The use of a wave prediction model for driving a near-surface current model
Tóm tắt
The current near the sea surface is an important influence on the drift and dispersion of plankton, fish eggs and larvæ and other marine organisms, of oil slicks, chemical pollutants and other substances, and contributes significantly to the forces on marine structures. Since it is intimately associated with air-sea momentum and energy transfer, it affects, and is affected by, the atmospheric conditions, and, in the long term, the global climate. Since surface gravity waves are almost always present at the air-sea interface, and carry significant energy and momentum, they should be taken into account when estimating near-surface currents. A one-dimensional current model is presented which does take waves into account. It uses a Lagrangian coordinate system, and the source terms from a directional spectral wave prediction model act to transfer momentum from the wave field to the current. After the wave model is run, but before it is used to drive the current, the source terms in the tail of the wave spectrum are parameterized in a way which deals consistently with the balance of wave energy, momentum and action. Numerical integration is performed by using a finite difference grid with a fine resolution near the surface, and an implicit time-stepping scheme is used. The model results are consistent with present knowledge of the behaviour of the surface current. Within the upper 1 metre, they depend strongly upon the behaviour of the wave spectral tail. Oscillations in the modelled high-frequency wave components, which arise when the wind forcing is changed rapidly, give rise to corresponding oscillations in the current: this effect may be physical or it may be due to a slight instability in the wave model.
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