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

Surface waves play important roles in the determination of air-sea fluxes and the overall dynamics of marine boundary layers. One of the strong couplings between atmospheric turbulence and sea state occurs in light winds with fast moving swell. Low-level jets, positive (upward) vertical momentum flux, and negative mean gradient profiles in the surface layer are signatures of a wave driven PBL. These features invalidate the use of Monin-Obukhov similarity theory most often used to predict air-sea fluxes.

To model the impact of surface waves on the PBL, we have recently developed a new large-eddy simulation (LES) code with the capability of imposing a moving sinusoidal wave at its lower boundary. This LES code is used to simulate PBLs with light winds, varying stratification, and different $z_o$ surface conditions. A surface fitted grid and variable vertical spacing are used to resolve features near the lower boundary.

Flow visualization of the LES solutions shows that in the case of fast moving swell (wave age c/U(z=10m) = 2.2) a coherent pattern of accelerated winds occurs downwind of each wave crest. At the same time, the vertical velocity is biased towards negative (positive) values upstream (downstream) of the wave crest, respectively. This organization induces positive vertical momentum flux which accelerates the near surface winds and leads to the formation of a low-level jet. The form stress, obtained by integration of the surface pressure, is positive, consistent with the sign of the vertical momentum flux. Our LES is thus able to replicate important features of a wave-driven boundary layer. We find that fast moving swell upsets the turbulence production mechanism in the marine surface layer which in turn impacts the whole PBL. In the absence of shear production, turbulence in the upper PBL tends to collapse and the wave-driven PBL differs from its counterpart with stationary surface roughness. The appearance of a low-level jet and vertically varying vertical momentum flux make surface layer measurements dependent on wave state and vertical distance above the surface. Comparisons with the detailed surface layer observations from the recent Coupled Boundary Layers Air-Sea Transfer (CBLAST) field campaign are also presented.