Developing A Performance Measure For Snow-level Forecasts

Abstract

The snow level, or altitude in the atmosphere where snow melts to rain, is an important variable for hydrometeorological prediction in mountainous watersheds; yet, there is no operational performance measure associated with snow-level forecasts in the United States. To establish a performance measure, it is first necessary to establish the baseline performance associated with snow-level forecasts. Using data collected by vertically pointing Doppler radars, an automated algorithm has been developed to detect the altitude of maximum radar reflectivity in the radar bright band that results from the precipitation melting process. This altitude can be used as a proxy for the snow level, partly because it always exists below the freezing level, which is defined as the altitude of the 0°C isotherm. The skill of freezing-level forecasts produced by the California–Nevada River Forecast Center (CNRFC) is evaluated by comparing spatially interpolated and forecaster-adjusted numerical model freezing-level forecasts with observed freezing levels estimated by radars operating at 2875 MHz (S band). The freezing level was chosen instead of the snow level as the comparison parameter because the radar algorithm and the CNRFC have different interpretations of the snow level. The evaluation occurred at two sites: one in the coastal mountains north of San Francisco and the other in the Sierra Nevada. The evaluation was conducted for forecasts made during the winter wet season of 2005/06. Although the overall mean freezing-level forecast bias is small enough not to be hydrologically significant, about 15% of the forecasts had biases greater than 300 m (forecast too low). The largest forecast biases were associated with freezing levels above 2.3 km that were underforecasted by as much as 900 m. These high freezing-level events were accompanied by the heaviest precipitation intensities, exacerbating the flood threat and making the forecast even more challenging.