| dc.description.abstract | Potential Vorticity (PV) is a quantity that combines both dynamic and thermodynamic information, and it is conserved under the adiabatic and frictionless condition. Therefore it is suitable for analyzing tropical cyclones’ (TC) dynamic. In recent years, there have been considerable studies between TCs and the PV. However, it appears that only a few studies using the potential vorticity budget to analyze a real TC case. Thus, what kind of dynamical / thermodynamical interaction processes could be obtained from the PV budget analysis, and what phenomena will be caused by the steep topography of Taiwan are worthy of study.
A compressible nonhydrostatic PV budget equation, derived base on Pedlosky (1987) and Schubert et al. (2001), is used here to gain insights into the PV budget evolution of a typhoon from its oceanic stage to landfall stage. The budget is conducted using high spatial resolution (2-km horizontal grid size) hourly outputs from Yang et al. (2008), in which the Pennsylvania State University / National Center for Atmospheric Research Fifth Generation Mesoscale Model was used to simulate Typhoon Nari (2001) and reproduced reasonably-well results as verified against observations. Subsequently, a series of terrain-sensitivity tests were performed to examine the effect of Taiwan’s topography on PV budget.
When Nari was located on the ocean, its PV distribution exhibited the typical feature in a mature oceanic TC. By the time of landfall, its eyewall was contracted and convection was intensified due to the topography. From the budget perspective, PV was redistributed via horizontal and vertical advections. Latent heating term accounted for major PV generation in lower levels during the oceanic and early landfall stage. And it also acts as a major PV sink term at mid-upper levels. The friction term included both effects of eddy mixing and surface friction; hence, it did not just act as a PV sink term.
In the terrain-sensitivity experiments, if the Taiwan topography was removed, the friction term began to reduce PV over the Taiwan area in lower levels, which was opposed to that for the full-terrain run. As a result, the existence of Taiwan topography could enhance vertical eddy mixing. When comparing latent heating term and friction term, it is evident that both the Taiwan topography and surface friction are prone to trigger convection, releasing more latent heat and leading to the increase of PV. And the cut-off of ocean fluxes such as sensible heat and latent heat flux will cause the dissipation of the PV ring. In the no-terrain experiment, after the typhoon moves into the ocean again, a larger new PV ring formed. The asymmetry latent heating effect occurred on the land-sea interface not only contributed to the formation of this new PV ring, but make this new PV ring became polygonal as well. This phenomenon may also be a reason that causes the typhoon move in a trochoidal manner afterward.
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