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TAMEX IOP - 13颮線個案特徵之觀測研究

An Observational Study on the characteristics of the TAMEX IOP-13 Squall Line Case


本文使用1987年TAMEX期間之密集觀測資料,探討IOP-13颮線個案(6月24-25日)之結構特徵及地面附近伴隨之天氣現象,並根據Parker and Johnson (2000)之分類,3與IOP-2個案作一比較。木顯示,低層西南噴流、地形效應以及梅雨面提供輻合與舉升機制,三者結合引性,降水下衝流與環境西南氣流輻合,在舊對流胞南側引發新對流胞,使颮線由上逐漸南移至前暖區並增強發展,但由於其移速遠較一般副熱帶颮線系統,地面附近之陣風面、中尺度高壓、冷池、尾流低壓等擾動均可辨認。陣風面通過時,則伴有風向轉變、風速先減弱後增強、氣壓上升、溫度與露點下降、以及層狀降水。至於對流性降水,則在陣風面通後約1h始達最大強度。 另外,IOP-13颮線屬於無明顯層狀降水區伴隨之平層狀型,其低層環境具垂直於系統之且主要由前向後垂直於颮線主軸。兩者走向與環境風切之關聯,以及移速快慢之差異,均顯示與北美地區此兩型個案之平均特徵相似。雖IOP-13颮線移速緩慢,但由於西南底層噴流提供水氣與對流能量來源以及強底層風切,使其能維特較長生命期。最後,地面分析顯示IOP-13所伴隨之颮前低壓與尾流低壓均較IOP-2者不顯著,由於IOP-13個案並無伴隨拖曳層狀降水區此結果似亦與理論預一致。

Parallel abstracts

Using data collected by the dense observational network in the 1987TAMEX (Taiwan Area Mesoscale Experiment), the present study inverstigated the squall line during Intensive Obsreving Period (IOP)#13 (24-25 June). The structural characterisitics and associated werther phenomena were discussed, and compared with another case during IOP-2 based on the classification by Parker and Johnson (2000). Results suggest that a strong low-level jet (LLJ), the topographic effect, and the arrival of a surface Mei-yu front all contributed to the initiation of the IOP-13 squall line. This convective line, aligned fromwest-southwest to east-northeast, exhibited multi-cell behavior, with new cells developing to the south of old ones due to interactions between precipitation downdrafts and environmental southwesterlies. As a result, the line gradually moved southward into the pre-frontal warm sector. Its speed of about 5 m sΛ(-1), however, was significantly slower than those of most subtropical systems, ans heavy rainfall was caused along the northwestern and western coast of Taiwan. The low-level cold outflow induced by the IOP-13 case also showed signs to affect ambient airflow and subsequently the movement of the Mei-yu front. Neat the surface, the gust front, mesohigh, cold pool, and wake low were all identifiable. Weather phenomena associated with the passage of the gust front included a shift in wind direction, a reduction in wind speed followed by an increase, a rise in atmospheric pressure, as well as a decrese in both temperature and dew point. The gust front was accompanied first by stratiform and later by convective precipitation, which can reach a peak intensity of 20-30mm/30 about 1 h later. In addition, IOP-13 squall line can be classified as a parallel stratiform (PS) type, with strong wind shear at low levels in the direction perpendicular to the line and shear nearly parallel to the line in middle troposphere. The IOP-2 case, on the other hand, is a typical trailing stratiform (TS) event since the environmental shear was deep and more perpendicular to the main line. The orientation of the lines relative to environmental wind shear, as well as the propagation speed of the two squall lines are all consistent with the general characteristics of these two types in the United States. The duration of IOP-13case (bout 16 h) was unusually long, and was a result of the pre-frontal LLJ that provided strong low-level shear and abundant supply of moisture as source of latent energy needed to drive the convection. Finally, both the pre-squall low (trough) and wake low associated with the IOP-13 squall line appeared less visible than those in IOP-2. This result also appears to be consistent with the theoretical expectation, since there was no trailing stratiform region in the IOP-13system.

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