Parameterization of turbulent viscosity over orography

Turbulent viscosity is an important parameter in numerical flow models that controls the vertical turbulent exchange. Vertical and temporal variations of turbulent viscosity have been determined from SODAR data taken over the Black Forest and the Alpine foreland in southern Germany in summer 2002. The turbulent viscosity - an analogon to the molecular viscosity - has been computed from the ratio between the variance of the vertical velocity and the mean shear of the horizontal wind. Turbulent viscosity shows a clear diurnal course with daytime peaks up to 1000 m(2)/s. An orographical influence on this quantity becomes visible from the presented data. A first step towards a parameterization of turbulent viscosity with mountain-specific length and velocity scales is shown.