Convective Boundary Layer - Overview Of Its Role In Mountainous Terrain
Mountainous topography significantly impacts the Earth's atmosphere, influencing atmospheric transport and mixing at several temporal and geographical scales.
The height of the atmospheric boundary layer determines the vertical scale of this transport and mixing, making it a significant parameter in air pollution research, weather forecasting, climate modeling, and many other uses.
When compared to level terrain, the spatiotemporal structure of the daytime convective boundary layer height is dramatically changed and more complicated in hilly and mountainous terrain.
While turbulent convection dominates the convective boundary layer over flat terrain, advection from multi-scale thermally induced flows plays an essential role in the convective boundary layer development across hilly terrain.
However, detailed observations of the convective boundary layer structure and comprehension of the underlying mechanisms remain restricted.
COPYRIGHT_SZ: Published on https://stationzilla.com/convective-boundary-layer/ by Alexander McCaslin on 2022-08-04T18:57:31.264Z
Convective boundary layers in lowlands and basins, where hazardous pollutant deposition is of particular concern, are better understood than convective boundary layers on slopes, ridges, or mountain summits.
Determining Convective Boundary Layer Heights
There is frequently a rise in potential temperature and wind speed near the top of the convective boundary layer, as well as a significant fall in humidity and pollution concentration.
A stable layer is one in which the potential temperature rises with height. A layer of this kind might be surface-based or raised.
Convective boundary layer heights have traditionally been calculated using vertical temperature profiles. Alternatives have been supplied by remote sensors such as sodars and lidars.
Lidars detect vertical aerosol distribution, whereas sodars detect temperature variations (turbulence), winds, and wind shear.
Sharp vertical fluctuations in turbulence and temperature stratification generate a higher maximum in the return signal from sodar emitted acoustic waves.
A vertical gradient further distinguishes the top of the convective boundary layer in aerosol concentrations, which may be detected by a sudden shift in lidar-emitted laser pulses.
Convective Boundary Layer Height In Mountainous Terrain
Flows interacting with convective boundary layer heights across hilly and mountainous terrain are often thermally driven and occur at various geographical and temporal dimensions.
The thermal structure of the convective boundary layer in hilly terrain is the subject of this study, although the accompanying flow structure is crucial for the inquiry.
Many of the observational studies analyzed did not use vertical soundings of turbulence parameters.
The base of the raised stable layer is used as a frequently available criterion for convective boundary layer height detection in the conceptual diagrams of convective boundary layer top behavior displayed for different terrain situations.
Convective Boundary Layer In Slope
The emergence of thermally driven upslope winds is the most noticeable aspect of the convective boundary layer over slopes during good daytime weather.
Near-surface airflow over slopes is often down-valley (in draining valleys) just before dawn, although stagnant conditions may also occur (in pooling valleys or enclosed basins).
Downslope flows usually weaken over the night and are generally invisible soon before morning. An upslope wind layer forms over insolated slopes after daybreak, with a return circulation toward the valley center above.
Advection of cold air counteracts heating caused by turbulent and radiative heat flux convergences on the slopes.
This negative feedback process reduces the strength of daytime slope wind circulation and may explain non-stationary flow behavior.
Slope flows become less well-defined in the afternoon and are often overridden by up-valley or regional winds owing to downward turbulent currents.
Convective Boundary Layer In Valley
Convective boundary layer development is comparable in basins, valleys, and plateaus. Basins are concave landforms with few or no low-lying exits to the surrounding landscape.
In the basin, cold air drainage from the surrounding hills collects and stagnates. Late-afternoon cold air causes shallower convective boundary layers across both basins and plateaus.
No plain-to-basin winds occur when the convective boundary layer height over the plains is lower than the mountain height.
Air movement from the plains to the basin atmosphere is still possible. Surface heating over plateaus produces a relatively homogeneous convective boundary layer spanning the raised flat ground.
Mesas and buttes may be too tiny to establish their convective boundary layers. There have been few observational studies of convective boundary layer development on well-defined plateaus.
A heat low over the Tibetan Plateau is connected with Plateau convective boundary layers and plain-to-plateau circulation.
The interplay of low-temperature heat and thermal circulation is critical for forming moist convection.
Turbulence over the plateaus may damage upslope winds coming on the plateau throughout the day.
Convective Boundary Layer In Basin And Plateau
Convective boundary layer development resembles that of basins, valleys, and plateaus. Basins are concave landforms with no or few low-lying exits to the surrounding landscape.
The basin collects and stagnates cold air drainage from the surrounding mountains. Late afternoon cold air causes shallower convective boundary layers across basins and plateaus.
When the convective boundary layer height over the plains is lower than the mountain height, plain-to-basin winds do not occur.
Air may still be transferred from the plains to the atmosphere of the basin. Surface heating over plateaus produces a relatively homogeneous convective boundary layer that spreads throughout the raised flat ground.
Mesas and buttes, plateau-like topographic features, may be too tiny to create their convective boundary layers.
There have been few observational studies of convective boundary layer development across well-defined plateaus.
A heat low over the Tibetan Plateau is connected with plateau convective boundary layers and plain-to-plateau circulation.
The interplay of heat low and thermal circulation is critical for forming moist convection. Upslope winds coming on the plateau throughout the day may be dissipated by turbulence over the plateaus.
Convective Boundary Layer In Mountain Range
While numerous efforts have been made to define the convective boundary layer height spanning valleys, slopes, basins, and plateaus, many observations demonstrate the integrative influence of the many mechanisms at work.
The ensuing convective boundary layer behavior over a mountain range might vary significantly in space and time.
Convective boundary layer heights were primarily calculated using in-situ aircraft and radiosonde observations collected during intense measurement programs.
Upslope wind lifting may amplify this bulging convective boundary layer height behavior.
Other findings imply that the convective boundary layer across surrounding areas is thinner.
This is because the orography is flat and does not follow the landscape.
The Bernoulli effect may explain a drop in convective boundary layer height, which causes the winds over the mountain top to accelerate up.
Various spatial convective boundary layer top behaviors have been found, which may be classified into four main patterns.
Atmospheric stability, synoptic wind speed, and vertical and horizontal orographic scales are discovered to be relevant.
Advection, entrainment, and friction cause the convective boundary layer to follow the topography, although gravitational forces tend to level it.
The effect of advection in making the convective boundary layer height more or less terrain following is unclear, although its potential significance is acknowledged.
Convective Boundary Layer And The Free Troposphere
The diurnal boundary layer cycle over flat terrain transports air pollutants vertically into the atmosphere in two ways.
During the day, pollutants are mixed upward by convection via a deepening convective boundary layer and entrained downward into the convective boundary layer at night.
Aerosols are a subgroup of air pollutants, although their physical transport and mixing mechanisms are the same as other air pollutants.
While the aerosol and convective boundary layer heights are almost identical over level ground, mountainous topography significantly impacts aerosol dispersal in the atmosphere.
Mixed transport mechanisms often intensify when opposed to flat and horizontally uniform terrain.
People Also Ask
Why Is Boundary Layer Important For The Analysis Of Convection Transfer?
If an insulating layer of enough thickness and low temperature is present on exterior surfaces, heat transmission is mainly driven by convection, and the effects of radiation may be neglected.
As a result, calculating heat transport in the convective boundary layer is critical.
What Is The Stable Boundary Layer?
A cool layer of air next to a cold surface of the earth, with temperature stratification within that layer.
Why We Use Convective Boundary Conditions?
A linear heat transfer model is used between the boundary entities and the external environment with the convective heat flux boundary condition.
This helps simulate general heat losses or gains caused by natural/forced convection or conduction between bodies of generally constant temperature.
What Is Convective Boundary Layer Height?
This layer spans from the earth's surface to a capping inversion, usually occurring at the height of 1-2 km over land by midafternoon.
What Is Boundary Layer In Convection?
A boundary layer is a thin layer of fluid created by the fluid flowing along the surface of a bounding surface in physics and fluid dynamics.
The interaction of the fluid with the wall results in a no-slip boundary condition (zero velocity at the border).
Conclusion
Observations made in situ and remote sensing are critical for expanding our understanding of boundary layers in hilly terrain.
Remote sensors like lidars and sodars have been beneficial in determining the multilayered structure of the alpine atmosphere.
According to observational and numerical data, aerosol layer heights generated from lidar typically surpass convective boundary layer heights across hilly terrain.
Complex terrain wind systems and convective boundary layer development are inextricably linked. In the late afternoon and evening transition phase, convective boundary layers become more level.
Some first ways of quantifying convective boundary layer height across hilly terrain have been established, but they need to be tested and extended.
Historically, daytime thermally driven flows have received less attention than their midnight counterparts.
Many potentially significant consequences of upslope flow on convective boundary layer behavior are yet unknown. The combined influence of land cover and orography on heights is similarly poorly understood.
In hilly terrain, convective boundary layer behavior is directly related to vertical transport and mixing processes.
Thermally-induced flows through steep terrain may contribute to the passage of pollutants over the convective boundary layer top.
The layer impacted by mountain-induced circulations and venting processes is also referred to as the mountain convective boundary layer.
