Characteristic frequencies of boundary-layer turbulence over flat, heterogeneous terrain under different stratification conditions

In spring 2013, extensive measurements with multiple wind Doppler lidar systems were performed in a moderately flat, agriculturally used terrain in northwestern Germany. The instruments were arranged in a triangle with edge lengths of about 3 km, and were setup on vertical stare mode to retrieve temporally high-resolved vertical velocity measurements. The goal of the experimental deployment was to improve our current understanding of the land heterogeneity effects on the atmospheric boundary layer structure. Within this project we intend to provide new understanding on the relationship between characteristic length scales related to turbulent eddy-sizes and background atmospheric stratification and cloud cover. For this purpose, three different characteristic scenarios have been selected for this study: convective boundary layers with clear skies, cumulus-topped , and stratocumulus-topped boundary layers. Using Proper Orthogonal decomposition (POD), the wind lidar signals are decomposed on an orthogonal set of basis functions. The optimality of this procedure is that the modes conforming the orthogonal basis are ordered with decreasing energy, meaning that the first n modes obtained through the POD analysis contain more energy than any other n first modes of a different orthogonal basis found through another linear decomposition. Through a posterior Fourier transform analysis of the most energetic (relevant) modes present in each wind lidar signal, it is possible to extract the frequency of the characteristic turbulent structures for a given characteristic time period. Upon application of this analysis for each individual it is possible to investigate spatial correlations of the different atmospheric boundary layer turbulent structures. Results will illustrate the existence of different characteristic turbulent length scales for each of the cases considered. For the cumulus cloud day, turbulent eddies can be detected that were related to the cloud sizes, and for the stratocumulus day, an additional frequency occurs which is believed to relate to a Kelvin-Helmholtz instability produced in the entrainment zone of the boundary layer.