| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 102 === When Typhoon Megi (2010) was still distant from Taiwan, located to the west of Luzon islands, it took a sudden track change from westward to northward over the South China Sea, leading to heavy rainfall in northeastern Taiwan and the adjacent seas during 0000 UTC 19-23 October. Since previous studies often highlight the accuracy of the Tropical Cyclone (TC) track as a key factor for accurate forecast of typhoon-related rainfall, the uncertainties of 1) the sudden recurvature of Megi, and 2) precipitation over Taiwan under Megi’s remote effect are both examined with ensemble simulations based on ensemble Kalman filter data assimilation system and the Advanced Research WRF (ARW) model in this study.
Based on the piecewise PV diagnosis and steering flow analysis, our results show that the eastward retreat of subtropical high (SH) and the westward retreat of monsoon trough (MT), the former located to the northeast and the latter located to the southwest of Megi, provide a favorable synoptic environment for Megi’s sudden recurvature. In addition, an anticyclone (AC) developing over the tropical area, expanding northward to the east rear of Megi, also contributes northward steering flow. After Megi’s sudden poleward turn, the approaching mid-latitude upper-level trough (UTR) helps enhance Megi’s northward movement. The diversity of simulated track among different members is affected by the uncertainty in simulating the expanding areas of AC and MT, the timing of a break of the band-like subtropical high pressure system, and the strength of UTR, all of which can further lead to different steering flows.
The quantitative evaluation of precipitation forecast based on the equitable threat score (ETS) shows that the TC track is not the dominant factor affecting the performance of rainfall forecast in this remote rainfall event. The uncertainty of remote rainfall simulation is affected by the area covered by the northeasterly monsoon, TC size, the direction of prevailing wind, mountain orientation, and the slopes of windward mountains. The results show that the low-level moisture convergence of northeasterly monsoon and the outer circulation of TC provides a moisture-abundant environment, which is favorable for the occurrence of rainfall. Then, it is the prevailing wind that further advects the moisture inland, and causes strong low-level convergence and vertical motion over the steep mountain areas at the south side of Yilan plain, where stronger orographic lifting results in heavy precipitation.
Based on ensemble simulations, our study not only investigates the dominant factors leading to Megi’s sharp recurvature but also evaluates the uncertainty in TC’s remote effect on precipitation and associated detailed mechanism, which are relatively limited in other studies. Our results also provide new insight into the precipitation under the typhoon–monsoon–terrain interaction, and can be applied in assessment of numerical products for real-time forecast, especially for the remote rainfall events occurring in northeasterly monsoon season.
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