Nearly all of the momentum transferred from wind to waves comes about through wave-induced pressure acting on the slopes of waves: known as form drag. Direct field measurements of the wave-induced pressure in airflow over water waves are difficult and consequently rare. Those that have been reported are for deep water conditions and conditions in which the level of forcing, measured by the ratio of wind speed to the speed of the dominant (spectral peak) waves, is quite weak, U10/cp < 3. The data reported here were obtained over a large shallow lake during the Australian Shallow Water Experiment (AUSWEX). The propagation speeds of the dominant waves were limited by depth and the waves were correspondingly steep. This wider range of forcing and concomitant wave steepness revealed some new aspects of the rate of wave amplification by wind, the so-called wind input source function, in the energy balance equation for wind-driven water waves. It was found that the exponential growth rate parameter (fra...

https://researchbank.swinburne.edu.au/file/01a7b074-7bc5-4c97-8a1e-068a2daa75e1/1/PDF (Published version).pdf

Wave-Follower Field Measurements of the Wind-Input Spectral Function. Part II: Parameterization of the Wind Input

Roughness in the Earth Sciences

In winds approaching hurricane strength, spray droplets proliferate. Once created, these droplets accelerate to the local wind speed in 1 s or less and thereby extract momentum from the wind. Because these droplets have substantial mass, they eventually plunge back into the ocean, delivering their horizontal momentum to the surface in the form of a spray stress. Inadequate information on the production rate and size distribution of spray droplets, however, hampered previous attempts to estimate the magnitude of this spray-mediated momentum exchange. This paper therefore uses recent estimates of the spray generation function to reconsider spray's ability to alter air–sea momentum exchange. Conservation of momentum requires that spray cannot enhance the air– sea stress beyond what the large-scale flow dictates. However, spray can redistribute stress in the near-surface atmosphere since the wind must slow if the spray droplets accelerate. For a wind of 30 m s−1, spray supports about 10% of the surfa...

Spray Stress Revisited

The extreme high winds, intense rainfall, large ocean waves, and copious sea spray in hurricanes push the surface-exchange parameters for temperature, water vapor, and momentum into untested regimes. The Coupled Boundary Layer Air–Sea Transfer (CBLAST)-Hurricane program is aimed at developing improved coupling parameterizations (using the observations collected during the CBLAST-Hurricane field program) for the next-generation hurricane research prediction models. Hurricane-induced surface waves that determine the surface stress are highly asymmetric, which can affect storm structure and intensity significantly. Much of the stress is supported by waves in the wavelength range of 0.1–10 m, which is the unresolved “spectral tail” in present wave models. A directional wind–wave coupling method is developed to include effects of directionality of the wind and waves in hurricanes. The surface stress vector is calculated using the two-dimensional wave spectra from a wave model with an added short-wave s...

Directional Wind-Wave Coupling in Fully Coupled Atmosphere-Wave-Ocean Models: Results from CBLAST-Hurricane

Based on the solution of the turbulent kinetic energy balance equation for the airflow in the regime of limited saturation by suspended sea-spray droplets, some experimental evidence, and simple arguments, a resistance law of the sea surface at hurricane winds is derived. It predicts the reduction of the drag coefficient for the wind speed exceeding hurricane values of 30–40 m s -1 in agreement with field data.

A note on the drag of the sea surface at hurricane winds

An approach that allows assessment ofthe impact of air-flow separation (AFS) fromwave breaking fronts on the sea-surface drag is presented. Wave breaking fronts are modelled by the discontinuities of the sea-surface slope. It is assumedthat the dynamics of the AFS from wave breaking crests is similar to thatfrom the backward facing step. The form drag supported by an individualbreaker is described by the action of the pressure drop distributed alongthe forward face of the breaking front. The total stress due to the AFS isobtained as a sum of contributions from breaking fronts of different scales.Outside the breaking fronts the drag of the sea surface is supported by theviscous surface stress and the wave-induced stress. To calculate the stressdue to the AFS and the wave-induced stress a physical model of the wind-wavespectrum is used. Together with the model of the air flow described in termsof surface stresses it forms a self-consistent dynamical system for the seasurface-atmosphere where the air flow and wind waves are strongly coupled.Model calculations of the drag coefficient agree with measurements. It is shownthat the dimensionless Charnock parameter (roughness length normalized onthe square of the friction velocity and the acceleration of gravity)increases with the increase of the wind speed in agreement with fieldmeasurements. The stress due to the AFS normalized on the square of thefriction velocity is proportional to the cube of wind speed. At low windsthe viscous surface stress dominates the drag. The role of the form drag,which is the sum of the stress due to the AFS and the wave-induced stress, isnegligible. At moderate and high winds the form drag dominates. At windspeeds higher than 10 m s-1 the stress supported by the AFS becomescomparable to the wave-induced stress and supports up to 50% of the totalstress.

The Impact Of Air-Flow Separation On The Drag Of The Sea Surface

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The Impact Of Air-Flow Separation On The Drag Of The Sea Surface