page_title: Alpine planetary boundary layer
section_title: Diurnal variation
hierarchical_section_title: Alpine planetary boundary layer / PBL growth due to wind systems / Diurnal variation
caption_attribution_description: English: Diurnal variation of the PBL over a mountainous terrain.
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: Overall, plain to mountain, up-valley and up-slope winds occurring during the day  locally increase the height of the PBL.
  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.
  Temporally speaking, the convection ends around the early evening. Clouds then starts dissipating and the Mountain-Plain circulation starts reversing into a sinking motion. The transition builds up from the surface and becomes deeper and deeper with time.
  The morning transition is slightly different and is the result of the combination of both the growth of the PBL and the sinking of the nocturnal temperature inversion.
  At nighttime, there is a limited residual layer since advection by the mountain synoptic wind system dominates.
caption: Diurnal variation of the PBL over a mountainous terrain.
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