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

To measure wind in the atmospheric boundary layer, Doppler lidar has proven to be a reliable tool. Deploying two Doppler lidar systems in a synchronized way enables the measurement of the two-dimensional wind field in a plane with a spatial resolution of several ten meters and a temporal resolution of about 10 to 15 seconds. In the presentation, we will show results from dual Doppler lidar measurements which were performed during the HD(CP)2 Observational Prototype Experiment (HOPE) in spring 2013 in Jülich, Germany. The obtained horizontal wind fields show different forms of self-organization: During situations with considerable background wind, bands of several hundreds meters lengths and 100 to 500 m widths of enhanced and reduced wind velocity, so-called low speed streaks, can be observed. During calm situations, narrow belts of convergence zones develop. These could join to rings similar to open cell convection. To characterize the observed wind field quantitatively, a two-dimensional integral length scale analysis was performed. The length scales in stream-wise direction and in cross-stream direction 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, the eddies are on average quite isotropic. Whereas during unstable conditions a considerable increase in anisotropy from one for calm situations to two to three during situations with background wind of 8 to 10 m per s can be observed.