Characterization Of Small Scale Roughness Elements On A Water Surface

Air-water exchanges of momentum, heat and water vapor were investigated from a tower on a fresh water lake at low to moderate wind speeds, 3 less than U(sub 10) less than 8 m/s. The tower is located 15 m offshore where water depth is about 4 m. For t... Read more

Air-water exchanges of momentum, heat and water vapor were investigated from a tower on a fresh water lake at low to moderate wind speeds, 3 less than U(sub 10) less than 8 m/s. The tower is located 15 m offshore where water depth is about 4 m. For this purpose, surface fluxes were determined directly from atmospheric turbulence measurements using the eddy correlation technique and water surface elevations due to long gravity and short gravity-capillary waves were measured by a resistance wire wave gauge. The observed wave field was generated purely by the local winds over a well-defined fetch of 7 km. Experimental results showed that for the conditions of light winds and nonneutral atmospheric stratification encountered in this study, atmospheric turbulence statistics could not provide an estimate of the surface fluxes with great accuracy. For similar environmental conditions the neutral drag coefficient, C(sub DN), and the surface roughness length parameter, z(sub 0), determined from flux-profile relationships were found to show large scatter mainly due to the significant variations in the contribution to total covariances by the low-frequency atmospheric phenomena. However, on the average they were found to be in good agreement with other independent estimates of C(sub DN) and z(sub 0), obtained from the wave measurements. The technique of determining C(sub 0) and z(sub 0) from wave spectra, devised in this study, was successfully used not only during steady state but also during rapidly changing wind speed conditions. Less