One-year-long turbulence measurements and modeling using large-eddy simulation domains in the Weather Research and Forecasting model

We present an intercomparison of a full year of turbulence measurements and simulations at Østerild, a site in northern Denmark with relatively flat terrain and high surface roughness, where a high-quality tall meteorological mast is deployed. Both sonic and cup anemometers are mounted on booms on the mast from 7 up to 244 m, thus covering the range of heights in which modern wind turbines operate. The simulations were performed using the Weather Research and Forecasting model in a multiscale setup, with large-eddy simulations (LESs) nested one-way within mesoscale simulations. The mesoscale domains thus simulated the evolving weather, while the two innermost domains used an LES closure, intending the largest scales of turbulence to be explicitly resolved. For a selected day, we show that the simulated turbulence is accurately resolved within the innermost LES domain, and agrees well with the observations at all vertical levels. For the full year, we show that the innermost domain accurately reproduces mean wind speed, direction, and turbulence levels, whereas the mesoscale simulations have difficulties matching the frequency of occurrence of both low and high turbulence ranges when compared to the observations. The largest differences between the simulated turbulence from the innermost domain and the observations are found under low wind speed conditions close to the surface, particularly during nighttime where the simulated mean wind and turbulence levels are higher and lower, respectively, than the observations, potentially due to the inability of the LES mesh to resolve the very small scales of turbulence and associated momentum transport that those features support.