Date of Award

2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Environmental Science

First Advisor

Lin, Yuh-Lang

Abstract

With Geophysical Fluid Dynamic Model and Weather Research and Forecasting model, two tropical weather systems, African easterly wave (AEW) and Typhoon Morakot (2009), are studied, respectively. With regards to AEW, wave-like disturbances were generated on the lee side by vortex shedding for an easterly flow over an idealized mountain, with a wavelength of about 2000 km, a period of ~2-3 days, and a propagating speed of ~9.6 ms-1 which mimic an AEW. Thus an AEW may be generated by vortex shedding. Furthermore, these orographically generated hydrostatic, continuously stratified inertia-gravity waves on a β-plane are characterized as mixed Rossby-gravity waves based on the dispersion relationship. The dispersion relationship gives a phase speed of about 10 ms-1, which is comparable to numerically simulated 9.6 ms-1 and the observed 8 ms-1 . Hence, the AEW-like disturbances are associated with orographically forced Rossby waves. As for Morakot, I found that in the ocean stage the upper outflow of the secondary circulation is not very clear. The eye disappears within two hours after landfall. The deep convection, secondary circulation and tilting maximum wind speeds disappear, and the eye wall corrupts within five hours after landfall. The wind speed, convergence, upward motion, and relative humidity close to the Morakot center are limited when it crossed northern Taiwan; whereas large wind speed, convergence, upward motion, and humid regions are away from the Morakot center to the south with heavy rainfall because of the interactions between Morakot circulation, the high and steep Central Mountain Range, and the prevailing southwesterly wind. In the comparison with five typhoons in recent years causing large accumulated rainfall in Taiwan, the abundant water vapor around Taiwan outweighs translation speed and orographic lifting in leading to the record-breaking precipitation.

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