| Summary: | 博士 === 國立臺灣大學 === 大氣科學研究所 === 101 === The thesis investigates the influence of topographic induced convection on the landfall typhoon motion in Taiwan. We investigate the track deflection by the topographic induced convection before the typhoon landfall and the translation speed change after the landfall. The pre-landfall typhoon track deflection is studied when the typhoon is within 200 km from the coast. The track deflections are investigated in terms of the deflection points and deflection angles, which represent the location where the largest curvature and the most significant deflection occur. Our analysis indicates the geographic asymmetric distribution of deflections. Namely, of the cases with deflection angle greater than 20°, there are 75% (9/12) with deflection points located northern than 23.5°N. For deflection angles smaller than 10 degrees, the ratio of south cases exceeds 71% (5/7). Further analysis of translation speed indicates that cases with larger angle and the looping track type usually has a lower average translation speed than small angle subgroups in the early stage 12 hours before landfall.
For the landfall typhoons, our analysis also suggests the geographic asymmetry. Data for 84 typhoons that reached Taiwan’s eastern coast from 1960 to 2010 are analyzed, with motions compared to the long-term average overland translation speed. For 61 continuous track typhoons among all cases, we find that 77% of the slow-moving tropical cyclones (TCs) made landfall on the northern end of Taiwan’s eastern coast, while 60% of the fast storms had southeast coastal landfalls. This geographic asymmetry with respect to typhoon translation speeds widened after landfall, as the slow-movers typically decelerated during the overland period while the faster TCs sped up. In particular, the average overland duration was 16 hours for the slow class, compared to only 3 hours for the fast-moving typhoons. The combination of slower translation with longer duration for the northern class of TCs led to large rainfall on the southwestern slope of the island’s Central Mountain Range.
Weather Research and Forecasting Model experiments are used to study the effect of convection on storm motion over a mountainous island resembling Taiwan. The topographic induced asymmetric diabatic heating can alter the typhoon track before and after the landfall. We also find that the topographically phase-locked convection acts to slow down (speed up) the northern (southern) landfalling typhoons. Our model results also suggest a positive feedback mechanism that exists for the slow storms, in which the convective heating pattern forced by topography acts to reduce the TC motion, leading to even more prolonged precipitation and heating, yielding further speed reductions after landfall for northern landfalling typhoons.
Our experiments also suggest that the asymmetric diabatic heating may contribute to the cyclonic typhoon track deflections before landfall. The mechanical lifting at windward slope as well as the down slope wind at leeside of terrain both contribute the asymmetric convection and diabatic heating near typhoon before landfall. In addition to the mechanical forcings in the presence of mountain, diabatic heating may play an important role affecting storm motion. In particular, the impact of the diabatic heating is significant when the environmental mean flow is weak.
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