Momentum flux structures and statistics in low-wind marine surface layers: Observations and large-eddy simulations

A primary objective of the CBLAST LOW program was to investigate the transfer of momentum at low wind speed conditions where the drag coefficients are characterized by significant variability. A large fraction of this uncertainty is believed to be due to sampling variability at low winds. However, previous investigations have indicated that physical processes are responsible for some of the scatter seen in direct measurement of the drag coefficients at low winds. These studies have identified wind-swell interaction and surfactant modulation of the gravity-capillary waves as possible causes. To improve marine forecasts and coupled atmosphere-ocean models, we need to quantify the effects of these physical process and develop parameterizations so as to include their effect in numerical models. In the present work, we analyze results obtained from two field campaigns that focused on the low-wind marine boundary layer: the Coupled Boundary Layers Air-Sea Transfer (CBLAST) and the Ocean Horizontal Array Turbulence Study (OHATS) both of which were carried out at the Air Sea Interaction Tower, a component of the Martha's Vineyard Coastal Observatory. High resolution turbulence data was collected using arrays of sonic anemometers located at several heights above the sea surface and simultaneous wave measurements were obtained from downward pointing laser altimeters. These datasets allow us to establish a correlation between the turbulent fields and wave state over a range of atmospheric conditions. We find that the marine surface layer dynamics are frequently dominated by fast running waves (swell). Under these conditions, we observe positive upward momentum flux and low-level jets, signatures of wave driven winds. The talk compares these measurements with recent Large Eddy Simulations (LES) of turbulent flow over a wavy surface. The LES models decaying wave conditions where the phase speed of the waves (i.e., swell) is moving faster than the wind. Appropriate boundary conditions are applied to correctly simulate energy and momentum exchange at the surface and thereby its effect on the overlying atmosphere. LES profiles of mean and turbulent variables show significant differences compared with classical boundary layers and flow over hills (i.e., stationary waves). The LES suggests that this is a result of a momentum flux divergence that accelerates the flow and a retarding pressure gradient which are opposite to the momentum balance in classical boundary layers. The LES is supported by the observations which provide clear evidence that variability in the drag coefficients at low winds is at least partially explained by this stress-swell interaction mechanism. Methods to incorporate these effects in numerical models are explored.