Variations that occur during the day in the planetary boundary layer are called diurnal variation.
The following phenomena affect the formation of the component layers:
- Solar radiation heats the Earth's surface, causing thermals of warm air to rise from the ground.
- Radiative cooling from the clouds at the top of the planetary boundary layer creates thermals of cool air that sink down.
- Wind shear across the top of the convective boundary layer contributes to the generation of turbulence.
During the day, diurnal variation has the following effects on the planetary boundary layer:
- The turbulence in the air is caused by solar radiation and radiative cooling, both of which occur simultaneously. At night, the radiative cooling of the surface controls the boundary layer, creating the nocturnal layer. The nocturnal layer blocks the interference between the surface layer and the residual layer. Before sunrise, the nocturnal boundary layer height is the mixing height.
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After sunrise the solar radiation destabilizes the situation on the surface, creating thermals of warm air that rise upwards. The thermals continue to rise until their temperature has dropped to the same temperature as the surrounding air.
At the same time thermals of cool air sink down from the top of the clouds. The resulting turbulence causes heat, moisture, and particles to mix uniformly in the convective boundary layer. The convective boundary layer reaches its maximum mixing height in the late afternoon.
The entrainment zone acts as an interface between the convective boundary layer and free atmosphere. Turbulent air from above is mixed into the convective boundary layer in this region of the atmosphere.
When the convective boundary layer reaches the level of the residual boundary layer, both layers are mixed together. This is an important process for air pollution transportation in time and space. The horizontal dispersion and transportation have a strong influence on the air quality.
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When the sun sets, radiative cooling causes the convective boundary layer to collapse. A new nocturnal boundary layer is formed, and it is again replaced with a new convective boundary layer during the next day.
