Observations of the westerlies over the sea

Abstract

Abstract Observations of wind speed and direction at 50 m intervals of height in the first few hundred metres have been made over the NE. Atlantic during 10 winter days of westerly winds. The following results were obtained: The vertical gradient of wind speed is of the same order at all levels and there is no identifiable transition from a layer of frictional influence to a layer of friction‐free wind. The wind speed at 300 m was on the average 21 per cent greater than the surface wind, though in 24 per cent of cases the latter was greater than the former. The mean change of direction in the same interval of height was 3·6° veer but the spread was much greater than the mean. On over 30 per cent of cases the wind at 300 m was backed on the surface wind and the speed at the upper level was then only 5 per cent greater than the surface speed. The vector variation of wind velocity with height in the layer 200–500 m is accountable as thermal wind. Turbulence of scale greater than about 10 min is of order 10 per cent of the mean motion and increases on average with height. The mean inclination of the surface wind to the isobar is not more than 1° or 2° and is more likely to be towards high pressure than low. It is inferred that the classical conception of the frictional boundary layer in a barotropic atmosphere has little validity in the atmosphere over the sea. In particular the shearing stress in the first few hundred metres of the westerlies probably varies only slightly with height and may increase if the thermal wind is strong. Measurement of surface drag by the method of frictional geostrophic departure is hardly possible in the baroclinic westerlies.