Observations of the westerlies over the sea

Abstract

SUMMARYObservations 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 2 1 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.(iii) The vector variation of wind velocity with height in the layer 200-500 m is accountable as thermal wind.(iv) Turbulence of scale greater than about 10 min is of order 10 per cent of the mean motion and increases on average with height.(v) 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.(i)(ii)

Specification of the Wind Distribution in the Marine Boundary Layer for Wave Forecasting

“…This supports also the results of Sheppard, Charnock and Francis (1952) who studied the variation of wind with height in the first few hundred meters over the Northeast Atlantic during ten winter days of westerly wind. They showed that on over 30% of the cases analyzed, the 300 meter wind was backed on the surface wind and these cases corresponded …”

The vertical transfer of momentum in the general circulation

“…The angle ~ then determines, for given t o J, the rate of change of the downwind shearing stress with height (Eq. 2.1a) and we infer that the above-mentioned result in [2] regarding ~ implies no material change in T~ with height near the surface.…”

“…Observations in the westerlies of the NE Atlantic in winter by S~EP-PAre), C~A~ocK and F~a~zms [2] showed that the surface wind direction had a mean inclination to the surface isobar of not more than 1 ~ or 2 ~ The mean surface motion was in fact deduced to be up the pressure gradient, but in view of the difficulty in determining the mean direction of the pressure gradient to better than about ~ 3 ~ one can 0nly infer with confidence that there was no appreciable down-gradient motion. Now, if we assume that, over a period which includes the passage of many disturbances, the accelerations of the air are self-cancelling or small in resultant compared with the pressure, shearing stress and Coriolis forces, we may write the equations of motion for any level z…”

The Oceanography of the Celtic Sea I. Wind Drift

“…Nevertheless, in a careful study, spanning 7 days in January, 1951 at Scilly (Sheppard et at. 1952), the conditions of air flow required for equations (I) and (2) to hold were not found.…”

“…A further advantage of concentration of effort at Scilly is that there have already been a number of meteorological and current studies there (inter alia, Sheppard, Charnock & Francis, 1952;Carruthers, Lawford & Veley, 1951 b).…”