Turbulence and Local Circulations

Real surfaces are relatively complex compared to better understood idealized simple surfaces.

This sketch indicates a number of possible complications with typical surfaces.
For nocturnal conditions with clear skies and weak large scale flow, cold pools will form in low lying areas (upper sketch).  Small cold pools may be embedded within larger cold pools.  Patches of trees or rougher surfaces, in general, may induce an internal boundary layer where the flow within the internal boundary layer is influenced primarily by the local surface. Flow above the internal boundary layer is influenced primarily by the upwind boundary layer.  Isolated buildings are too local to generate an internal boundary layer but disturb the flow through an induced perturbation pressure field.  Waves in the stable nocturnal boundary layer induced by surface features can lead to instabilities and turbulent mixing.

With computer models, the entire region in the sketch may be represented by one grid box and formulated in the model by effective parameters such as effective roughness lengths.  Downscaling refers to approaches that transform the prediction of atmospheric conditions for a given grid box (horizontal area) down to atmospheric conditions at a point.

The bottom of this sketch focuses on smaller scale variations choosing an individual bush as an example.  If the bush is sufficiently dense,  flow approaching the bush may lift over the top driven by a local pressure perturbation.   If the air between the bushes is colder, as might occur on a clear night with weak winds, the overriding air does not mix down to the surface. If the air is moving very slowly, the perturbation pressure is too weak to deflect the air and it “creeps” through the bush.  If the air is strongly stratified as on a clear night with weak winds, the airflow can no longer lift over the bush and flows around it.  In the daytime with clear skies and very weak winds, individual bushes can induce upward convective plumes, particularly if the evapotranspiration is not too large.

Individual obstacles.  The following photos show flow around a Sodar and flow around a building in very stable conditions.

Natural fog moving from left to right tends to rise up over a group of trees in the center of the video as a thin filament of fog. Forest edges perturbed the flow through modification of the local pressure field in which case the forest acts analogously to an obstacle, albeit a porous one.