Alignment of stress, mean wind, and vertical gradient of the velocity vector

In many applications in the atmospheric surface layer the turbulent-viscosity hypothesis is applied, i.e. the stress vector can be described through the vertical gradient of velocity. In the atmospheric surface layer, where the Coriolis force and baroclinic effects are considered negligible, this is supposedly a good approximation. High resolution large-eddy simulation (LES) data show that it is indeed
the case. Through analysis of WindCube lidar measurements accompanied by sonic measurements we show that this is, on the other hand, rarely the case in the real atmosphere. This might indicate that large scale mechanisms play an important role in the misalignment observed in the atmosphere. Baroclinicity is one candidate of a such, instationarity another. In this contribution we will present ongoing work: data from both a WindCube lidar, sonic anemometers and LES and discuss the results in the context of atmospheric boundary layer modeling. The measurements are from the Danish wind turbine
test sites at Høvsøre. With theWindCube lidar we are able to reach heights of 250 meters and hence capture the entire atmospheric surface layer both in terms of wind speed and the direction of the mean stress vector.