The influence of two-dimensional surface heterogeneities on the structure and development of the atmospheric boundary layer
Responsible: Marcus Herold
Project type: Diplomarbeit (equivalent to master thesis)
Duration: 01/08/2001 - 23/08/2002

Heterogeneities of the surface vertical heat flux may induce mesoscale circulations. This has been confirmed by recent observations and numerical studies. In the studies of this diploma thesis a parallelized large-eddy simulation model has been used to examine the influences of two-dimensional continuous and discontinuous inhomogeneities arranged in a chessboard-type pattern. Previous numerical studies revealed strong influences of a number of parameters on the strength and type of the secondary circulations. The most important are perturbation wavelength, perturbation amplitude, shape of the inhomogeneity, wind speed and wind direction. Inhomogeneities with a horizontal extent of the boundary layer height or larger were found to have the largest effects on the mean thermal and dynamical properties of the boundary layer, such as vertical temperature profiles and horizontal diffusion. Thus, the purpose of this study was to examine the effects of heterogeneities at scales of tens of kilometers, the scales of which the strongest effects had been reported.

The simulation results revealed some significant differences to previous studies on one-dimensional heterogeneities of the same size and two-dimensional perturbations of the size of the boundary layer height (and smaller). For example, the development of secondary circulations was found to be strongly dependent on the shape of the inhomogenity. The temporal oscillations found in studies on one-dimensional inhomogeneities could not be observed in runs with discontinuous perturbations of this study. The two-dimensionality also triggers more complex circulation patterns caused by near-surface convergence lines over the areas of high heat flux values and corresponding divergences at boundary layer height. Due to the nature of the velocity field in the circulation, increased horizontal diffusion and velocity variances can be observed in the lower and upper part of the boundary layer, while their values in the middle remain almost unchanged compared to cases with uniform surface heating. (See also the studies by Schröter and Uhlenbrock.)