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On Dynamics Near an Idealized Tropopause

出版	McGill University Libraries, 2018
URL	http://books.google.com.hk/books?id=lZKYtgEACAAJ&hl=&source=gbs_api

註釋"The near-tropopause energy spectrum obeys a -5/3 power law at mesoscales. Some theories accommodating this fact assume ageostrophic dynamics but ignore the abrupt stratification jump characterizing the tropopause. Alternatively, it has also been proposed that the mesoscale spectrum results from tropopause-altered geostrophic turbulence. This thesis investigates the respective roles of the stratification jump and of ageostrophic dynamics in shaping the near-tropopause flow. To do so, a numerical model is constructed that integrates the non-hydrostatic Boussinesq equations in the presence of an arbitrary stratification profile. Decaying turbulence simulations are performed over a wide range of Rossby numbers both with and without a tropopause, i.e. a sharp change in the background stratification profile. The dynamics are first examined under the quasigeostrophic (QG) approximation. Near sharp changes in stratification QG flows develop comparably sharp vertical gradients of perturbation buoyancy. For realistic atmospheric parameters, not only is the small Froude number assumption underlying QG theory violated, but also statically unstable conditions may develop. Thus, QG strictly holds near the tropopause only if Ro “e, where Ro is the Rossby number and e is the ratio of the tropopause thickness to the vertical scale height. The QG approximation is then relaxed, and leading-order Boussinesq dynamics are analyzed in the weak-flow regime, e ~ Ro “1. Under such circumstances the conservation of Ertel's potential vorticity reduces to the conservation of total stratification. Static stability is thus preserved, and the tropopause undergoes oscillations with amplitudes proportional to Ro. Such weak flows exhibit characteristic features of tropopause-altered geostrophic turbulence, but these vanish rapidly with increasing Ro and distance from the tropopause. For Rossby numbers more typical of the near-tropopause flow, the energy spectrum develops a shallow, near -5/3 tail, which is unambiguously associated with ageostrophic motion. Crucially, this shallowing is apparent at any height, irrespective of the tropopause. Further diagnostics reveal that shallow-range motion is better characterized as unbalanced turbulence, not as inertia-gravity waves. Potential implications for the real atmosphere are discussed."--