The imbalance of the surface energy budget in eddy-covariance measurements is still an unsolved problem. A possible cause is the presence of land surface heterogeneity. The influence of surface heterogeneities on the atmospheric boundary layer has been intensively investigated since two decades. Previous studies found that heterogeneities of the boundary-layer scale or larger are most effective in influencing the boundary layer turbulence. Subsequent large-eddy simulations showed that also the turbulent fluxes are influenced by large-scale organized structures in the boundary layer. However, the precise influence of the surface characteristics on the energy imbalance of measurements in the surface layer and its partitioning is still unknown. To investigate the influence of surface variables on all the components of the flux budget under convective conditions, we set up a systematic parameter study by means of large-eddy simulation. For the study we use a virtual control volume approach, which allows the determination of advection by the mean flow, flux-divergence and storage terms of the energy budget at the virtual measurement site, in addition to the standard turbulent flux. We focus on the heterogeneity of the surface fluxes and keep the topography flat. The surface fluxes vary locally in intensity and these patches have different length scales. Intensity and length scales can vary for the two horizontal dimensions but follow an idealized chessboard pattern. Our main focus lies on heterogeneities of length scales of the kilometer scale, and length scales of one order of magnitude smaller. For heterogeneities of these two types, we investigate the average response of the fluxes at a number of virtual towers, when varying the heterogeneity length within the length scale and when varying the contrast between the different patches. For each simulation, virtual measurement towers were positioned at functionally different positions (e.g. downdraft region, updraft region, at border between domains, etc.). Furthermore, we seek correlators for the energy balance ratio and the energy residual in the simulations. Besides the expected correlation with measurable atmospheric quantities such as the friction velocity, boundary-layer depth and temperature and moisture gradients, we have also found an unexpected correlation with the temperature difference between sonic temperature and surface temperature.
De Roo, F., & Mauder, M. (2017). The influence of idealized surface heterogeneity on virtual turbulent flux measurements. Atmospheric Chemistry and Physics Discussions, 1–24. https://doi.org/10.5194/acp-2017-498