Large–Eddy Simulation of Turbulent Flux Patterns over Oasis Surface

Abstract

Abstract. Large-eddy simulation (LES) is widely used for studying surface fluxes over heterogeneous areas, but the parameterizations based on the Monin-Obukhov Similarity Theory (MOST) often used violate the basic assumptions of the very theory and generate inconsistencies with the LES model closures. Experience shows that LES simulated surface fluxes are strongly dependent on model resolution. Here, we propose a novel scheme for turbulent flux estimates in LES models, which computes the fluxes locally using the LES sub-grid closure scheme (here a k-l scheme) and constrained on the macroscopic scale using the MOST. In comparison with several other schemes, the new scheme performs better for various types of land surfaces tested. The surface fluxes estimated with the new scheme are compared with the field measurements over an oasis surface at different height levels. Other quantities related to surface energy balance, including net radiation, ground heat flux and surface skin temperature, simulated using the new scheme are also found to be consistent with the measurements. Sensitivity tests show that the uncertainty of MOST flux vary little with horizontal resolution, while the macroscopic constraint of MOST increases with horizontal resolution.