page_title: Alpine planetary boundary layer
section_title: Snow
hierarchical_section_title: Alpine planetary boundary layer / Effect of landcover / Snow
caption_attribution_description: English: Snow sublimates and creates a cold layer directly above the surface, which leads to winds blowing down-slope.
context_page_description: The alpine planetary boundary layer is the planetary boundary layer associated with mountainous regions. Due to its high spatial and temporal variability, its behavior is more complex than over a flat terrain. The fast changing local wind system directly linked to topography and the variable land cover that goes from snow to vegetation have a significant effect on the growth of the PBL and make it much harder to predict.
  Understanding the processes inducing changes in the mountain PBL have critical applications for predicting air pollution transport, fire weather and local intense thunderstorm events. While some processes, such as mountain waves, have been well studied in the mountain PBL due to their importance for aviation, most of the behaviors of the alpine PBL are relatively unknown.
context_section_description: Due to wind, fresh snow does not stay at the surface but is blown into the first few meters of the surface layer. This blowing snow usually sublimates due to the insulation and has a significant meteorological effect . The sublimation of blowing snow leads to a modification of the energy budget and an overall temperature decrease of 0.5 °C combined to an increase of water vapor has been observed. This forms a stable cold and moist layer of air above the snow surface, even if the surrounding air temperature is above freezing point.
  This cold layer induces downslope winds, which  damper the growth of the PBL.
  This surface winds are also drifting the snowpack, leading to an increase of surface roughness and therefore an increase of wind shear (Forced convection).
caption: Snow sublimates and creates a cold layer directly above the surface, which leads to winds blowing down-slope.
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