Improving the Quantitative Precipitation Estimate for Hydrometeors Classified as Dry Snow by Polarimetric Radars

Between 2011 and 2013, National Weather Service (NWS) Weather Surveillance Radar 1988 Doppler systems (WSR-88D) were upgraded with a dual polarization capability. The polarimetric upgrade is a significant enhancement that provides new and improved information about precipitation type, intensity, and size. Much work has gone into improving quantitative precipitation estimates (QPE), but the dual polarization QPE system only uses a modified version of the legacy reflectivity - rain relation for returns classified as dry snow that applies a multiplication factor of 2.8 to take into account the lower reflectivity returns associated with dry snow. NWS Forecast Office Buffalo, NY was upgraded with dual polarization during April 2012 and together with surrounding offices noticed an overestimation in the dual polarization QPE for several cold season events when the radar beam was above the melting layer. This study used gauge-to-radar comparisons for 722 hourly cases to test whether the coefficient was causing the overestimation. The results showed that the default coefficient of 2.8 was too high and led to a high bias in QPE. The mean dual polarization QPE was nearly double the gauge measured precipitation. When the coefficient was lowered to 1.4, the mean dual polarization QPE was still 19% higher than measured precipitation, but much improved over the initial values.