Observations of non‐dimensional wind shear in the coastal zone

Abstract Vertical profiles of the time‐averaged wind stress, wind speed and buoyancy flux from the off‐shore tower site in the Risø Air Sea Experiment are used to evaluate similarity theory in the coastal zone. the observed dependence of the non‐dimensional wind shear on stability is compared to the traditional parametrization. Relationships between the non‐dimensional shear, development of internal boundary layers and wave state are explored. We find that the largest‐scale turbulent eddies are suppressed in shallow convective internal boundary layers, leading to larger non‐dimensional shear than that of the traditional prediction based only on stability. In shallow stable boundary layers, elevated generation of turbulence leads to smaller non‐dimensional shear compared to the traditional prediction. Over young growing waves in stable stratification, the observed non‐dimensional shear is less than that over older more mature waves in otherwise similar conditions. the non‐dimensional shear is a function of wave state for stable conditions even though the observations are well above the wave boundary layer. We conclude that development of shallow internal boundary layers and young growing‐wave fields, both of which are common in the coastal zone, can lead to substantial departures of the non‐dimensional shear from the prediction based only on stability.