A new nocturnal boundary layer model

The nocturnal boundary layer is generally modelled in terms of eddy diffusivities, which are a function of z/h where h is the boundary layer depth. This approach is modified in this study to include turbulence generated by local shear both within and above the nocturnal boundary layer. The locally generated turbulence is formulated in terms of a stability-dependent mixing length and Prandtl number, which are not tied to the existence of a boundary layer. Under certain conditions, the modeled boundary layer is allowed to couple with turbulence generated above the boundary layer. These modifications have been guided by nocturnal data from the BORRIS95, Microfronts and CASES99 field programs.

The modified model produces a more realistic nocturnal jet, generates layers of isolated turbulence and simulates a very stable early evening period, which has been observed in previous studies. The modeled nocturnal boundary layer sometimes engulfs turbulence generated above the boundary layer, which eventually strengthens the turbulence near the surface.