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Leslie M. Hechtel, Roland B. Stull, and Chin-Hoh Moeng

Abstract

Most land surfaces are quasi-randomly nonhomogeneous, yet most boundary-layer studies assume homogeneous or simply varying surface conditions. In this study, nonhomogeneous surface fluxes of realistic scale and amplitude are applied to a large-eddy-simulation (LES) model. The boundary conditions are representative of conditions observed near Chickasha, Oklahoma during the Boundary Layer Experiment 1983 (BLX83), while the model is a modified version of Moeng's LES model.

The afternoon of 28 May 1983, which had light winds and cloud-free skies, is simulated by using data collected during BLX83 to provide initial and boundary conditions; the simulation verifies fairly well against observations. A second simulation of the case-study afternoon uses horizontally homogeneous surface fluxes. The results of the two runs are compared to see what effect the quasi-random nonhomogeneous conditions have on mixed-layer development.

The inclusion of realistic size (450–900 m) and amplitude (∼2 (°C)2 variance in surface temperature) non-homogeneities in a model of this resolution (50 m in the vertical, 150 m in the horizontal) does not in this case change the development of the mixed layer. The two runs show no significant differences in area-averaged statistics, possibly because of the nonzero wind which was present during the simulated afternoon. In the nonhomogeneous case, there is no evidence that thermals are anchored to or are preferentially forming over certain surface features, and there is no change in the thermal structure as evidenced by the power spectra of temperature, moisture, or vertical velocity. Until our understanding of the physical processes involved in ground-to-atmosphere heat and moisture transfer improve, and until LES models are made more sensitive to such transfer, it appears that the use of horizontally homogeneous bottom boundary conditions is sufficient to adequately simulate the development of the boundary layer during combined free and forced convection.

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