THE INFLUENCE OF THERMAL EFFECTS ON THE WIND SPEED PROFILE OF THE COASTAL MARINE BOUNDARY LAYER BERNHARD LANGE 1,⋆ , SØREN LARSEN 2 , JØRGEN HØJSTRUP 2 and REBECCA BARTHELMIE 2 1 Institute of Physics, University of Oldenburg, D-26111 Oldenburg, Germany; 2 Wind Energy Department, Risø National Laboratory, P.O. Box 49, 4000 Roskilde, Denmark (Received in final form 4 November 2003) Abstract. The wind speed profile in a coastal marine environment is investigated with observations from the measurement program Rødsand, where meteorological data are collected with a 50 m high mast in the Danish Baltic Sea, about 11 km from the coast. When compared with the standard Monin– Obukhov theory the measured wind speed increase between 10 m and 50 m height is found to be systematically larger than predicted for stable and near-neutral conditions. The data indicate that the deviation is smaller for short (10–20 km) distances to the coast 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, a capping inversion typically devel- ops. The air below is constantly cooled by the water 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 correlated with the estimated height and strength of the inversion layer. Keywords: Atmospheric stratification, Coastal influences, Marine boundary layer, Monin–Obukhov theory, Wind speed profile. 1. Introduction Monin–Obukhov theory, although developed from measurements over land, has been found to be generally applicable over the open sea (Edson and Fairall, 1998). In homogenous and stationary flow conditions, it predicts a log-linear vertical profile of the wind speed in the atmospheric surface layer: u(z) = u ∗ κ  ln  z z 0  −  m  z L   . (1) The wind speed u at height z is determined by friction velocity u ∗ , aerodynamic roughness length z 0 and Obukhov length L; κ denotes the von Karman constant, taken as 0.4, and  m is a universal stability function. Thus, if the wind speed is known at one height, the friction velocity can be derived from Equation (1) and the vertical wind speed profile is determined by two parameters: the surface roughness z 0 and the Obukhov length L. ⋆ E-mail: Bernhard.Lange@uni-oldenburg.de Boundary-Layer Meteorology 112: 587–617, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.