Mesoscale controls on the mountainside rain-snow line

One of the most fundamental aspects of mountain weather and climate is the rain-snow line, the boundary between lowland rainfall and mountain snowfall. Precise knowledge of the rain-snow line is of value, since modest changes of 100-200 m in its elevation can have major impacts on snowpack accumulation and flooding.

Perhaps the simplest expectation is that the mountainside rain-snow line will reside at about the elevation of the 0°C isotherm upwind of a mountain. However, as air passes over a mountain the temperature and distribution of precipitation is profoundly altered; as a result, on the mountainside the rain-snow line is often located at an elevation hundreds of meters different from the rain-snow line in the free air upwind of the mountain. This mesoscale modification is poorly understood, is not resolved by global models, and is large enough to have major impacts on a variety of natural and human systems.

Semi-idealized simulations with a mesoscale numerical weather prediction model (WRF) are used to simulate the rain-snow boundary over mountains for stably stratified orographic precipitation. These simulations allow the identification of the physical mechanisms responsible for mesoscale structure of the rain-snow line. Results reveal that latent cooling from melting precipitation, pseudo-adiabatic cooling from vertical motion, and spatial variations in microphysical timescales all make important contributions to lowering the rain-snow line over the windward slopes of mountains. The relative importance of these processes depends on properties of the incoming flow and terrain geometry in ways that can be understood from simple theories. Implications of these results for regional climate change impacts will be discussed.