Effects of Mesoscale Surface Thermal Heterogeneity on Low-Level Horizontal Wind Speeds |
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| Authors: | Song-Lak Kang Donald Lenschow Peter Sullivan |
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| Institution: | (1) Department of Biological and Agricultural Engineering, The University of Georgia, 1109 Experiment st., Griffin, GA 30223, USA;(2) Lab for Environmental Physics, The University of Georgia, 1109 Experiment st., Griffin, GA 30223, USA;(3) Department of Geography, University of Florida, Gainsville, FL 32611, USA |
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| Abstract: | 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. |
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