An experimental study of Reynolds stress and heat flux in the atmospheric surface layer

The paper describes an experimental programme to study the characteristics of momentum and heat transport in the first 22‐6 m of the atmosphere. Sonic anemometers and fine platinum wire thermometers were used for flux computations by the eddy correlation technique. Two drag plates were used to measure the surface stress. The results indicate that the fluxes are constant with height to within ± 20 per cent provided a long enough averaging period is chosen. Shorter averaging periods (15 min or less) show considerable variation in the fluxes and in their vertical gradients especially during unstable conditions. It appears that much of this variability can be attributed to the strong effect on the fluxes of submesoscale circulations. The momentum flux gradient seems to be particularly sensitive to these circulations. The most consistent estimates of u * were obtained from the drag plates. They show much less scatter when used in similarity relationships than estimates obtained from the eddy correlation technique at higher levels. However, because of inherent difficulties in installing the drag plates in such a rough surface, their absolute reading was obtained by calibration against the eddy correlation measurement. The correlation γ UW , between the longitudinal and vertical components of the wind, is generally much smaller than γ WT , the correlation between the vertical component and temperature. With increasing instability γ WT increases to a value of about 0‐6 while γ UW drops to negligible levels.