Long-Term Measurements of Drag Coefficients and Waves in High Winds

The WaveWatch III model over-predicts wave heights at high winds speeds. One potential reason is that the momentum flux to the waves is overestimated within the model. Recent results indicate that the momentum flux at wind speeds above 30-40 m s-1 plateaus [Powell et al., 2003] and may lead to a reduction in the drag coefficient. Here we describe the physical relationships between wind history, wave field development, and atmospheric drag during high wind speed events, through the determination of wind speed and stability dependence of drag coefficients from long-term measurements at the US Army Corps of Engineers Field Research Facility (FRF) in Duck, NC. Our approach is to develop a long-term robust measurement suite to capture the conditions during tropical storms and nor'easters. Accurate measurements of 3-D turbulent wind speed components and turbulent water vapour concentration fluctuations are measured at 2 different heights simultaneously with wave height measurements to determine the friction velocity and drag coefficients as a function of wind, waves, and atmospheric boundary layer stability during high wind speeds. Initial results from micrometeorological measurements show that in onshore winds the conditions are representative for open ocean conditions (comparison with TOGA-COARE). However, wind speeds in these experiments were only up to 20 m s-1, whereas for the long-term deployment we are aiming for extreme wind speeds up to at least 40-50 m s-1 that may occur at FRF during winter storms and hurricane conditions. The long-term objective is wave model prediction at high wind speed using improved parameterization of atmospheric inputs.