Mesoscale controls on the mountainside snow line

Observations show that the elevation at which rain changes to snow, (i.e., the snow line, Z_S), and the 0^o Celsius isotherm elevation (Z_0C), frequently drop over the windward slopes of mountains by several hundred meters relative to the free atmosphere upwind. The magnitude of this mesoscale descent is large enough to have important implications for the hydroclimate of mountainous basins. A number of physical mechanisms have been cited as responsible for this behavior: 1) adiabatic cooling of stably stratified air rising over a mountain's windward slopes; 2) orographic enhancement of precipitation resulting in a localized cooling of the air by melting of frozen hydrometeors; 3) orographic enhancement of precipitation resulting in larger snowflakes which fall farther below Z_0C before melting.

This study uses semi-idealized simulations with a mesoscale numerical atmospheric model (the Weather Research and Forecasting model) to diagnose the processes responsible for determining Z_S on the mesoscale, quantify their relative importance, and investigate their sensitivities to atmospheric conditions and terrain geometry. The sensitivity of model results to choice of turbulence and microphysical parameterization is also characterized.

Results of 2D simulations reveal that all three of the above mechanisms play an important role in determining Z_S. The strength of each mechanism varies with mountain shape and incoming airflow properties in ways that can be largely understood with simple theories, helping to explain some of the observed behavior of Z_S. The simulated drop in Z_0C and Z_S is found to increase with temperature. This is of interest, since such a tendency, if present in nature, may act to buffer mountainous basins against the impacts of climate warming on snowpack accumulation and flooding. 3D simulations are also presented that reveal how Z_S is affected by the blocking of low level winds that frequently occurs for sizable mountain ranges. Implications of these results for hydrological forecasting and regional climate modeling will be discussed.