A Large Eddy Simulation using the WRF model over the sea: a real case study of a storm

There are very few studies in which the WRF model is used under Large Eddy Simulation (WRF-LES) mode for real time, offshore conditions. This study utilizes WRF-LES to investigate the wind characteristics during a real storm over the North Sea, west of the Danish Jutland coast. A WRF-based multiscale simulation was conducted to examine the storm, which is characterized by strong south-westerly winds, representing open ocean conditions. The simulation setup comprised four nested domains: three outer domains at mesoscale resolution (9.9 km to 1.1 km, domain 1 to 3) and an innermost domain running in LES mode (spatial resolution 100 m, domain 4). ERA5 reanalysis was used to drive the outermost mesoscale domain, while the other were one-way nested domains.

Results from the LES domain, domain 4, were compared to those from the finest mesoscale domain, domain 3. Lidar measurements of wind speed and direction from 40~m to about 250 m inside the studied domain are used to evaluate the simulations. Compared to the results from domain 3, the simulated vertical profiles of wind speed from the LES domain aligned more closely with the measurements up to a height of 150 m. At higher elevations, the profiles from the mesoscale and WRF-LES output converged, with both outputs overestimating the wind speeds. The wind directions were well simulated in both mesoscale and Les domains. The performance of the mesoscale and WRF-LES output in comparison with measurements is further explored using time series analysis at multiple heights.