| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 96 === Both observation and numerical studies showed that typhoons are prone to be deflected when approaching high topography. Sometimes, the track could be sharply deflected, and looping motion might occur. The warning areas, which are lashed by severe rainfall and gust, could shift greatly due to the sharp turn. This is a very important typhoon forecast problem. However, research issues with such unusual typhoon motion have not been well addressed in literatures. Few idealized numerical experiments have been conducted based on a full-physic model with high resolution. The mechanisms leading to the sharp turn, which is an interesting scientific problem, have not been well investigated.
High resolution (3-km) simulations are conducted using MM5 model to investigate the mechanisms leading to the loop motion of the typhoon, and the role of Taiwan topography. The results show that high topography plays an important role in Typhoon Krosa’s looping. The southward track deflection is reduced when the terrain elevations are lowered, and vice versa. An experiment with coarser resolution (9 km) cannot well capture the southward turn. Differences in the topography from two model resolutions appear not critical to the simulated track. Impacts of the model resolution on the vortex structure and physics processes still need to be addressed in the future.
Idealized experiments are also conducted using MM5 model with a 3-km mesh. These simulations show that the more the vortex approaches (leaves) the northern (southern) topography, the more southward (northward) deflection occurs.
Both the real-case and the idealized studies indicate that strong winds at the channel between the storm and topography are enhanced when great deflection occurs. The asymmetric flow within the inner-core appears to contribute to the track deflection. Meanwhile, the backward-trajectory calculation of the low-level air parcels shows the confluent trajectories when the air column passes over the channel between the high terrain and the vortex. The results of the backward trajectory analysis are consistent with the concept of the channeling effect.
To fully investigate this issue, more sensitive experiments will be designed in the future. The impact of the magnitude of the ambient flow, vortex intensity and structure, and the impinging angle will be further studied to identify the flow and parameters regimes leading to the sharp track deflection and the looping motion.
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