Observed spatial variability of boundary-layer turbulence over flat, heterogeneous terrain

In spring 2013, extensive measurements with multiple Doppler lidar systems were performed. The instruments were arranged in a triangle with edge lengths of about 3 km in a moderately flat, agriculturally used terrain. For six mostly cloud-free convective days, vertical velocity variance profiles were compared for the three locations. On the average over all considered cases, differences between variances at different sites were about three times higher than between those derived from measurements by different lidars at the same site. For all investigated averaging periods between 10 min and 4 h, the differences were not significant on the average when considering the statistical error. However, statistically significant spatial differences were found in several individual cases. These could not be explained by the existing surface heterogeneity.

In some cases, nearby energy balance stations provided surface fluxes that were not suitable for scaling the variance profiles. Weighted-averaged values proved to be more applicable, but even then, the scaled profiles showed a large scatter for each location. Therefore, it must be assumed that the intensity of turbulence is not always well-determined by the local heat supply at the Earth's surface. ... mehrInstead, a certain dependency of turbulence characteristics on mean wind speed and direction was found: thermals were detected that travelled from one site to the other with the mean wind when the travel time was shorter than the large-eddy turnover time. At the same time, no thermals passed for more than two hours at a third site that was located perpendicular to the mean wind direction in relation to the first two sites. Subsidence prevailing in the surroundings of thermals advected with the mean wind can thus partly explain significant spatial variance differences existing for several hours.