates from X-Band Radar Reflectivity Measurements

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 Z(e) and the liquid equivalent snowfall precipitation rate S (Z(e) = AS(B)). 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 Z(e)-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 (similar to 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 Z(e) is in mm 6 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.