Temporal Oscillations of Low−level Horizontal Winds Induced by Two-dimensional Surface Heat Flux Heterogeneity

Large eddy simulation (LES)-based studies (e.g., Kang et al. 2012; Letzel and Raasch 2003) have suggested that surface heat-flux variations specified by one-dimensional (1-D) sinusoidal functions may induce temporally oscillating horizontal winds at about 100 m height, a typical wind turbine hub height, with no background mean wind. As an extension of previous work, this study investigates whether temporal oscillations still occur in low-level horizontal winds that are induced by surface heterogeneity specified by two-dimensional (2-D) sinusoidal functions, and with non-zero background mean wind. Surface heat-flux variations are 2-D sinusoids with amplitudes of 0, 50, and 200 Wm-2, and wavelengths of 4 and 32 km. A vertically constant background flow of 0, 2, or 5 m s-1 is prescribed. Using two-dimensional Fourier spectral techniques, we investigate the energy cascade from the horizontal scale of the surface heterogeneity to smaller scales and its association with the temporal oscillations, which has been suggested by the previous LES studies with 1-D surface heterogeneity (e.g., Kang et al. 2012; Kang 2009). By applying low-pass, high-pass, and band-pass filters, the simulated low-level horizontal winds are decomposed into the components on the mesoscale forcing scale, the isotropic turbulence scale, and intermediate scales. The characteristics of the low-level wind are investigated for each of these scales. The effects of non-zero background wind on the characteristics are also considered. This is part of an ongoing effort to understand ABLs that include both mesoscale circulations and turbulence, with a primary focus on abrupt changes in low-level wind speed induced by local surface heterogeneity in multiscale ABLs, with progressively more realistic surface and atmospheric conditions.