A Study of Rain-Snow Transitions in the Kananaskis Valley, Alberta

The precipitation phase has a major impact on water resources, especially during spring when snow melts. Major precipitation events occur on the eastern side of the Canadian Rockies and can lead to major disasters such as the 2013 Calgary flooding. Rain-snow boundaries along mountainsides are the region bound by snow at higher elevations and rain at lower elevations. The width and precipitation type distribution within that boundary depend strongly on the weather conditions such as temperature, wind field and relative humidity because these could be altered by the temperature feedbacks from phase changes. For example, cooling due to melting snow can modify the direction of the valley flow and can lower the position of the rain-snow line on the mountainside. The present study examines the relative importance of snow sublimation, melting and accretion on the temperature and wind fields in a valley associated with a precipitation type transition. The Weather Research and Forecasting model is used to simulate well-observed cases associated with rain-snow transitions during a field campaign in spring 2015 in the Kananaskis valley, Alberta. Preliminary results show that sublimation of snow has a greater impact on the temperature tendency and wind direction, compared to melting or accretion. The accretion process leads to a warming of the environment and forms denser solid particles, which can reach the surface at high temperatures. For example, solid particles were observed at temperatures up to +8°C. Finally, it is also showed that rain-snow transitions occur sometimes at temperatures above 0°C on the eastern side of the Canadian Rockies. Overall, this study will contribute to a better understanding of microphysical processes that can lead to extreme meteorological events in mountainous regions.