Effect of Ocean Waves on Air-sea Momentum Fluxes and Hurricane Intensity

Recent observational studies demonstrate that airsea momentum flux at high wind conditions in hurricanes strongly depends on the wave field and that the drag coefficient (Cd = u*/U10) levels off or even decreases at very high winds (Powell et al. 2003). Yet most hurricane research and forecast model utilize the bulk parameterization, i.e., the boundary layer parameterization based on the Monin-Obukhov similarity theory with the behavior of the drag coefficient based on extrapolations from field measurements in much weaker winds. It assumes that the Charnock coefficient (zch) is constant, i.e., is independent of the sea state. (Here, the Charnock coefficient is defined as zch = z0g/u*, and z0 is the equivalent surface roughness.) Therefore, this parameterization implicitly assumes that the surface wave field is fully developed. These extrapolations describe a monotonic increase in Cd with wind speed.