Large-eddy simulations (LES) of neutrally and stably stratified turbulent flows over urban-type surfaces with relatively low plan area ratios are presented. Numerical experiments were performed for different shapes of streamlined objects and at different static stability. A new method for setting up a numerical experiment aimed at studying the heat and momentum transfer within the roughness layer and investigating the thermal and dynamic interaction between the turbulent flow and the surface as a whole has been proposed. This method enables us to obtain an equilibrium state for values of parameters determining the characteristics of the external turbulent flow chosen beforehand. A strong dependence of the thermal roughness length on stratification was found. We also discuss the physical mechanisms that lead to the maintenance of turbulence above the canopy when the ground surface is strongly cooled.
Using LES, we identify a mechanism that contributes to the maintenance of turbulence in the atmospheric boundary layer under the condition of strong surface cooling. Although these results are obtained for an urban canopy, we believe that the qualitative conclusions should be general for a wide type of surfaces with large-scale roughness elements. We hope that the new results will be useful for improving surface flux schemes in NWP and climate atmospheric models that suffer from attenuated mixing in a very stable boundary layer and the effect of “surface decoupling.” The found effect gives a physically justified alternative way to parameterize the air–surface exchange under strong stability compared to the often ad hoc modification of the MOST universal functions.
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