Given the rising number of offshore wind farms, the effect of wakes (the area downwind of wind farms characterized by a wind speed deficit) on downwind wind farms and their impact on the regional climate is discussed. This work investigates the spatial dimensions of wakes and the micrometeorological and regional climate impacts of offshore wind farms on the marine boundary layer based on mesoscale simulations using a wind farm parameterization (WFP) and airborne observations.
WFPs act as a momentum sink for the mean flow. However, WFPs differ on whether or not they add additional turbulent kinetic energy (TKE) to represent the enhanced mixing caused by wind farms. This thesis uses for the first time aircraft observations taken above and behind offshore wind farms to answer this uncertainty for stable conditions. The airborne measurements reveal that a TKE source and a horizontal resolution in the order of 5 km are necessary to represent the enhanced TKE (i.e. 20 times higher than in the ambient flow) above offshore wind farms.
Further, this thesis evaluates the simulated spatial extent of a wake by the use of airborne measurements taken on 10 September 2016. Observations and simulations show a wake longer than 45 km associated with a warming and drying at hub height in the order of 0.5 K and 0.5 g kg-1, respectively. Vertical cross-sections perpendicular to the wake reveal that warmer and dryer air was mixed towards the surface due to an inversion located within the rotor area. An analysis of 23 additional airborne measurements executed within the far-field of offshore wind farms suggests that an impact on the temperature is only visible in case of inversions in the vicinity of the rotor area and wind speeds over ≈ 6m s-1.
Based on the successful evaluations above and downwind of offshore wind farms, this thesis explores a future scenario including all offshore wind farms possibly installed at the German Bight to discuss potential impacts of large offshore wind farms on the regional climate by considering two case studies. The simulations suggest that the wakes of large offshore wind farms clusters are longer than 100 km associated with changes in the sensible and latent heat flux. The net impact on the MABL depends on the inversion height and the temperature gradient between sea surface temperature (SST) and air temperature. Therefore, the dominating impact of offshore wind farms can only be determined by simulations covering several years with the constraint that the inversion height is captured by the driving mesoscale model.