Dual-Polarization Radar Snow QPE in MRMS

Radar-based quantitative precipitation estimation (QPE) for snow is complicated by the numerous factors which impact storm total snow accumulation, such as varying snow crystal types, densities, and fall speeds. Within the Multi-Radar Multi-Sensor (MRMS) system, snow QPE is calculated using one reflectivity (Z) to snow water equivalent (SWE) rate (S) relationship, Z = 75S². However, many studies have shown that one reflectivity value may correspond to different SWE rates depending on the aforementioned factors and one Z-S relationship is insufficient to obtain accurate storm total snow QPE. Recent studies have shown that the use of dual-polarization variables in addition to reflectivity can provide more accurate estimations of snow QPE than traditional Z-S relationships.

This study evaluates the performance of snow QPE calculated through the use of an S(K_DP, Z) relationship, S = 1.48K_DP^0.68Z^0.33. Multiple cases of heavy snowfall across the CONUS were collected for investigation to provide geographic and environmental variability. Additionally, the impacts of environmental parameters on the performance of this S(K_DP, Z) relationship was investigated through the analysis of quasi-vertical profiles (QVP), and modifications to the S(K_DP, Z) relationship were examined. QPE performance was evaluated based on hourly and 24-hr SWE reports from quality-controlled automated gauges and manual observations from CoCoRaHS (Community Collaborative Rain, Hail and Snow Network, www.cocorahs.org/) volunteers. Results show that this S(K_DP, Z) relationship reduced the overall spread in the gauge value vs snow QPE bias, providing an overall improvement in snow QPE within the MRMS framework.