Improving Vertically Profiling Radar Bright Band Measurements to Support Snow Level Forecasts in the Western U.S.

The snow level is the elevation above which snow will fall, and below which precipitation falls as wet snow or rain with no accumulation. Accurate knowledge of this elevation is important to hydrometeorological forecasts because the partitioning of rain vs. snow and knowing if rain is falling on an existing snowpack influences the timing and magnitude of runoff generation. For example, atmospheric rivers (ARs) are a source of extreme precipitation and floods along the western U.S. The impacts of a given AR can depend on the snow level: depending on the distribution of topography of a given watershed and soil moisture characteristics, there is potential for warm ARs to result in increased runoff since rainfall at high elevations may be associated with greater storm runoff. The altitude of maximum reflectivity in the radar bright band, otherwise known as the bright band height, has been demonstrated to be a useful proxy for the snow level. The Micro Rain Radar (MRR) is a vertically profiling radar that can monitor the bright band height. Compared to other radars such as NOAA’s frequency modulated, continuous wave (FMCW) vertically-profiling radars or 3-GHz S-band profilers (SPROF), the MRR is a low-cost and portable option. The Center for Western Weather and Water Extremes (CW3E) has deployed a network of eight MRRs along the western U.S. to enhance snow level observations, with several more sites planned. This study aims to take initial steps toward meeting two objectives: 1) identify whether the MRR provides accurate and frequent measurements of bright band height up to an elevation of 3,000 meters above the radar, and 2) determine whether these observations are useful for improving snow level forecasts in the western U.S., such as from CW3E’s West-WRF model. The primary dataset used in the study is from an MRR placed in Cazadero, CA, located in the coastal mountains north of San Francisco, from December 2014 to March 2015 as part of the Chilean Coastal Orographic Precipitation Experiment (CCOPE). We use this dataset to quantify the accuracy of the MRR observed bright band height vs. the colocated NOAA SPROF at Cazadero, comparing results between several different bright band detection algorithms. Finally, we provide examples of how observations from this radar network can be used to support West-WRF snow level forecast verification efforts and develop decision-support tools.