Turbulence Characteristics of Different Flow Regimes in Complex Terrain

Orography presents a significant forcing to the flow over it. As the orographic features span different scales, thus also the response in the atmosphere is found at all scales of motions, extending down to turbulence. Thus the complexity of the atmospheric response increases with increasing terrain complexity. This complexity is also visible near the surface where it impacts the exchange processes within the boundary layer. Motions in the boundary layer are thus disturbed by mesoscale influences unknown in flat terrain. In situations with low synoptic forcing valley/slope wind systems are the dominant mesoscale driver, while under strong synoptic forcing downslope windstorms can develop and disturb the boundary layer. Since some of these motions are still smaller than (or comparable to) the resolution of most operational numerical models, they need to be parameterized and therefore their interaction with turbulence needs to be well understood.

Here we use a multi-year dataset from mountainous terrain to investigate different scales on which orography impacts boundary layer characteristics. The scales of inhomogeneity are significantly dominated by the type of forcing (e.g., radiatively driven downslope flow or dynamically driven foehn winds), which in turn is strongly impacted by the orography. For radiatively driven downslope flow for example it is found that local shear is the dominant generator of turbulence and flow is relatively homogeneous along a 200m slope, whereas for foehn winds the turbulence response is very inhomogeneous even at local scales of several tens of meters. On the contrary the near-surface response (e.g., the non-dimensional wind speed variance in the lowest meters above the surface) is found to correspond better to ideal-terrain behavior under foehn conditions than for pure downslope flow.