摘要(英) |
Precipitation during the Mei-Yu season is closely related to the economic activities, agriculture and human daily life in Taiwan. Because the rainfall during Mei-Yu season comprises about a quarter of the Taiwan’’s annual rainfall amount, the management and the use of water resources often rely on the amount of the rainfall during this period. However, the Mei-Yu rainfall is frequently continuous and sometimes accompanied with thunderstorms and severe heavy rainfall which often cause serious damages in Taiwan.
In order to understand what weather systems triggering heavy rainfall in central and southern Taiwan areas during the Mei-Yu season, this study attempts to classify the weather types which producing heavy rainfall events in the past 20 years by the use of surface and upper-air weather maps. Besides, we use the composite analysis method to reveal the main features of the various weather types in association with the heavy rainfall events, and the scale-separation technique to study the roles of large-scale and synoptic scale atmospheric circulations in producing heavy rainfall from the dynamic, thermodynamic and precipitation mechanism viewpoints. Because the weather systems involved in the Mei-Yu heavy rainfall events occurring in central and southern Taiwan areas are rather complicated, only two of the weather types will be discussed in this study, namely type FSWH and type FSW, in which F stands for the Mei-Yu front, SW stands for the southwest airflow, and H stands for the western Pacific subtropical high pressure.
Type FSWH is mainly affected by the Mei-Yu front, the southwesterly flow, and the western Pacific subtropical high pressure. The meteorological and wind fields depict that, on the day of heavy rainfall event, the western Pacific subtropical high is strengthening and its outflow converges with the southwesterly flow causing strong southwesterly wind in central and southern Taiwan areas. The kinematic fields also show enhancements of convergence and positive vorticity along the Mei-Yu front. The thermodynamic and moisture fields further reveal convective and conditional instability in the lower troposphere over central and southern Taiwan areas. The southwesterly flow will bring warm and moist air to the vicinity of Taiwan from Bay of Bengal, Indo-China Peninsula and South China Sea. It converges with the outflow of the western Pacific subtropical high, which will cause a widely moisture-convergence region. All of the conditions mentioned above are favorable of the occurrence of heavy rainfall.
Type FSW is mainly affected by the Mei-Yu front and the southwesterly flow. From the wind field, we can find that a series of anti-cyclones located to the southeastern and the northwestern sides along the Mei-Yu front, which make the frontal wind shear even enhanced. The kinematic fields also show enhancement of convergence and positive vorticity along the Mei-Yu front on the day of heavy rainfall event. The strong divergence and negative vorticity on the southeastern and the northwestern sides of Mei-Yu front help the development and maintenance of the front. The thermodynamic and moisture fields further show that the southwesterly flow will bring the warm and moist air to central and southern Taiwan areas from Bay of Bengal and Indo-China Peninsula, and then heavy rainfall occurs due to strong moisture convergence. |
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