High-wind atmospheric boundary layers above a spectrum of surface waves are simulated using a large-eddy simulation (LES) model that uses a non-orthogonal, time dependent, wave fitted mesh. The three-dimensional wave surfaces h(x,y,z) are imposed, and built from observations of surface wave spectra for wind speeds U(10) = [15 – 35] m/s for wave age Cp/U(10) ~ 1 with cutoff frequency 0.5 Hz. Large and small LES domains are used to examine the sensitivity of the solutions to the airflow grid and wave resolutions.
Peter Sullivan,
NCAR
In both domains, flow visualization shows the near surface layer flow tends to skim over the wave troughs and the highly intermittent flow separation observed for breaking dominant waves is not found; this suggests intermittent breaking is removed in the averaging of the wave spectra. In the small domain LES, the average momentum flux computed along the wave following coordinates shows approximately a 25\% contribution from the fluctuating pressure field correlated with the slope of the grid lines. This pressure contribution potentially impacts near surface observations of momentum flux at high winds. Small scale waves k > 1 rad\ m contribute to the overall surface drag in agreement with theory. Arguments are offered to illustrate how the LES momentum fluxes can be interpreted in a “motion-averaged” framework similar to field observations. The surface drag computed from the LES sub-grid-scale roughness model and surface pressure flux agrees with wind-wave tank observations collected for strongly forced waves. In agreement with the observations, LES shows a saturation Cd ~ 0.003 for winds greater than 25 m/s.
Peter Sullivan is a Senior Scientist in the Mesoscale and Microscale Laboratory of NCAR. Peter received his Bachelor’s and Ph.D. Degrees in Civil Engineering from Colorado State University and a Master’s Degree in Mechanical Engineering from the University of British Columbia (Vancouver, CA). Prior to coming to NCAR, Peter worked for six years as a Senior Specialist Engineer in Aerodynamics Research at the Boeing Company. Peter’s research interests are: simulations and measurements of geophysical turbulence, sub-grid-scale modeling, air-sea interaction, effects of surface gravity (water) waves on marine boundary layers, impacts of stratification, and numerical methods. He uses large-eddy and direct numerical simulations to investigate turbulent processes in both the atmospheric boundary layer and the ocean mixed layer. Peter has participated in and planned field campaigns, Horizontal Array Turbulence Study, and Advective Horizontal Array Turbulence Study focused on the measurement of sub-grid scale variables. His current interests include developing a large-eddy simulation model of wind marine boundary layers with a resolved spectrum of time dependent surface waves, computing turbulent flow over breaking waves, simulating stable atmospheric boundary layers using LES, sub-mesoscale density fronts and ocean boundary layer turbulence, and coupling atmospheric-oceanic boundary layers using LES. In 2025, Peter was honored by AMS with “Outstanding Contributions to Boundary-Layer Meteorology” award.