User:Liz.Baldo/Mountain planetary boundary layer

The planetary boundary layer (PBL) behavior over flat terrain is relatively well known and represented. However its behavior over mountainous terrain is more complex due to a fast changing local wind system directly linked to topography. A variable land cover that goes from snow to vegetation also has a significant impact on the growth of the PBL. Understanding the processes inducing changes in the mountain PBL have critical applications for predicting fire weather, local intense thunderstorm events and air pollution transport. Some processes such as mountain waves, which are oscillations produced by winds flowing towards a mountain, are already well studied processes due to their importance for aviation but most of the behavior of the PBL over mountainous terrain remains unknown.

Wind systems
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. The height of the PBL can be observed using LIDAR, which measures the backscatters of the aerosols.

Mountain-Plain winds
This is the largest scale phenomenon going across the mountain range. During daytime, incoming solar radiation heats up the mountain top faster than the plain, creating a mean low pressure zone at the top. Winds are then coming into the mountains from all sides, go up the slopes and meet at the ridge top. A return flow occurs aloft and comes back down into the plains. The exact opposite happens during nighttime, when the top cools down faster than the plain, which creates a mean high pressure zone leading to winds coming from the mountain top down to the plain. This is of course the idealized situation since many complications can arise from cross currents, forced or pressure driven channeling or even cold fronts approaching the mountain barrier.

Valley winds
Valley winds are best developed on clear summer days and are driven by horizontal pressure gradients. During the day, the valley is warmer compared to a flat terrain (because it is a smaller volume of air receiving the same amount of radiation), which creates a lower pressure zone entraining the air up the valley from the plains. The opposite process occurs at nighttime.

Slope winds
Slope-winds are produced by the temperature gradient between the valley and the air layer aloft. During daytime, the air above the valley on the slopes is warmer than at the bottom, which leads to upslope flows converging at the ridge tops (and can lead to cloud formation depending on the humidity of the air parcel). At night, the air above the valley cools down faster than the surface leading to a down-slope motion. This means that a temperature inversion occurs at night. The temperature increases from the bottom of the valley to the ridge top and the starts decreasing only when the air parcel is free from the influence of the topography. Again, this ideal circulation can often vary due to the complex topography. Insulation of the slopes is affected by shade, aspect and sky view factor. For instance, east facing slopes receive radiation earlier in the morning than west facing slopes, which has an impact on how the PBL grows with time and space. Slope winds are necessary for the valley winds to develop because a vertical motion is needed to compensate for the air mass carried up or down by the valley winds.

Diurnal PBL variation due to wind systems
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 the 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 start 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, the general observations are that there is not much residual layer since it is advected off by the mountain synoptic wind system.

Mountain venting
The daytime increase of the PBL due to up-slope winds is called mountain venting. This phenomenon can sometimes cause a vertical exchange of the PBL air into the free troposphere. Similarly to the daytime situation, during summer the top of the mountain is warmer than its surroundings creating a low pressure zone. 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.

Impact of landcover
Bare soil is not the only landcover type found in high elevation, a more complex combination of snow and/or ice and/or vegetation is often observed. On such surfaces, the energy budget is highly temporally and spatially variable and so is the growth of the PBL.

Snow
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 impact. The sublimation of blowing snow leads to a modification of the energy budget. An overall temperature decrease of 0.5C 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).

Vegetation
Low vegetation cover such as grass or shrubs are usually covered by snow during winter time or at very high elevation and the modification in surface roughness is therefore of a limited magnitude. However dense and high forests have a significant impact on surface roughness and also on the energy budget. The turbulences created by the vegetation canopy increases the forced convection in the surface layer. The top of the canopy is also warming up faster than the air at the bottom of the valley, which means that the presence of high vegetation has a tendency to produce upslope winds.

To summarize, the presence of snow creates down-slope winds hindering the growth of the PBL while the presence of forest creates up-slope winds facilitating the growth of the PBL.