Spatial variability of the snow level across the northern California Sierra Nevada

Snow level is defined as the lowest elevation in the atmosphere where snow completely melts and changes over to rain. Characterization of this parameter in mountainous regions is critical for several applications, such as river forecasting, highway maintenance, power generation, emergency management and recreation, to name a few. The elevation of the 0°C isotherm as deduced from rawinsonde data and numerical weather prediction model output has often been used to infer the elevation of the snow level. However, this approach usually provides an overestimate of snow level due to the unaccounted time required for snow to melt as it descends through temperatures above freezing. An empirical altitude offset of 150-450 m below the 0°C isotherm elevation is sometimes used as an estimate of the actual snow level. Profiling Doppler radars provide an alternative for snow level detection through identification of the brightband signature. Although the brightband is usually not coincident with the actual snow level, it provides a more accurate estimate than using the 0°C isotherm elevation by itself.

Profiling Doppler radars are increasingly being employed for snow level detection, particularly in California as part of the western Hydrometeorology Testbed (HMT-West) operated by NOAA. Over the past three cool season's (2005/6, 2006/7 and 2007/8) several profiling Doppler radars (915 MHz and 2875 MHz) were deployed across the northern California Sierra Nevada. The transect of these observing systems extended from the central valley near Sacramento, up the windward slope within the American River Basin and over the crest into the lee of the barrier near Truckee. This study examines the variation of snow level along this transect. The most notable finding to date is the tendency for higher snow levels to be observed over the upstream central valley compared to the windward slope. This difference has sometimes been observed to be in excess of 1 km. Possible explanations for this spatial variability will be discussed, including linkages to orographically forced ascent and the associated adiabatic cooling that would likely result.