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Abstract

A theoretical model of flow over a change in surface roughness shows that given an average geostrophic wind of between 8 and 13 m s−1 and near-neutral conditions the difference in equilibrium wind speed at 50 m height over roughness of 0.1 m and over the sea surface is about 20% or 2 m s−1. However, in non-neutral conditions the difference between on- and off-shore wind speeds is affected by changes in stability conditions. Using data from an off-shore monitoring project it is shown that flow over sea is typically associated with smaller stability corrections to the logarithmic wind speed profile owing to an increase in the number of near-neutral and unstable observations compared to flow over land. The presence of land is still detectable after flow of 1–2 km off-shore but its effect on stability and therefore the wind speed profile is reduced. The differences between stability climates on- and off-shore are not simple to elucidate since a number of factors such as the orientation of the coastline with regard to the prevailing wind direction, water depth and latitude are probably also important. Copyright © 1999 Royal Meteorological Society