The effects of an isolated mesoscale island on a stably-stratified airstream Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/12579v91t

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  • The perturbation of a stably-stratified flow by irregular terrain is studied utilizing a seven-layer, hydrostatic, and potential enstrophy and energy conserving primitive equation model. The Island of Oahu, Hawaii, and the surrounding ocean area of 130 km x 100 km is chosen as the model topography. The domain is covered with a 5 km x 5 km mesh of 26 x 20 grid points in the horizontal. A cyclic boundary condition is imposed at the lateral boundaries. In order to give finer resolution to the lower atmosphere, an irregularly spaced sigma-coordinate is used in the vertical. Uniform east-north-easterly large-scale geostrophic winds up to the 400 mb level are imposed in order to represent the typical trade-wind condition. The pressure perturbation related to the mass flux divergence associated with the terrain irregularities, and the land-sea temperature difference associated with the different responses to the insolation during the daytime, are found to play a key role in determining the velocity field in the horizontal as well as in the vertical. Asymmetric surface pressure arises, due to the topographically induced vertical motion, with high pressure at the windward slope and low pressure at the downwind slope of the island. Daytime heating of the island induces low pressure on the island surface. This effect is the most significant at the lee side and inland portion of the island where the effect of cold-air advection from the ocean is minimal. At the lee side of the island the negative pressure perturbation induced by the daytime heating favors the establishment of a reverse flow toward the island, but the mountain range at the lee side of the island and prevailing tradewind prevent this low-level inflow from penetrating further inland. At the upwind side of the island, the flow field is mostly determined by topographic slope rather than by the heating of the island surface. The perturbation to the basic flow decreases rapidly upward. But a vertical cross-section of the horizontal divergence and the vertical velocity fields shows well defined wave motions up to the domain top level. The wave motions also appear downstream and upstream of the island with reduced amplitudes away from the island. These suggest the existence of hydrostatic mountain waves forced by the island contour, as was predicted by previous linear studies. The windward tilt of the wave axis shows upward propagation of wave energy, but the wave momentum flux was negligible.
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