FY 1983

The variation of the drag coefficient in the marine surface layer due to temporal and spatial variations of the surface wind and sea state

Byrne, H.M.

NOAA Tech. Memo. ERL PMEL-49, NTIS: PB84-104348, Boulder, CO, 116 pp (1983)

The neutral atmospheric drag coefficient, C, and ocean surface wave spectra were measured on both sides of a strong atmospheric front. The front was part of a mature extratropical cyclone moving across the Gulf of Alaska on 15 November 1980. Results showed a large variation in C at constant mean wind speed in the cold sector which could not be explained by formulations found in the literature. Previous work which attempted to relate drag coefficients to surface wind speed paid slight attention to the coincident surface wave field. As a result, no explicit dependence of C on the wave field has ever been reported. This investigation, which measured the complete one-dimensional surface wave spectrum, constitutes the first time wave spectral measurements that have been made together with eddy correlation flux measurements in marine surface layer. Coincident wave spectral measurements showed an unexpected increase in energy in the six-second wave band (.15.17 hz), which was correlated with the variation C. The spatial distribution of wave spectral energy in the duration-limited sea behind the 15 November front was found to differ markedly from the spatial distribution of wave spectral energy in the fetch limited case. The wave spectra in the warm sector followed Phillips' model well. The wave spectra in the cold sector did not. Using only the wave spectral information, a representation of z was derived which reduced to Charnock's relation in situations of dynamic equilibrium between the wind field and the surface waves. In situations of disequilibrium between the wave field and the surface wind, the derived relation showed a dependence of z on the six-second wave field. We found that C calculated from measurements made during the 15 November 1980 frontal penetration, and the 7 November post-frontal flight followed the derived z relationship. This relationship of z to the surface wave spectra helps explain the high values of C behind moving fronts which have been reported in the literature.