Wind gusts are short-lasting (3 to 20 seconds) fortified winds with higher speed than the average wind (~ 10 min). They can occur during a front passage or thunderstorms. In particular, cold downwinds in severe thunderstorms are responsible for the largest wind damage and therefore of great interest in the weather forecast. However, the small-scale and short-lived nature of gusts impedes their explicit dissolution in weather models. Therefore, gusts as a sub-scale phenomenon have to be derived from other, resolved model variables. For this purpose, an exact gust detection and a detailed description of the environmental conditions are necessary. Although gusts occur at higher altitudes, previous measurements are typically limited to heights below 10 meters. Therefore, above 10 meters (for example, some meteorological masts) there are only few measurements available to validate and improve models.
The FESSTVaL campaign aims to measure wind gusts, especially at altitudes above 80 m, and to better understand the processes that lead to them. With the help of Wind lidars (light detection and ranging) the wind vector at different heights can be determined by considering the Doppler effect. Wind lidars are a proven and variably applicable method to determine wind profiles up to the upper edge of the mixture layer (about 1-3 km). As part of the FESSTVaL test campaign, various scanning patterns are being tested to find the appropriate configuration to use during the main campaign. Here, the comparison with ultrasonic anemometer gust measurements of the Falkenberg weather mast is decisive. During the main campaign, the focus is then on the synopsis of the different measurements that occur in combination with gusts.
During FESSTVaL some Halo Photonics Doppler lidars will be available. During the test campaign, two lidars with different measurement configurations will be measuring in parallel (pictured left) and a suitable configuration will be determined by comparison with the ultrasonic anemometer on the mast (90 m). Various scanning patterns (so-called "volume azimuth displays" - VAD) are being tested, which differ in terms of inclination, number of beams and temporal resolution. In addition, the combination of three lidars makes it possible to operate a so-called "Virtual Tower" (pictured right). The advantage here is that three measurements are directed to a common air package: no spatial and temporal homogeneity assumptions must be made and the wind vector can be determined with higher temporal and spatial resolution.