The Turbulence in the SBL
Turbulence in the SBL is driven by mechanical shear aloft and is usually localized and intermittent. Mechanical shear is caused by a change of wind speed with height. Another source of shear is from surface friction with terrain. Compared to the daytime convective boundary layer, turbulence in the SBL is weak. The weak, intermittent turbulence leads to higher concentrations of pollutants in the SBL.
The stable boundary layer is often divided into two subtypes: weakly stable and very stable boundary layer [2]. The following table summarizes the key differences:
The following video explains the difference of turbulence in a very stable and weakly stable boundary layer.
During the day, pollutants can be mixed more uniformly due to convective turbulence. At night, mixing is more limited due to the vertical structure of the SBL. Pollutants emitted near the surface, are dispersed little due to the weak mixing. Burst of turbulence can enhance vertical mixing. [2]
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