ABSTRACT |
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This study has examined two mesoscale phenomena, Karman vortex and
cloud streets, which are mesoscale disturbance in the lee of an isolated
mountain. Two kinds of numerical models (LCM and RAMS, referred below) are
used for the mechanism of the two mesoscale phenomena.
Well developed Karman vortex street were simulated by the numerical model (LCM) including an isolated mountain, uniform stratification and constant inflow with low Froude number (0.22) in the inflow boundary. The results are summarized as follows: 1) The numerical model (LCM) can simulate Karman vortex without surface friction. This mean the mechanism of Karman vortex in the atmosphere is different from the classic Karman vortex. 2) This founding agrees with the result of Schar and Durran (1997, hereafter referred to as SD). In this study, however, it is shown that the flow deceleration (horizontal shear instability) in the lee of the mountain comes from the divergence of total vertical momentum flux caused by mountain drag, which does not investigated in the previous study. 3) The divergence of momentum flux can be explained by the wave saturation theory given by Lindzen (1981) with some modification. Simulations in this study show that the momentum flux in the lower level is much larger than the saturated momentum flux, whereas it is almost equal to the saturated momentum at the upper levels as expected from the saturation theory. This means that large flux divergence is produced between surface layer and upper layer (about 2.5 km level). As a result, the mean flow is decelerated behind the mountain and the horizontal wind shear is formed. When the horizontal shear is strong enough, the Karman vortex will form by absolute instability as mentioned by SD. 4) In case of a three-dimensional bell-shaped mountain, the onset of wave breaking is observed if Fr (Froude Number) is less than 0.8 in the numerical model. 5) The Karman vortex forms if Fr is less than about 0.22, although stationary lee vortices forms if Fr is larger than this value. 6) Vortex formation also depends on Reynolds number which is estimated from the horizontal diffusivity. 7) The momentum budget calculated by hydrostatic model is almost the same as the nonhydrostatic result as long as horizontal scale of mountain is 10 km. And also, well developed Karman vortex similar to the hydrostatic result was simulated in the nonhydrostatic case. This means that the hydrostatic assumption is adequate for investigation of the origin of Karman vortex from the viewpoint of total vertical momentum budget. Cloud streets were successfully simulated by numerical model (RAMS) including an isolated mountain near the coast, large sensible heat flux from the sea surface, and uniform stratification and wind velocity with low Froude number (0.25) in the inflow boundary. The well-developed cloud streets between a pair of convective rolls are simulated at a level of 1 km over the sea. The following five results were obtained: 1) For the formation of the pair of convective rolls, both strong static instability and a topographically induced mechanical disturbance are strongly required at the same time. 2) Strong sensible heat flux from the sea surface is the main energy source of the pair of convective rolls, and the buoyancy caused by condensation in the cloud is negligibly small. 3) The pair of convective rolls is a complex of two sub-rolls. One is the outer roll, which has a large radius, but weak circulation, and the other is the inner roll, which has a small radius, but strong circulation. The outer roll gathers a large amount of moisture by convergence in the lower marine boundary, and the inner roll transfers the convergent moisture to the upper boundary layer by strong upward motion between them. 4) The pair of inner rolls form the line-shaped cloud streets, and keep them narrow along the center-line of the domain. 5) Both by non-hydrostatic and by hydrostatic assumptions, cloud streets can be simulated. In our case, non-hydrostatic processes enhanced somewhat the formation of cloud streets. The horizontal size of the topography does not seem to be restricted to within the small scale where non-hydrostatic effects are important. Key words: Karman vortex, cloud streets, mechanical disturbance, static instability. |