Characterization of Roughness Elements on a Water Surface

Air-water exchanges of momentum, heat and water vapor were investigated through measurements of surface fluxes and wind generated waves from a tower located 15 m offshore where water depth and fetch were about 4 m and 7 km, respectively. For the conditions of light winds and nonneutral atmospheric stratification encountered, turbulence statistics could not provide an estimate of the surface fluxes with great accuracy. For similar conditions, neutral drag coefficient, C_{DN}, and surface roughness length, z_{rm o}, determined from flux-profile relationships showed large scatter. However, on the average they were in good agreement with other independent estimates of C_{DN } and z_{rm o} obtained from wave spectra by a technique devised in this study. Increase in C_{DN} with U_{10} was typical of the results from other marine studies. Experimental findings indicated that z_{rm o} ~ zeta (U_{10}/C _{p}), where zeta is the ms wave height and C_{p } is the phase speed of the dominant waves. Neutral bulk transfer coefficients and characteristic roughness lengths for heat and water vapor did not change with wind speed and sea state, but they were larger in the unstable regime than in the stable regime. Sudden changes in their magnitudes in near neutral conditions could be better described by air-water temperature difference than atmospheric stratification parameter, z/L. Frequency spectra of observed long waves (5 m in wavelength) could be adequately described by the model of Donelan et al (1985). However, the relationships between fetch, wave energy and the U_{10} /C_{p} ratio deviated significantly from those suggested by Donelan et al (1985). These deviations are suspected to be due to the differences in fetch and duration between the two experiments. Measured frequency spectra of short gravity-capillary waves (3 to 38 cm in wavelength) were corrected for the Doppler frequency shifts. Portions of the data affected by breaking waves were excluded from the analyses. Spectral amplitudes of short waves increased with increasing U _{10} as expected. Spectral slope at high frequencies varied between -4 and -5, depending on the magnitude of and the distance from the peak frequency. A parameterization of this dependency requires further experimental evidence. These short wave components were strongly suppressed in the presence of surface films. Surface films were observed mostly during periods of stable stratification when intermittent nature of atmospheric turbulent transport is pronounced.