Alpine planetary boundary layer

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,[1] fire weather and local intense thunderstorm events.

[2] [3][4] The PBL in complex terrain is shaped by three local (non synoptic) wind systems occurring at different scales, which are closely related to the structure of the topography.

This represents the idealized situation since many complications can arise from cross currents, forced or pressure driven channeling or even cold fronts approaching the mountain barrier.

The temperature increases from the bottom of the valley to the ridge top and then starts decreasing only when the air parcel is free from the influence of the topography.

[6] The PBL starts rising on the east facing slopes and near the ridges (warmed up by sun first and not hindered by pockets of cold air accumulated down the valley over night) and becomes more spatially homogeneous during the afternoon.

Winds then blow up from the plains to the mountain top, which is an efficient lifting mechanism to carry PBL pollutants into the free atmosphere.

Bare or rocky soils are not the only land cover types found in high elevation, a more complex combination of snow and/or ice and/or vegetation is often observed.

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.

The top of the canopy has a tendency to warm up faster than the air at the bottom of the valley creating upslope wind conditions just from the presence of vegetation.