Mesoscale simulations of coastal boundary-layer transitions. Part 2: offshore wind speed development

The share of wind energy in the global electricity supply continues to grow, with offshore wind farms near coastlines forming a major part of this growth. High quality wind forecasts are needed for coastal areas, where wind directions from land towards the sea are associated with high variability. This raises two questions: How far offshore does the impact of the coastal transition reach, and how large is the impact on the wind resource? To address these questions, this article analyzes a 12-year data set (between 2010 and 2021) of mesoscale simulations performed with the Weather Research and Forecast Model (WRF) over the German Bight. The simulation results are evaluated using a comprehensive statistical approach for all wind directions and, additionally, only for wind directions from land to sea (offshore wind directions, defined here as wind directions from the South, Southeast, and East). The interannual variability of wind speed averaged over each of the 12 years is in the order of 1 m s -1 for all wind directions and up to 1.5 m s -1 for only offshore wind directions. On average, the coastal impact decreases with height and with increasing absolute wind speed. For all wind directions, the wind speed gradients are most pronounced close to the coastline, and decrease with distance from the coast until reaching a new equilibrium wind speed. For the selected offshore wind directions, which occur approximately 30% of the time for the 12-year data set, a consistent linear increase in wind speed with distance was observed above sea. For all wind directions, 95% of the reference wind speed at 200 km distance from the coast was reached after 60 km (40 km, 30 km) for a height of 10 m (100 m, 200 m). For only offshore wind directions, 95% of the wind speed at 200 km distance from the coast was reached after 100 km (120 km, 130 km) for a height of 10 m (100 m, 200 m). The results of the spring, summer, and daytime simulations are similar in terms of the horizontal extent of the coastal effect and the increase of wind speed. The horizontal extent increases with height and reaches a distance of 180 km at a height of 200 m. Winter, autumn, and nighttime simulations are similar in that the horizontal extent of wind speed increase is limited to a distance of approximately 50 km up to a height of 200 m. The impact of the coastal effect on wind speed decreases with height. For wind directions from Southeast, the linear increase in wind speed does not level off even at a distance of 200 km. The wind speed development for wind directions from the Southeast shows the most symmetric behavior for the coastal effect, with wind speed increasing gradually from the southern and eastern coastlines to the open sea. The wind speed development for wind directions from the East and South shows geolocation-specific features. The wind from the East shows a tunnel effect induced by the geometry of the Danish landmass, while the wind speed for wind directions from the South increases more quickly towards the West along the Dutch coast.