The Dynamics of Orographic Rainfall and Track Deflection Associated with the Passage of a Tropical Cyclone over a Mesoscale Mountain

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Title: The Dynamics of Orographic Rainfall and Track Deflection Associated with the Passage of a Tropical Cyclone over a Mesoscale Mountain
Author: Witcraft, Nicholas Charles
Advisors: Dr. Fred Semazzi, Committee Member
Dr. Gary Lackmann, Committee Member
Dr. Yuh-Lang Lin, Committee Chair
Abstract: This thesis is composed of two papers concerning tropical cyclones affecting Taiwan. The first paper investigates the dynamics of heavy orographic rainfall associated with the passage of a typhoon over the Central Mountain Range (CMR) of Taiwan. Included in this paper are sensitivity tests of various precipitation parameterizations. The second paper examines the dynamics of track deflection associated with the passage of typhoons over the CMR. In the first paper, the Penn State/NCAR MM5 Mesoscale Model was used to simulate Supertyphoon Bilis (2000) in order to investigate the dynamics of orographic rainfall associated with the passage of typhoons over the Central Mountain Range (CMR) of Taiwan. In the first part, we identified many factors present in this case to support heavy rainfall, based on Lin et al. (2001). The most important factors appear to be the presence of potential and convective instability, a very moist air impinging on the CMR, and a low level wind maximum associated with the outer circulation of the typhoon. A moisture flux model was also used to estimate rainfall and the relevant dynamics. The remaining portion of the first paper concerns the sensitivity of track, intensity, and rainfall to various subgrid-scale cumulus parameterization and resolvable scale microphysics schemes in simulations of Bilis at 21 and 7 km. The amount of rainfall over the CMR was sensitive to the cumulus scheme, even though most of the rainfall over the CMR was generated by the microphysics scheme. The more active schemes modified and stabilized the environment around Bilis, resulting in less rainfall over the CMR. The track and intensity were also highly sensitive to the cumulus scheme. Varying the microphysical parameterization had relatively minor effects on the simulation. In the second paper, the Penn State/NCAR MM5 Mesoscale Model was used to simulate Supertyphoon Bilis (2000) and Typhoon Toraji (2001) in order to investigate the dynamics of track deflection associated with the passage of typhoons over the Central Mountain Range (CMR) of Taiwan. Bilis was an intense fast moving storm with a continuous track over the CMR. The upper and lower level potential vorticity (PV) centers remained coupled as the center traversed the CMR. The forward speed of Bilis also helped prevent any significant lee-side cyclone reformation. Toraji was weaker and slower moving, and had a discontinuous track over the CMR. Partially due to the slower forward speed, Chinook winds (foehn), a combination of the release of latent heat over the CMR, followed by adiabatic downsloping and warming, had a longer time to generate lower heights in the lee of the CMR. The original low-level center made landfall and dissipated, while the upper level center continued to move northwestward. Without lower level support, the original upper level center also weakened and dissipated. Over time, PV banners/filaments wrapped into the secondary center, which ultimately became dominant. As the secondary center pulled away from Taiwan, it extended into the upper levels. Control parameters for track continuity from idealized studies are calculated for Bilis and Toraji. A conceptual model proposed by Lin et al. 2004 is applied to explain the behavior of the track for each storm.
Date: 2004-03-31
Degree: MS
Discipline: Marine, Earth and Atmospheric Sciences
URI: http://www.lib.ncsu.edu/resolver/1840.16/894


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