Effects of Mesoscale Surface Thermal Heterogeneity on Low-Level Horizontal Wind Speeds

Using large-eddy simulation, we investigate characteristics of horizontal wind speed at 100 m above the ground, with surface heat-flux variations that are sinusoidal with amplitudes of 0, 50, and 200 W m ^-2 and wavelengths of 16, 32, and 128 km, and no background flow. When the amplitude is 200 W m ^-2, wind speeds induced by the surface-flux variations on scales of 16 and/or 32 km have multiple temporal oscillations from 0600 to 1800 local standard time. The positive peaks first appear before noon. In contrast, for wind speeds induced by the 128-km surface heterogeneity, a single oscillation occurs in the late afternoon, which is much larger than those generated by the 16- and 32-km surface heterogeneity. In addition, at the oscillation onset the kurtosis of the velocity increment over a distance of 1 km significantly increases, which implies intermittency in the generation of 1-km scale eddies. The spatially intermittent energy cascade generated by surface heterogeneity scaled down to 1-km eddies is analogous to the well-known intermittent energy cascade in the inertial subrange. The kurtosis of the 1-km eddies is much larger with the 128-km surface heterogeneity than with the 16- and 32-km heterogeneities. Thus we conclude that localized rapid changes of low-level horizontal wind speed may be caused by significant local surface heterogeneity on scales between a few tens and a few hundreds of kilometres.