Low Froude Number stratified flows interacting with an isolated obstacle
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| Autores: | , |
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| Formato: | artículo original |
| Fecha de Publicación: | 1997 |
| Descripción: | In previous studies it has been postulated that vortices, which are observed in the lee of mesoscale mountains, may be due to a variety of interactions of the upstream flow with the mountain including the inviscid baroclinic production of vorticity, the influence of the Earth's rotation and the role of frictional processes in the boundary layer. Here the relative importance of these three mechanisms is quantified for an upstream flow, with constant static stability, past a mountain with dimensions such that the Rossby number is 0.4 and the Froude number is 0.2. A simple Ekman-type boundary layer is used for simulations in which the role of friction is calculated. The full vorticity budget is evaluated so that the dominant sources of vorticity can be found from the numerical simulations, which use a non-hydrostatic, three-dimensional model. It is found that if a frictional boundary layer is included the lee vortices are formed at the flanks of the orography due to viscous mechanisms. The (inviscid) baroclinic mechanism exists but is a small contribution. The asymmetry introduced by the Earth's rotation accentuates the tendency for shedding of the lee vortices. The frictional effects also act as a source of potential vorticity. However caution needs to be applied when interpreting the potential vorticity budget because there is a source due to the implicit numerical dissipation inherent in this, and any other model. This implicit source makes the dynamical interpretation of the so-called inviscid simulations in potential vorticity terms difficult if not impossible. For such problems the use of vorticity, rather than potential vorticity thinking is less ambiguous. The production of vorticity and potential vorticity is studied in a low Froude number stratified flow over and around an isolated bell-shaped mountain with a circular base. The effects of background rotation and surface friction have been considered. Without background rotation and shear and using a free-slip lower boundary condition for inviscid flow the baroclinic mechanism is the main source of horizontal vorticity. This horizontal vorticity is tilted into the vertical and generates two counter-rotating lee-vortices. With background rotation the baroclinic mechanism remains an important source of horizontal vorticity but it is closely followed by additional tilting terms which depend on the Coriolis parameter. Simulations of viscous flow using a no-slip boundary condition show that the baroclinic term is then not an important contributor to the horizontal vorticity budget. Much larger values of vertical vorticity are found on the upwind side of the orography than in the other two cases, due to the tilting of the horizontal vortex tubes created by the no-slip boundary condition. A quantity called normalised potential vorticity is defined to allow the relation between the amount of vorticity and potential vorticity being produced to be quantified. It is produced from early in the integration for both non-rotating and rotating inviscid flows, due to the implicit diffusion implied by the numerical method. In the viscous flow more normalised potential vorticity is produced due to the inclusion of friction. For a esterly flow in the northern hemisphere, negative potential vorticity is produced on the northern flank of the orography and positive potential vorticity is produced on the southern flank.The onset of vortex shedding due to asymmetries in the flow is also studied.The asymmetries were introduced by including background rotation creating an asymmetry in the upwind flow allowing the lee eddies to be shed. |
| País: | Kérwá |
| Institución: | Universidad de Costa Rica |
| Repositorio: | Kérwá |
| Lenguaje: | Inglés |
| OAI Identifier: | oai:kerwa.ucr.ac.cr:10669/89811 |
| Acceso en línea: | https://hdl.handle.net/10669/89811 |
| Palabra clave: | METEOROLOGY FLOW |