Abstract
We conduct a high-resolution large-eddy simulation (LES) case study in order to investigate the effects of surface heterogeneity on the (local) structure parameters of potential temperature \(C_T^2\) and specific humidity \(C_q^2\) in the convective boundary layer (CBL). The kilometre-scale heterogeneous land-use distribution as observed during the LITFASS-2003 experiment was prescribed at the surface of the LES model in order to simulate a realistic CBL development from the early morning until 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. In the analysis, particular attention is given to the Monin–Obukhov similarity theory (MOST) relationships and local free convection (LFC) scaling for structure parameters in the surface layer, relating \(C_T^2\) and \(C_q^2\) 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 the local structure parameters reflect the surface heterogeneity pattern up to heights of 100–200 m. The assumption of a blending height, i.e. the height above the surface where the surface heterogeneity pattern is no longer visible in the structure parameters, is studied by means of a two-dimensional correlation analysis. We show that no such blending height is found at typical heights of scintillometer measurements for the studied case. Moreover, \(C_q^2\) does not follow MOST, which is ascribed to the entrainment of dry air at the top of the boundary layer. The application of MOST and LFC scaling to elevated \(C_T^2\) data still gives reliable estimates of the surface sensible heat flux. We show, however, that this flux, derived from scintillometer data, is only representative of the footprint area of the scintillometer, whose size depends strongly on the synoptic conditions.
Highlights
The turbulent surface fluxes of sensible and latent heat play an important role in the vertical transport of energy and water vapour in the atmospheric boundary layer (ABL)
The vertical profiles of CT2 and local free convection (LFC) predictions according to Eqs. 8–9 are shown in Fig. 6 and reveal AT,LIT2E = 3.0 ± 0.2 and AT,HOM = 2.8 ± 0.2 (95 % confidence interval), which is in agreement with the predictions of Monin–Obukhov similarity theory (MOST) scaling in the free convection limit as shown above
A case study for May 30 2003 of the LITFASS-2003 experiment was conducted using a set of large-eddy simulations (LES) that allow surface-layer turbulence to be resolved
Summary
The turbulent surface fluxes of sensible and latent heat play an important role in the vertical transport of energy and water vapour in the atmospheric boundary layer (ABL). Local point measurements over all relevant land-use types would be necessary to derive area-averaged surface fluxes based on a suitable averaging strategy (Gottschalk et al 1999; Beyrich et al 2006). Maronga and Raasch (2013a) simulated the CBL over the heterogeneous LITFASS-2003 terrain (see Fig. 1a), based on the former LES runs of Uhlenbrock et al (2004), for four selected days during the LITFASS-2003 experiment (see Beyrich and Mengelkamp 2006) They found that secondary circulations developed that were superimposed on the randomly distributed convection, partly taking over the vertical transport of heat and moisture. The structure parameters are for the first time derived from surface-layer turbulence resolving LES over such an irregular surface heterogeneity, and they are compared with LAS measurements performed during the LITFASS-2003 experiment.
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