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Title: The role of asymmetric structure in tropical cyclone intensity change
Other Titles: Fei dui cheng jie gou dui tai feng qiang du de ying xiang
Authors: Kwok, Ho Yin (郭浩賢)
Department: Dept. of Physics and Materials Science
Degree: Master of Philosophy
Issue Date: 2005
Publisher: City University of Hong Kong
Subjects: Cyclone forecasting
Cyclones -- Tropics
Notes: CityU Call Number: QC942.K85 2005
Includes bibliographical references (leaves 85-88)
Thesis (M.Phil.)--City University of Hong Kong, 2005
xii, 92 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: This study attempts to investigate the influence of asymmetric structure on the intensity changes of a tropical cyclone (TC). Two sets of numerical experiments, which generate asymmetric structure by either dynamic or thermodynamic forcing, are performed using the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model Version 5 (MM5). In the dynamically-forced experiments, a uniform flow is imposed to a TC on either an f plane or a beta plane. A strong uniform flow on an f plane results in a weaker vortex due to the development of a vertical wind shear induced by the asymmetric vertical motion and a rotation of the upper-level anticyclone. The asymmetric vertical motion also reduces the secondary circulation of the vortex. On a beta plane with no flow, a broad anticyclonic flow is found to the southeast of the vortex, which expands with time. Similar to the f-plane case, asymmetric vertical motion and vertical wind shear are also found. This beta-induced shear weakens the vortex significantly relative to that on an f plane. When a uniform flow is imposed on a beta plane, an easterly flow produces a stronger asymmetry whereas a westerly flow reduces it. In addition, an easterly uniform flow tends to strengthen the beta-induced shear whereas a westerly flow appears to reduce it by altering the magnitude and direction of the shear vector. As a result, a westerly flow enhances TC development while an easterly flow reduces it. The vortex tilt and mid-level warming found in the above experiments agree with the previous investigations of vertical wind shear. A strong uniform flow with a constant f results in a tilted and deformed potential vorticity at the upper levels. For a variable f, such tilting is more pronounced for a vortex in an easterly flow, while a westerly flow reduces the tilt. In addition, the vortex tilt appears to be related to the mid-level warming such that the warm core in the lower troposphere cannot extend upward, which leads to the subsequent weakening of the TC. In the experiments with thermodynamic forcing, a TC is placed over an asymmetric sea surface temperature (SST) distribution. The two major factors affecting the TC intensity are the surface heating and the vertical wind shear. The asymmetric SST provides an asymmetric heating under the TC. Subsequent increase (decrease) of latent heat flux is found in the warmer (colder) region, which results in asymmetric convection. Such an asymmetry leads to subsidence and asymmetric vertical motion, which reduces the secondary circulation through subsidence so that a vertical wind shear is induced. The induced shear has a negative impact on the TC intensification. Potential vorticity tendency (PVT) analyses show that the TC moves towards the maximum azimuthal wavenumber-1 PVT, which appears to be the warmer region of the SST anomaly. The TC further intensifies from gaining surface heating from the warm sea. The combining effects of surface heating and vertical wind shear determine the intensity change of the TC. A strong SST differential (> 8 ºC) induces a strong shear that dominates over the effect of the surface heating, which results in weakening. For a TC over a weak SST differential (< 8 ºC), the effect of surface heating more than compensates that of the vertical wind shear, which leads to a second stage intensification. Asymmetric structure therefore, in general, has a negative impact on TC intensification. A strong asymmetric dynamic or thermodynamic forcing leads to the induction of vertical wind shear through the weakening of secondary circulation as a result of subsidence.
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