Significance of mesoscale surface heterogeneity in wind speed forecasting

Using large eddy simulation (LES), we investigate characteristics of horizontal wind speeds at the height of 100 m above the ground level, a typical wind-turbine hub height, over surface heterogeneity on scales between a few 10s and a few 100s of kilometers under a zero background wind condition. First we confirmed that thermally-induced mesoscale winds may trigger temporal fluctuations (up-and-down events of wind speed). Within the daytime diurnal cycle between 6 and 18 h local standard time (LST), the horizontal wind speeds induced by the specified 16- and 32-km scale surface heterogeneity have multiple up-and-down events and the peaks of the up-and-down events first appear before noon. In contrast, the wind speeds induced by the 128-km surface heterogeneity have only one up-and-down event and its peak appears in the late afternoon. However, the amplitude of the up-and-down event of wind speed induced by the 128-km surface heterogeneity is much larger than those of the 16- and 32-km surface heterogeneity. In addition, the kurtosis obtained from the probability density function (PDF) of the velocity increment demonstrates that spatial intermittency of turbulence rapidly and significantly increases at the peaks of the up-and-down events, which is likely associated with an intermittent energy cascade. The increase of turbulence intermittency is much more significant in horizontal wind speeds induced by the 128-km surface heterogeneity than by the 16- and 32- km heterogeneity. In conclusion, we suggest that some extreme and rapid changes in wind power output may be associated with significant, local surface heterogeneity in particular on scales between a few 10s and a few 100s of kilometers.