Negative shear gusts in complex terrain

We observe violent and wind-turbine damaging gusts from two meteorological masts erected at a wind turbine park on a steep and complex ridge in Southern Europe. The masts are equipped with cups, vanes, sonics, and temperature sensors. Gusts with very fast rise times of the order of a fraction of a second, which cause problems for the turbines control systems, are observed with a much higher probability than in flat terrain. Also, so-called negative shear gusts, which again are very problematic for the wind turbine, are very powerful. The strongest of these gusts develop over of the order of a second where the wind speed at hub height (approximately 48 meters) decreases to 5 m/s while the wind speed measured at the lower tip of the turbine (at 18 meter above the ground) increases to 25 m/s.

Modeling of turbulence and gusts in very complex terrain has not developed to a stage where we can predict which particular sites are prone to such gusts. Linear flow models predict mean shears on the top of ridges and other good wind turbine sites reasonably well. However, the average flow is completely insufficient to model the loads on the wind turbine. We show that rapid distortion theory, which works well in moderately complex terrain with not too steep slopes, fails in predicting the structure of turbulence at our site.

We give tentative empirical rules for the occurrence of large negative shear gusts.