Direct Measurements of Stress and Spectra of Turbulence in the Boundary Layer Over the Sea

Abstract

A pair of hot wire anemometers in an X-configuration was used in the boundary layer over the sea. Ten runs were analyzed to yield spectra and co-spectra of vertical and downwind components of velocity over a frequency range from 0.016 to 60 Hz and for average wind speeds from 1.4 to 10 m sec⁻¹. The downwind spectra agreed in shape with those measured simultaneously with a single wire, although the levels differed by up to 35%.The spectra of vertical components and of stress both tended towards zero at lowest wave numbers suggesting insignificant contributions from larger scales. In no case was there a dominant peak at the frequency of the surface waves, although all spectra had appreciable energy about this frequency. Although local isotropy was approached, it was not reached even at the smallest observed scales, which were within a decade of the scales of maximum dissipation. Various parameters estimated from the spectra are compared with the results of other workers. The kinematic stress u_*² was estimated both directly and indirectly. Ten direct estimates of the drag coefficient appropriate to the 5-m height were obtained; no trend with mean wind speed was observed and the mean value was 1.5 × 10⁻³. Measurements made with three vertically spaced single wires gave a direct confirmation of the Monin-Obukov similarity theory in the boundary layer up to a height of about 5 m.