Abstract
The nocturnal atmospheric boundary layer (ABL) poses several challenges to standardturbulenceanddispersionmodels,sincethestablestratificationimposedbytheradi- ative cooling of the ground modifies the flow turbulence in ways that are not yet completely understood. In the present work we perform direct numerical simulation of a turbulent open channel flow with a constant (cooling) heat flux imposed at the ground. This configuration provides a very simplified model for the surface layer at night. As a result of the ground cool- ing, the Reynolds stresses and the turbulent fluctuations near the ground re-adjust on times of the order of L/uτ ,w hereL is the Obukhov length scale and uτ is the friction velocity. For relatively weak cooling turbulence survives, but when ReL = Luτ /ν 100 turbulence collapses, a situation that is also observed in the ABL. This criterion, which can be locally measured in the field, is justified in terms of the scale separation between the largest and smallest structures of the dynamic sublayer.
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