The diurnal variation of the marine surface wind in an offshore flow

Data from a number of light vessels moored off the English coast have been used to show that the marine near-surface wind within a distance of 40 km to 80 km from the coast varies diurnally when the wind is blowing in an offshore direction and the sea is cooler than the maximum overland air temperature. This variation is such that the maximum wind speed is overnight while the minimum wind speed is during the afternoon. This is in distinction to overland surface winds where the diurnal maximum in the wind strength occurs during the afternoon. A numerical model of the stable boundary layer has been used to simulate the variation, and the mechanisms involved are compared with those operating in stable layers over the land. This gives evidence that the main reason for the variation is the difference in boundary-layer temperature profiles between the daytime and night-time offshore flows. During the day the stable layer formed advectively over the sea reduces downward momentum transfer and hence the surface wind. During the night the stable layer preformed over the land evolves during advection to form an elevated inversion over the sea which, for reasons discussed, causes the overnight maximum. As well as having implications for maritime forecasting, model results show that vertical wind shears in the stable layers affect the performance of sailing vessels.