HR: 15:05h
AN: OS52G-05    [PDF]
TI: Large-Eddy Simulations of Low-Wind Boundary Layers in the Presence of Waves
AU: * Sullivan, P P
EM: pps@ucar.edu
AF: NCAR, P.O. Box 3000, Boulder, CO 80503-3000 United States
AU: McWilliams, J C
EM: jcm@atmos.ucla.edu
AF: Atmospheric Dept. UCLA, 405 Hilgard, Los Angeles, CA 90095-1565 United States
AU: Moeng, C
EM: moeng@ucar.edu
AF: NCAR, P.O. Box 3000, Boulder, CO 80503-3000 United States
AB: Surface waves play important roles in the determination of air-sea fluxes and the overall dynamics of marine boundary layers. Recent observations (Smedman etal, 1999; Sjoblom \& Smedman, 2003; and the CBLAST field campaign) hint that fast moving waves (swell) can generate spectacular behavior in the low-wind neutral planetary boundary layer(PBL). Low-level jets, positive (upward) vertical momentum flux, and negative mean gradient profiles are often observed. These features are signatures of a wave-driven PBL and 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, neutral stratification, and different surface conditions. The simulations utilize 250x250x96 gridpoints in a domain of size 1200x1200x800m. A surface fitted grid and variable vertical spacing are used to resolve features near the lower boundary. The properties of the imposed wave are amplitude = 1.6m, waveslope = 0.1, wavelength = 100m, and wave phase speed c = 12.5m/s. The friction velocity based on the surface stress is 0.2m/s, typical of light wind conditions. 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 modifies the usual 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. Results for stratified boundary layers are also discussed.
DE: 3307 Boundary layer processes
DE: 3379 Turbulence
DE: 4560 Surface waves and tides (1255)
SC: Ocean Sciences [OS]
MN: 2004 Ocean Sciences Meeting