Large-Eddy Simulations and Observations of Wave-Driven Boundary Layers

Air-sea interaction occurs over a wide spectrum of scales ranging from millimeters (spray droplets and air bubbles) to hundreds of kilometers (synoptic scale storms) and even larger (global climate). The coupling among water waves and atmospheric turbulence is one of the important components of air-sea interaction. Wind generated waves influence the flux of momentum and scalars at the air-sea interface and represent a visible signature of coupling between the atmosphere and ocean. Also, the fundamental difference between atmospheric boundary layers over land and over water derives from the scale and mobility of the water surface. An outstanding question in wind-wave interaction is the impact of swell on the atmospheric planetary boundary layer (PBL). Swell dominated wave fields occur after the passage of storm fronts and can propagate long distances without significant dissipation, e.g., see estimates in Cohen and Belcher (1999). In the open ocean, the wave height variance is typically dominated by swell (i.e., by old waves) and as a result it is difficult to measure and isolate the contributions of young (short) waves to the roughness and surface stress. The impact of swell on surface drag parameterizations is correspondingly unsettled. A recent study by Donelan et al. (1997) suggests that swell influences are strong and that wind-swell alignment is an important factor for the measured drag coefficients (e.g., they report that the drag increases by a factor of 3 for swell opposing the wind). Thus, the surface stress depends on at least three factors, wind speed, wave age, and swell. The coupling between light winds following fast running swell is of particular interest here. Harris (1966) first reported the formation of a wave-driven wind in a laboratory wave tank. Later, Holland et al. (1981) and other observational campaigns noted an increase in surface winds in the presence of swell. Since then a growing body of experimental evidence has documented