The Influence of Thermal Effects on the Wind Speed Profile of the Coastal Marine Boundary Layer

The wind speed profile in a coastal marine environment is investigated withobservations from the measurement program Rødsand, where meteorologicaldata are collected with a 50m high mast in the Danish Baltic Sea, about 11 kmfrom the coast. When compared with the standard Monin-Obukhov theory themeasured wind speed increase between 10m and 50m height is found to besystematically larger than predicted for stable and near-neutral conditions. Thedata indicate that the deviation is smaller for short (10-20 km) distances to thecoast than for larger (>30 km) distances.The theory of the planetary boundary layer with an inversion lid offers a qualitative explanation for these findings. When warm air is advected over colder water, acapping inversion typically develops. The air below is constantly cooled by thewater and gradually develops into a well-mixed layer with near-neutral stratification.Typical examples as well as scatter plots of the data are consistent with this explanation.The deviation of measured and predicted wind speed profiles is shown to be correlatedwith the estimated height and strength of the inversion layer.