A climatology of snow density at upper-elevation locations in the western United States

The prediction of snow density of freshly fallen snow is difficult as there are a number of factors that affect the density in any particular storm, which include temperature, in-cloud crystal habit and size, subcloud processes, such as sublimation and melting, as well as surface processes, such as compaction (Roebber et al., 2003). A recent study by Baxter et al. (2005) provides a comprehensive snow density climatology for COOP sites in the contiguous United States. Despite the usefulness of regional climatologies, a more detailed climatology for upper-elevation sites in the western United States is necessary as there is high variability in snow density over small spatial scales.

As part of a class study, the climatology of snow density at 23 upper-elevation COOP sites in the western United States is provided. Using data such as the maximum and minimum daily temperature, and daily liquid precipitation and snowfall amount, the mean snow density at coastal, intermountain, and continental sites, as well as histograms of snow density for 2 in., 5 in., and 10 in. events (also meeting a criteria of maximum temperature less than or equal to 32°F) are provided. Likewise, the relationship between mean daily temperature (and precipitation) versus snow density is evaluated. Results show for coastal upper-elevation sites, such as those in the Cascades and Sierra Nevadas, the mean snow density is 170 kgm^-3 and is skewed towards higher density. For intermountain sites, such as those in southwestern Montana and northwest Wyoming, the mean snow density is 94 kgm^-3 and slightly skewed towards higher density. For continental sites, such as those in Colorado, the mean snow density is 84 kgm^-3 and has little skewness.

In addition to the above climatologies, composites of 500 hPa geopotential heights, 700 hPa winds, temperature and relative humidity, and sea level pressure from NCEP/NCAR Reanalysis are evaluated at observation time, as well as 12 and 24 hours prior, for both high density and low density events. From the composites, the differences in synoptic patterns between high and low density events at upper-elevation coastal, intermountain, and continental sites are outlined.