A Two-Layer Model for Hurricane-Driven Currents on an Irregular Grid

G. Z. Forristall Shell Development Company, Houston, TX 77001

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Abstract

Measurements made during Hurricanes Carmen and Eloise revealed some features of wind-driven currents which have been incorporated into a numerical model. In the summer, near-surface waters on the continental shelf off Louisiana are usually strongly stratified by river runoff. The passage of a hurricane provides enough energy to mix the surface layer down to a depth between 30 and 45 m. At the same time, a two-layer current system develops, with the mixed layer responding much more directly to the wind shear than the bottom layer. This system was modeled by parameterizing the mixed layer with a relatively high eddy viscosity and the lower layer with a much lower eddy viscosity. A modification of a previously developed convolution integral scheme permits calculation of the detailed structure in both layers without requiring a three-dimensional grid. To eliminate some problems with lateral boundary conditions, the vertically integrated calculations were performed on an irregular grid system covering the entire Gulf of Mexico. Comparisons with the storm measurements show that the model is reasonably accurate, but there are still some unmodeled processes.

Abstract

Measurements made during Hurricanes Carmen and Eloise revealed some features of wind-driven currents which have been incorporated into a numerical model. In the summer, near-surface waters on the continental shelf off Louisiana are usually strongly stratified by river runoff. The passage of a hurricane provides enough energy to mix the surface layer down to a depth between 30 and 45 m. At the same time, a two-layer current system develops, with the mixed layer responding much more directly to the wind shear than the bottom layer. This system was modeled by parameterizing the mixed layer with a relatively high eddy viscosity and the lower layer with a much lower eddy viscosity. A modification of a previously developed convolution integral scheme permits calculation of the detailed structure in both layers without requiring a three-dimensional grid. To eliminate some problems with lateral boundary conditions, the vertically integrated calculations were performed on an irregular grid system covering the entire Gulf of Mexico. Comparisons with the storm measurements show that the model is reasonably accurate, but there are still some unmodeled processes.

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