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Title: Evolution of tropical cyclone structure during landfall under the influence of a uniform flow
Other Titles: Zai jun yun liu ying xiang xia re dai qi xuan deng lu shi de jie gou yan bian
Authors: Shum, Chi Tai (沈志泰)
Department: Dept. of Physics and Materials Science
Degree: Master of Philosophy
Issue Date: 2006
Publisher: City University of Hong Kong
Subjects: Cyclones -- Tropics
Notes: CityU Call Number: QC942.S55 2006
Includes bibliographical references (leaves 82-86)
Thesis (M.Phil.)--City University of Hong Kong, 2006
xiv, 87 leaves : ill. (some col.) ; 30 cm.
Type: Thesis
Abstract: This study attempts to investigate the structural evolution of a tropical cyclone (TC) during landfall under the steering of a uniform flow. Idealized numerical experiments are performed using the fifth-generation Pennsylvania State University – National Center for Atmospheric Research Mesoscale Model (MM5) version 3.7 on an f plane. The spun-up vortex is steered towards the coastline under easterly flows of uniform strength of 4, 6 or 8 m s-1. Asymmetries in total wind distribution, vertical motion, rainfall and potential vorticity (PV) are observed in all cases. Prior to and during landfall, the location of low-level maximum wind at σ = 0.995 is found in the offshore quadrant while that at σ = 0.87 is found in the onshore quadrant, which implies an increase in low-level vertical wind shear. The maximum rising motion at σ = 0.91 is just downshear of the southeasterly σ-level shear vector and is located in the northwest quadrant. Maximum rainfall is observed in the south to southwest quadrant of the vortex just before landfall, just downstream of such strong rising motion. As the storm moves nearer to the coastline, moisture from the land surface is advected up by such stronger rising motion and rotates cyclonically to spread over the whole western quadrant of the vortex. The distribution of specific humidity at higher levels of σ = 0.825 and σ = 0.775 shows a more moist region in the western quadrant and relatively drier region in the eastern quadrant. Such moisture transport explains the location of maximum rainfall of the storm prior to landfall. After landfall, increased instability of the atmosphere results in more rainfall in all quadrants of the vortex. The distribution of specific humidity over land at the levels σ = 0.825 and σ = 0.775 becomes more asymmetric after landfall, with more mixing of moist and dry air masses over land. Such mixing has resulted in a more unstable atmosphere. The 850-200 hPa shear vector is generally northeasterly in all cases before landfall. The maximum rainfall location is approximately in the frontal-left quadrant of such shear vector. After landfall, the stronger the uniform flow, the faster the weakening of the vortex. Tilting in the low-level PV is observed in all cases, but not obvious in mid- to upper levels. Comparing the magnitudes of both the σ-level and pressure level vertical wind shear with their respective controls, the shear in the landfall cases is obviously larger during and after landfall. An associated mid-level warming is found such that the vortices in the landfall cases have an increased temperature anomaly in the mid-levels relative to their respective control runs. It is noted that, in addition to frictional dissipation, the development of stronger vertical wind shear and the subsequent mid-level warming developed during and after landfall may be some of the mechanisms contributing to the weakening of TC over land.
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