Wednesday, June 1, 2016
The Shifting Wasatch Snow Climate
A new study examining the shifting snow climate of the Wasatch Range and western United States that has been accepted for publication in Geophysical Research Letters has gotten some coverage this week by the Utah press, including a this report by Judy Fays on KUER. Jason Scalzitti, Court Strong, and Adam Kochanski, colleagues of mine in the University of Utah Department of Atmospheric Sciences, conducted the study, which was recently accepted for publication in Geophysical Research Letters and is available here.
Two aspects of the study's methodology are important for understanding their findings. First, it is based on a relatively high-resolution (4-km grid spacing) regional climate model. That means it accounts reasonably well for local influences of terrain on precipitation and temperature. Second, it compares a historical (1985–2010) simulation by the climate model, which is driven by observed analyses, with what is known as a pseudo-global-warming simulation in which the historical moisture and temperature, and sea surface temperatures being fed to the model are perturbed based on a global climate model projection for 2085–2094 assuming a medium-to-high future greenhouse gas emissions scenario. This allows one to conduct a thought experiment asking how global warming would influence local climate if storm-track characteristics were largely unchanged. What it does not account for are changes in precipitation and storm characteristics due to large-scale shifts in the storm track.
Using this modeling configuration, they examine the correlation between mean April snowpack water equivalent (SWE) and (1) temperature during March and April and (2) cool-season (October to April) precipitation. In our current climate, precipitation is the primary driver of April SWE at the highest elevations in the Wasatch Range. Bad snow years reflect drought and good snow years reflect wet periods. At lower elevations, temperature plays a more important role and the snowpack SWE reflects both the March to April temperature (i.e., an inverse correlation with warmer periods yielding lower SWE) and the amount of precipitation during the cool season. SNOTEL and model data for the 1985–2010 period suggest that the threshold elevation at which precipitation correlates more strongly than temperature to mean April snowpack SWE is around 1980 m (6500 feet). This doesn't mean that temperature doesn't matter above that level, just that precipitation plays the more dominant role.
In the pseudo-global warming simulation, the threshold elevation increases by about 250 m (800 feet) to 7300 feet. Above this level, the inverse correlation of mean April SWE with temperature increases and the correlation with precipitation decreases, although precipitation remains the primary driver. These results are consistent with temperature playing an increasing role in the Wasatch snow climate through a decrease in the fraction of cool-season precipitation that falls as snow, an increase in cool-season melt events, and an earlier and more rapid snowmelt season.
There are a few caveats that must be noted. Perhaps the most important is that the modeling approach used does not consider the effects of aspect (e.g., what direction a slope faces) on snowmelt. The former, in particular, is quite important in the Wasatch Range and I suspect one might find that on south aspects the threshold elevation is higher than on north elevation (both in the historical and future climates) and that the change in this threshold elevation might vary by aspect. For example, portions of Park City Mountain Resort near the base that face east or south likely are already below the threshold elevation at which temperature plays an important role.
Ultimately, most of the energy for melting snow comes from the sun and thus this is an important factor to consider in future work. Similarly, the possible influence of dust, which increases the amount of solar radiation absorbed by the snowpack, should also be explored.
One aspect of the study that might not be apparent in press releases is that it reinforces the view, which we have noted frequently on this blog, that the sensitivity of snowpack to global warming varies with elevation. In our part of the world, snowfall and snowpack vulnerability decreases with elevation. Northerly aspects are also less vulnerable. In a warming world, this will likely further amplify winter recreation pressure in the Cottonwood Canyons due to their relative abundance of high altitude, north facing terrain.