Title: Effect of topography on tropical cyclone tracks
Authors: Tang, Chi Kit (鄧志傑)
Abstract: The effect of Taiwan, China coastal and Philippines topographies on tropical cyclone (TC) tracks are investigated by conducting idealized experiments using the Weather Research and Forecast model. In each experiment, different parts of the terrain are set to be either present or absent in the domains and an artificial TC is inserted at a location far away from the topographies so that the full mechanism of the track changes due to the topographies can be investigated. The potential vorticity tendency (PVT) diagnostic approach is employed to study the relative contributions of various processes to the movement of the TC. In each set, two experiments are conducted, one having real topographies and the other without but the land areas are replaced by grass lands. The physical processes are investigated by subtracting the parameters in the without-topography experiment from those in the with-topography experiment. Several sets of experiments with different initial latitudes of the TCs, TC sizes and steering flow strengths are conducted.
In the experiments without a steering flow and on a beta plane, a terrain-induced gyre pair is clearly found after the subtraction between with- and without-topography experiments once the TC circulation encounters the topography. This gyre pair is then advected cyclonically around the TC centre by the TC circulation. The gyre-associated flow, which contributes to the horizontal advection term in the PVT calculation, advects the TC northward leading to a northward deflection.
The intensities and the movement of the terrain-induced gyres vary with TCs initially at different latitudes, which subsequently lead to different extents of the northward deflection of the TCs. A southwestward deflection before the onset of the northward deflection is even found for a TC passing south of Taiwan. In the presence of China terrain, an additional larger but weaker anticyclonic gyre is induced, which enhances the southwestward but reduces the northward deflection.
For TCs with different sizes, there is an apparent optimal TC size with the largest northward deflection. Also the onset location of the northward deflection is closer to Taiwan for smaller TCs. Near-shore southward deflection is found in some smaller TC cases making landfall on Taiwan, which is caused by a near-shore induced gyre on the southeastern side of Taiwan. The flow pattern of the TC over the western side of Taiwan eventually leads to a looping or V-shape track on the eastern side of Taiwan.
In some cases, diabatic heating term in PVT calculation is also important near landfall, which results from several mechanisms: an incursion of dry air from the mountain, low-level convergence induced by the terrain-altered wind field in the southwestern part of the TC, the development of convergence cyclonically inward from the eastern side of the mountain to the TC core and the vertical wind shear induced by the gyre-associated flow in smaller TC cases.
In the cases with an easterly steering flow, the terrain-induced gyre pair is also found in addition to the lee vortices generated by the steering flow over the topography. A small near-shore southward deflection is found for some cases and larger TC under a weaker steering flow is more likely to have such a deflection.
Notes: CityU Call Number: QC959.E18 T36 2015; xxv, 146 pages : illustrations 30 cm; Thesis (Ph.D.)--City University of Hong Kong, 2015.; Includes bibliographical references (pages 140-146)
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