Coastal Flux-Gradient Dynamics: Insights from an Anemometer Array

The flux-gradient relationship in the atmospheric boundary layer is described by Monin-Obukhov Similarity Theory, which assumes stationary and horizontally homogenous conditions (i.e., flat regular terrain, low uniform roughness, and fair weather). It is generally thought these assumptions are met at sea under typical quiescent conditions and that the vertical gradients of momentum fluxes are negligible. However, the assumption of horizontal homogeneity clearly breaks down near shore where surface properties are altered by shoaling and breaking waves and the transition between water and land. This is a problem because a constant flux layer is necessary for equating flux measurements collected at different height, times, and locations, furthermore, it limits the accuracy of operational coastal models that use flux estimated from bulk parameterizations. Precise quantification of the atmospheric boundary layer is also needed for offshore wind resource characterization where wind speeds aloft are often estimate from surface measurements and extrapolated to hub height (O 100 m) using an assumed logarithmic profile. The energy produced by a wind turbine is proportional to the square of the wind speed, so even small wind speed uncertainty considerably increases the uncertainty in energy production. Consequently, a comprehensive understanding of the profile of wind speed in the lower marine boundary layer is essential for accurate site placement and energy forecasting. Using months of data from a four-anemometer array deployed upon a coastal tower in Duck, North Carolina, as part of the DUring Nearshore Events eXperiement (DUNEX), we begin the tackle this complex problem. We will quantify wind speed and friction velocity profiles, characterize the conditions that affect them, and discuss the implications of our findings.