Ground‐layer snow clouds

Abstract

Abstract Snow clouds forming directly over the ground and reaching depths of up to a kilometre have been observed with a W‐band airborne radar. The phenomenon is referred to here as ‘ground‐layer snow cloud’ (GSC). Most of the cases were observed over mountainous terrain but some were seen over nearly flat terrain. These snow clouds occurred over snow‐covered ground. The temperature at their point of first appearance on rising slopes was in the range 0 to −10° C with humidity close to water saturation. GSCs form within windy and highly turbulent surface layers, with specific features linked to sudden rises or dips in terrain. The most plausible explanation for the formation of the GSC is that snow particles lofted from the surface grow in the ice‐supersaturated environment and mix throughout the turbulent layer. Sufficiently high humidity for crystal growth distinguishes these clouds from blowing snow in dry air. On the downwind sides of mountains, the snow clouds precipitate and evaporate. As for blowing snow, visibility within GSCs is greatly reduced. The GSC represents a form of horizontal water mass transport. When a GSC forms with other clouds present above it, as is frequently the case, there is a clear potential for those clouds to be seeded from below, thereby altering their evolution, precipitation distribution, and other impacts. Copyright © 2012 Royal Meteorological Society