An Investigation of Wind Structure in the Trades: Anegada 1953

This paper describes an observational study of the mean and larger-scale turbulent structure of the wind in the lowest 1500 m of the North-East Trades. The observed motions are used both alone and in conjunction with the horizontal pressure field to deduce values of the vertical transport of momentum; the pattern of cumulus cloud convection is borne in mind throughout. Sections 1 and 2 provide a brief survey of the background to the expedition and of the simplified equations by which the observations are interpreted. Section 3 describes the site and observations in detail. 466 double-theodolite pilot-balloon soundings were made in the spring of 1953 from the small flat island of Anegada (18°N, 64° W). Soundings were made on 15 days over a 27-day period, balloons being released at intervals of 5 to 15 min. The balloons, rising at about 150 m/min, were observed every 20s for 9 min, to obtain the three components of the motion in 50 m layers over the lowest 1350 m. Special observations of pressure were made in a network of neighbouring islands. The derivation of component air velocities and of the horizontal pressure gradient as a function of height is described in §4. Difficulty was experienced in obtaining the pressure field with requisite accuracy. Surface observations of the weather in relation to the main aim of the study are discussed in §5. The mean angle between surface wind and isobar over the 15-day period was 13°, notably less than the climatological value of about 33°. Section 6 discusses the properties of the mean horizontal motion for the whole period of observation. The easterly component of wind velocity was greatest at 350 m, and the wind veered with height through 24° in the first 1350 m. There was also a veer of geostrophic wind in this layer of about 13° so that some down-gradient motion remained at the top of the layer. It is shown that the mean values of the local and advective components of acceleration were negligible compared with others terms in the momentum balance. Section 7 uses the wind profiles of §6 together with the mean horizontal pressure field to find the distribution of shearing stress with height, assuming that ageostrophic flow is balanced by internal friction. The mean stress in the direction of the surface wind varied from 0-41 dyn/cm2 at the surface to —0.37 dyn/cm2 at 1300 m. The former provides a coefficient of surface stress, based on the anemometer windspeed, c = 1.5 x 10-3. The mean stress in the direction normal