Graduate Thesis Or Dissertation
 

A numerical study of mesoscale motion in the atmospheric mixed layer

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/xk81jn83h

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  • The numerical modeling of motions in the atmosphere's planetary boundary layer (PBL) is a challenging task. In general, the boundary layer interacts with both the overlying atmosphere and the underlying land or water surface in a complex manner. Random turbulence is also present in the PBL which precludes exact prediction by numerical models. Nonetheless, expensive three-dimensional numerical models have been developed which, with several parameterizations and assumptions, can give a good idea of the PBL structure in many situations. However, on certain occasions, there is strong mixing evident in the PBL which may enable one to describe the structure of the boundary layer in a much-simplified theoretical model. By eliminating the vertical dimension from consideration, this two-dimensional mixed-layer model can be applied to mesoscale phenomena (horizontal length scale < 100 km) at greatly-reduced costs. The equations for motion and mixed-layer height are derived for such a situation and methods appropriate to the numerical modeling of the atmospheric mixed layer are discussed. Using an energy-conserving finite-difference analog of the model equations, the model is integrated in time to simulate the motions which were associated with the atmospheric vortex street observed near Cheju-do, South Korea on 17 February 1975. Experiments were carried out which investigated the effects of lateral diffusion, horizontal resolution, and mixed-layer depth. It is concluded that, given proper representation of prognostic variables on a staggered finite-difference grid, only small, realistic values of eddy diffusivity need be utilized. It also appears evident from the numerical experiments and atmospheric observations that the vortex street will form only when the obstacle which triggers its formation protrudes above the mixed layer. Although the wind fields in the simulations sometimes lack clear, fully rotational cells well downstream of the island, the characteristic sinusoidal pattern observed in laboratory experiments and cloud photographs is explicitly resolved by the model. The simulated vortex street also compares favorably with the observed in that the dimensionless governing parameters of the simulated vortex street (the Reynolds number, Strouhal number, Lin's parameter, the spacing ratio, and the speed ratio) closely match the observed values.
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