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Title: The role of convection during the early development of a tropical cyclone
Other Titles: Dui liu zai re dai qi xuan fa zhan chu qi de zuo yong
Authors: Wong, Sze Man (黃詩雯)
Department: Dept. of Physics and Materials Science
Degree: Master of Philosophy
Issue Date: 2006
Publisher: City University of Hong Kong
Subjects: Convection (Meteorology)
Cyclones -- Tropics
Notes: CityU Call Number: QC948.W66 2006
Includes bibliographical references (leaves 101-104)
Thesis (M.Phil.)--City University of Hong Kong, 2006
xvii, 104 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: This study represents a detailed analysis of observational data from satellites to understand the contribution of convection in the early development of a tropical cyclone (TC) and how mesoscale vortex interactions affect the length of time for TC early development (ED). Enhanced Geostationary Meteorological Satellite 5 (GMS-5) satellite imageries for the western North Pacific TCs in 2001 and 2002 are used to examine the convection distribution and to analyze the mesoscale vortex interactions during ED, which is defined as the time period when its maximum wind speed first reached 30 knots from 25 knots. The spatial and temporal distributions of convection are examined to analyze the physical processes of convection during ED and to substantiate the physical mechanisms proposed in theoretical studies and numerical modeling. Classification of the patterns, size, strength of convection and evolution of convection pattern are made to identify the different ED evolutions. The variation of distribution of convection indicates physical processes of convection and mesoscale vortex/convection interactions. Over 70% of the studied TCs have the single mesoscale convective system convection pattern (1MCS) appearing at the end of ED and 50% have the multiple mesoscale convective systems pattern (MMCS) appearing at the beginning of ED. About 90% cases with a shorter time period of ED (within 6 hours) have the 1MCS pattern, which also has an important role for ending ED. The physical processes under the 1MCS and MCSs convection patterns are investigated. Dissipation of convection leads to new eruptions of deep convection located near the edge of the dissipating convection core where positive Ekman pumping velocity is maximal. Ingestion of nearby convection, merging from MMCS into 1MCS (meso-α scale) and merging of deep convection (meso-β scale) within the mesoscale convective system (MCS) are associated with the aggregation of vorticity in fast ED. Waxes and wanes of convection associated with accumulation of vorticity were often seen during slow ED. Downdraft of deep convection would split the convection region within the MCS, which can be both conducive and detrimental. The downdraft would dry the air on the periphery of the convection core, thus the near-surface air is cooled and the overlying air is warmed and stabilized by the downdraft. However, the downdraft associated with precipitation would moisten the air at lower level at the mature and/or dissipating stage and that under stratiform precipitation would promote downward growth of mid-level vorticity at the dissipating stage as well. The physical processes of convection proposed in the ED are simulated in a numerical model. The model results show the longer the convective heating supply for the latent heat released between convective clouds, the longer the preconditioning period before TC intensification. A general evolution of the convection distribution patterns in ED, i.e. from the SCAT pattern to the MMCS pattern and then to 1MCS pattern, proposed based on observational results, are simulated in the model results. Thus, the convection distribution pattern could be used as a sign to determine the length of time for ED.
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