Assessing the sensitivity of Wasatch snowfall to temperature variations

Techniques to estimate the sensitivity of snow falling in the Wasatch Mountains of northern Utah to observed and projected changes in atmospheric temperature are evaluated. Daily precipitation data from 1 October to 30 April 1979-2008 are examined from selected snowpack telemetry (SNOTEL) sites in northern Utah. Profiles of wet bulb temperature from near crest level (700 hPa) to elevations along the mountain slopes of selected watersheds are estimated on days with precipitation from twice-daily radiosondes launched from the nearby Salt Lake City airport. The rain-snow line is assumed to correspond uniformly to the elevation of the wet bulb temperature equal to 1oC. Histograms of the fraction of precipitation falling as snow as a function of daily wet-bulb temperature are computed. The percentage of precipitation estimated to fall at wet-bulb temperatures in the range 0-1oC is assumed to switch from snow to rain if temperatures increase by 1oC. While the sensitivity to temperature depends strongly on elevation within the watershed, an increase in tropospheric temperatures of 1oC is estimated to lead to an 8-10% decrease in snow when averaged over the selected watersheds. Projected increases in tropospheric temperatures in northern Utah of 1-3oC over the next century are estimated from general circulation models after removal of model biases.