Turbulent Structures and Coherence in the Surface Layer as Observed by Dual Doppler Lidar Measurements

Laser-based remote sensing of wind (Doppler lidars) in the atmospheric boundary layer has become a common technique in research, but also in applied areas like e.g. wind industry and air traffic safety. A recent trend is the deployment of two or more Doppler lidar systems in a complementary and synchronized way. In the presentation, we will show results from coplanar dual Doppler lidar measurements during the HOPE field experiment in spring 2013 in Jülich, Germany. The measurement setup was conducted in a way that a temporal resolution of about 12 sec and a spatial resolution of less than 100 m was obtained. The horizontal wind fields show impressive forms of self-organisation: During situations with considerable background wind, low speed streaks i.e. bands of several hundreds meters lengths and 100 to 500 m widths of enhanced and reduced wind velocity can be observed. During calm situations, narrow belts of convergence zones develop, which could join to rings, similar to open cell convection. A two-dimensional integral length scale analysis shows the effects due to stability and background wind on the coherence and the isotropy of the wind fields. The length scales are in general about 200 m shorter during unstable situations compared to situations with a stable stratified surface layer. During stable and near neutral situations, a major part of the eddies are isotropic. Whereas during unstable conditions a considerable increase in anisotropy from one for calm situations and up to three during situations with background wind of 8 to 10 m s^-1 can be observed.