Structure parameters over heterogeneous terrain – a highresolution LES case study for the LITFASS2003 experiment

We conduct a high-resolution (grid spacing of a few meters) large-eddy simulation (LES) case study in order to investigate the effect of surface heterogeneity on the structure parameters of potential temperature CT² and specific humidity Cq² in the convective boundary layer (CBL). The kilometre-scale heterogeneous land use distribution as observed during the LITFASS-2003 experiment is prescribed at the surface of the LES model in order to simulate a realistic CBL development from the early morning until the early afternoon. The surface patches are irregularly distributed and represent different land-use types that exhibit different roughness conditions as well as near-surface fluxes of sensible and latent heat. These were obtained from eddy-covariance measurements during the LITFASS-2003 experiment.

Particular attention is given to the Monin-Obukhov similarity theory (MOST) relationships and local free convection (LFC) scaling in the surface layer for structure parameters, relating CT² and Cq² to the surface fluxes of sensible and latent heat, respectively. Moreover we study possible effects of surface heterogeneity on scintillometer measurements that are usually performed in the surface layer.

The LES data show that signals of the surface heterogeneity are still present in the structure parameters up to height levels of 100-200 m. The assumption of a blending height, i.e. the height above the surface where signals of the surface heterogeneity vanish, that is required by MOST is studied. We show that no such blending height at typical height levels of scintillometer measurements exists for the studied case. Moreover, it is found that Cq² does not follow MOST which is ascribed to entrainment of dry air at the top of the boundary layer. Nevertheless, it is found that the application of MOST and LFC for CT² still gives reliable estimates of the surface flux of sensible heat. It is shown, however, that this flux, derived from scintillometer data, will be more representative for the local footprint area of the scintillometer rather than for an area of several square kilometres.