Rain-snow boundaries along mountainsides

A major issue in winter is the occurrence and type of precipitation over mountains. In general, the precipitation can be either rain or snow although mixtures in the rain-snow boundary are also common. Such mixtures include snow which has just melted a little (wet snow) as well as snow that has almost completed melting (slush). The regions of a mountain experiencing the different types of precipitation is complicated by the fact that the air is cooled by melting snow and this can cause the 0ºC isotherm to bend down towards the mountain surface. Furthermore, dynamic effects operating in concert with thermodynamic and microphysical processes can conspire to produce the maximum amount of precipitation on the mountainside to occur within the rain-snow boundary.

The objective of this study is to better understand the transition of snow to rain (or vice versa) within the varying, and interacting, environmental conditions experienced over the mountainous terrain of, for instance, western North America. Using a cloud model coupled with a microphysics including detailed melting of snowflakes, the evolution of the precipitation types formed along a mountainside rain-snow boundary is investigated. Precipitation within the boundary is systematically arranged within four categories of wet snow; wet snow and slush; wet snow, slush and rain; slush and rain. The spatial extent and fractional mass content of the various categories vary as the boundary evolves due to the cooling process of melting. Particular attention will be paid to understanding the processes governing the lowest level on the mountain experiencing the various types of precipitation and their combinations. Attention will also be paid to the relation of the boundary to low visibility. This arises due to the precipitation as well as to fog that can be induced as the air in the boundary is cooled towards 0ºC by the melting snow.