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Title: The role of land-sea contrast in a tropical cyclone
Other Titles: Hai lu cha yi dui re dai qi xuan de ying xiang
Authors: Wong, Lap Mho (黃立武)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2006
Publisher: City University of Hong Kong
Subjects: Cyclone forecasting -- China, Southeast
Cyclones -- Tropics
Notes: CityU Call Number: QC941.W66 2006
Includes bibliographical references (leaves 149-157)
Thesis (Ph.D.)--City University of Hong Kong, 2006
xxii, 157 leaves : ill. (some col.) ; 30 cm.
Type: Thesis
Abstract: The influences of land-sea contrast through the different surface fluxes of momentum, heat and moisture on a tropical cyclone (TC) are studied. The results are useful for understanding the track, intensity, and structure change of TCs that stay close to a land-sea interface, and in particular for landfalling TCs. Starting from the analytical results of the non-divergent barotropic derivation, we hypothesize that the rotation of the asymmetries by the TC mean circulation could offset the continual creation of asymmetries by differential friction, and results in a much more symmetric TC. Results from the more realistic MM5 simulations suggest that the asymmetric momentum fluxes are responsible for much of the structure changes, while the sensible and latent heat fluxes have small influences. Idealized simulations on an f-plane in the Northern Hemisphere suggest that the creation of large-scale wind asymmetries by asymmetric friction could steer a TC towards land, when the TC is initially 150 km away from a straight coast. The primary reason is that the large-scale divergence in the free atmosphere is larger to the right (relative to the direction pointing land), and leads to smaller vorticity there. The asymmetric gyres thus formed then advect the TC. The asymmetries of the storm core are not the same as at outer radii. The strong inflow of air that moved offshore is responsible for the wind maxima to the left. Furthermore, boundary-layer inflow is strongest to the left and would also be responsible for more severe convection. It is argued that the asymmetric structure of the TC core is analogous to that due to movement. The intensity change of TCs that made landfall along the South China coast is also studied. Just prior the landfall of the TC center, the TCs already feel the influence of land, and none of the moderate/intense TCs intensified. After landfall, an empirical formula with considerable skill is devised to forecast the rate of pressure fill. The decay rate is found to be faster for more intense TCs, larger landward speed, and smaller 850-hPa moist static energy.
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