Effect of coastal boundary resolution and mixing upon internal wave generation and propagation in coastal regions |
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Authors: | Philip Hall Alan M Davies |
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Institution: | (1) Proudman Oceanographic Laboratory, 6 Brownlow Street, Liverpool, L3 5DA, UK |
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Abstract: | A non-linear two-dimensional vertically stratified cross-sectional model of a constant depth basin without rotation is used
to investigate the influence of vertical and horizontal diffusion upon the wind-driven circulation in the basin and the associated
temperature field. The influence of horizontal grid resolution, in particular the application of an irregular grid with high
resolution in the coastal boundary layer is examined. The calculations show that the initial response to a wind impulse is
downwelling at the downwind end of the basin with upwelling and convective mixing at the opposite end. Results from a two-layer
analytical model show that the initial response is the excitation of an infinite number of internal seiche modes in order
to represent the initial response which is confined to a narrow near coastal region. As time progresses, at the downwind end
of the basin a density front propagates away from the boundary, with the intensity of its horizontal gradient and associated
vertical velocity determined by both horizontal and vertical viscosity values. Calculations demonstrate the importance of
high horizontal grid resolution in resolving this density gradient together with an accurate density advection scheme. The
application of an irregular grid in the horizontal with high grid resolution in the nearshore region enables the initial response
to be accurately reproduced although physically unrealistic short waves appear as the frontal region propagates onto the coarser
grid. Parameterization of horizontal viscosity using a Smagorinsky-type formulation acts as a selective grid size-dependent
filter, and removes the short-wave problem although enhanced smoothing can occur if the scaling coefficient in the formulation
is too large. Calculations clearly show the advantages of using an irregular grid but also the importance of using a grid
size-dependent filter to avoid numerical problems. |
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