Mesoscale simulations of coastal boundary-layer transitions. Part 1: low-level jets

The impact of coastal low-level jets (LLJs) should be taken into account in the planning and operating phases of offshore wind farms as these elevated wind maxima introduce additional kinetic energy and vertical wind shear at operational heights of typical offshore wind turbines. In this study, the spatial distribution of LLJs in the German Bight was investigated using a twelve-year high-resolution data set computed with the Weather Research and Forecasting Model (WRF). The data set was divided into sectors of different wind directions, and the sectors with offshore oriented wind directions, i.e., wind blowing from land towards the open sea, are analyzed separately. The results show that the occurrence of LLJs strongly depends on wind direction. In general, offshore wind directions have higher probabilities of LLJ occurrence, with a systematically reduced LLJ probability immediately behind the coastline and the highest probability approximately 100–150 km offshore. On average, the probability of LLJ occurrence is highest around a height of 200 m in the vertical profile. The probability distribution of the strength of the LLJ wind speed follows the pattern of the LLJ probability, but the LLJ is less pronounced over water than over land. The seasonal changes in LLJ probability are driven by atmospheric stability, with larger probabilities in spring and summer and smaller probabilities in autumn and winter. On average, the highest LLJ probabilities occur in spring and summer during daytime and the lowest in autumn and winter during nighttime.