A Detailed Evaluation of the MRMS Snow QPE

The Multi-Radar Multi-Sensor (MRMS) system provides a radar-derived quantitative precipitation estimation (QPE) across the CONUS for various types of precipitation. Instantaneous observations of precipitation rate are calculated every two minutes, and are summed to provide observation totals at various time intervals ranging from 1 to 72 hours. Snow QPE is provided by the MRMS system, and is calculated using one reflectivity (Z) to snow rate (S) relationship, Z = 75S². There are numerous challenges with calculating accurate storm total measures of snow QPE. Some of these challenges with radar-derived QPE include beam overshooting at far distances from the radar, and varying snow crystal types which affect snow crystal density and fall-rates. These challenges can result in errors radar-derived QPE products, thus it is important to understand the factors that contribute to radar-derived snow QPE errors in order to correct for them within the MRMS system.

In this study, the performance of the MRMS snow QPE was evaluated. The primary goal of this study was to examine snow QPE biases compared to various MRMS products in order to develop a method to reduce MRMS radar-derived snow QPE biases. Eight cases of heavy snowfall occurring in New England were selected to examine how MRMS radar-derived snow QPE performed in comparison to hourly and 24-hr snow water equivalent (SWE) reports from both quality-controlled automated gauges and CoCoRaHS reports. It was found that there is no systemic bias in snow QPE based on gauge distance from the radar, or based on other MRMS products such as seamless hybrid scan of reflectivity height, radar quality index, reflectivity at the -15 °C level, etc. Rather, it was found that there is a systemic bias based on gauge reported SWE. Snow QPE tended to be underestimated (overestimated) for high (low) gauge SWE reports. The findings and analysis herein will be used to develop a correction for MRMS QPE for snow in the near future.