Optimized Measurement Techniques for Dual-Doppler Lidar Coherent Structure Detection in the Surface Layer

Combining two Doppler lidars in a dual-Doppler system offers the unique opportunity to measure the atmospheric wind field on a plane of several square kilometers. This enables detailed insights into the behavior of boundary layer turbulence, for example the development and decay of patterns in the wind field, so-called coherent structures. However, the limited time resolution due to scanning speeds of about five to ten degrees per second, and the limited spatial resolution due to inherent Doppler lidar averaging effects, lead to measured wind fields which appear strongly smoothed. Consequently, the length scales of coherent structure are severely overestimated.

Based on virtual dual-Doppler lidar measurements in high-resolution LES, this presentation discusses the applicability and limits of dual-lidar scan techniques and structure detection methods. We will introduce four different techniques to capture characteristics of coherent structures: an integral length scale approach, an approach using wavelet techniques, a cluster algorithm as well as a technique based on the Finite Time Lyapunov Exponent. The differences between results from high resolution LES output and smoothed lidar output will be illustrated and discussed theoretically. It will be shown how additional high-frequency wind measurements at a meteorological tower can be used to correct the lidar-measured length scales.