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The Composite Structure of Mesolows Accompanying Heavy Rainfall in the Taiwan Mei-Yu Season



本文選取1985~87年台灣梅雨季5個伴隨豪雨之中尺度低壓個案,利用每3小時之地面資料及每3小時之日本同步衛星(GMS)數據化雲頂溫度資料,以合成方法研究個別低壓、不同生命階段之低壓與不同尺度之低壓結構。合成場包括,氣壓、溫度、温度露點差、流線與等風速線、渦度、散度、平均雲頂溫度及不同雲頂溫度之深對流頻率。 結果顯示,個別中尺度低壓合成結構間存在顯著之相似性與相異性。在梅雨鋒上形成且沿鋒面在台灣海峽東移之低壓,具有較大水平尺度、較長生命期及較深中心氣壓,且豪雨發生在低壓以南之最大輻合區與最強西南氣流區。在台灣西北及西南地區產生之中尺度低壓,豪雨則發生在低壓區域。中央山脈對低層由東北方而來的冷空氣之阻擋效應,顯然為台灣西北部與西南部中尺度低壓形成之重要因子,而海陸摩擦差異導致之中尺度低壓區内之風切渦度,則為中尺度氣旋形成與維持之重要機制。 不同生命階段之合成結構顯示發生在低壓中心以南之深對流與豪雨,顯然透過西南氣流和近地層輻合之增強與中尺度低壓之形成和發展密切關連。深對流/豪雨與最大輻合區之密切關係,更顯示邊界層之摩擦輻合效應在決定對流發展之重要角色。此外,結果亦顯示地面摩擦在中尺度氣旋演變過程裡,有抵消渦旋拉伸之旋生效應。最後,不同尺度之結構異同亦存在於中α與中β尺度之低壓,其特徵則類似個別合成低壓之結構。


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Parallel abstracts

Five cases of mesolows accompanying heavy rainfall in the Mei-Yu season of 1985-1987 were studied. Data used consisted of the 3-h surface observations and the 3-hinfrared digital data of the cloud top temperature observed by the Geostationary Meteorological Satellite of Japan. A composite technique was used to obtain the structure of each individual mesolow, the structure at the different life stages, and the structure for the different space scales. The composite fields included pressure, temperature, dew-point depression, streamline and isotach, vorticity, divergence, mean cloud top temperature, and frequency of deep convections of different cloud top temperature. It was found that similarities and differences existed among all of the 5 cases. The mesolow which formed and migrated along the Mei-Yu front over the Taiwan Strait had a larger horizontal scale, a longer time duration, and a deeper center pressure as compared to that formed locally to the northwest and southwest of Taiwan. It had heavy rainfall occurrence to the south of the low center where the maximum convergence, the stronger southwesterly flows, and deep convections prevailed, as compared to the locally formed low which had heavy rainfall occurrence over the mesolow area. The blocking effect of the Central Mountain Range (CMR) on the low-level northeasterly cold air appeared to be important in the formation of mesolow to the west of the CMR. The shear vorticity due to the land-sea differential friction was suggested to be important for the formation and maintenance of the mesocyclone over the same area. The composite structure at different life stages showed that the deep convections and thus the heavy rainfall to the south of the low center were closely associated with the formation and intensification of the mesolow through the enhanced southwesterly flows and the boundary layer convergence. The close relationship between the deep convections/heavy rainfall and maximum convergence suggested the important role of the surface boundary layer (frictional) convergence in regulating the behavior of convections. The surface friction also possibly counteracted the vortex stretching process in the evolution of the mesocyclone. Finally, the scale dependent structure was obtained in the composite of the meso-α and meso-β lows, similar to that observed in the individual case.

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