2007年梅雨季期間之颮線個案分析

吳慧玲(Hui-Ling Wu)

指導教授 ： 陳台琦

國立中央大學/地球科學學院/大氣物理研究所/碩士(2008年)

摘要

摘要
針對2007年梅雨季降雨量較多的北部地區做各測站累積降雨量統計，發現在6月5日至6月10日有明顯的持續降雨事件，又進一步配合其他氣象資料，找出有興趣的個案發現6月6日1400UTC有一完整的颮線系統由大陸東南沿岸慢慢往台灣移動。所謂颮線為活躍且深厚之組織性對流窄帶，常發生於春夏季之中緯度與熱帶地區，為台灣地區梅雨季常見的劇烈天氣現象。本研究利用中央大學的雙偏極化都卜勒雷達資料來深入探討其颮線結構，除了與文獻上颮線內部流場結構特徵比較外，並加上由偏極化雷達參數推導之雲物理場，為國內首度利用偏極化雷達針對梅雨鋒面做雲物理特性分析。
綜觀天氣分析發現，地面天氣圖有一明顯的鋒面系統持續影響著台灣地區，850mb天氣圖則發現有低層噴流(LLJ)存在，500mb有短波槽，200mb的風有輻散的情形。在眾多有利於中尺度對流發展的條件下，系統隨之發生。
由垂直風場結構圖發現颮線上對流胞的西南側有暖濕的西南氣流移入，此暖濕氣流為主要提供系統發展的入流，而在主對流胞的東邊吹的是較乾冷的東北風，當此乾冷的東北風遇上主對流胞西南側暖濕的西南氣流，較暖濕的西南氣流則會沿著較乾冷的東北下層氣流而向上爬升，在此介面上因為氣流較不穩定，因而造成對流，形成新的對流胞。
本文也利用雙偏極化雷達所求得的偏極化參數ZH、ZDR、KDP來反演降雨粒徑分布DSD參數NW及D0，進而用這些參數來看颮線系統變化特徵。由垂直剖面圖發現在對流初生時期降水多由雨滴個數較少及中值體積直徑較小的雨滴所組成，而對流胞成熟階段發現各個參數都很大，此時期降水又分成兩部分，對流胞前緣(南邊)多由雨滴個數較少及中值體積直徑較大的雨滴所組成，對流胞後緣多(北邊)由雨滴個數較多及中值體積直徑較小的雨滴所組成。推斷前半段為前向後氣流的入流區，上升運動較強，小水滴不易往下掉，而後小水滴會隨著風場向上且向後掉入後半段區域內，後半段上升運動為較弱的情形，所以小的水滴就往下掉，使得後半段有較小且較多的水滴，此結果和風場是符合的。最後發現反演的DSD參數和原來的偏極化參數都很一致。

關鍵字

雙偏極化雷達 ； 雨滴粒徑分布

並列摘要

Abstract
According to the statistics of the accumulated rainfall at northern station in Taiwan during the Mei-Yu season 2007, there were obvious heavy rainfall events from June 5th to 10th . After thoughly investigating the data of CWB, it exhibited that a squall line moved slowly from southeast coast of China to Taiwan. We have used NCU C-band dual-polarization (C-Pol) radar to confer the structure of squall line in this thesis. In addition to the study of the inner flow structure characteristics, we also added the discussion of the cloud physical phenomena revealed by the dual-polarization variables. It is the first time in Taiwan to analyze cloud microphysics of the Mei-yu front precipitation systems using dual-linear polarization Doppler radar .
From weather analysis, the surface weather map show there was a well defined frontal system influencing Taiwan during the whole period. We detected a LLJ in 850mb weather map, a short wave trough in 500mb and divergence in 200mb. The system on top of the Taiwan island enhanced with the above-mentioned advantage conditions for meso-scaled development.
The vertical wind structure showed that there was warm and wet south-west flow moving into the southwest leading edge convective cells on the squall line. This flow provided the main inflow for the developed system, and the wind on north side of the main convective cell was colder and dryer northeast wind. When this northeast flow encounter with the southwest flow, the flow will move upward along the northeast in southwest flow and across it. It enhanced the new convective cells by the release of convective instability which was provided by the unsteady flow in this system.
From the vertical cross-section of the retrieved DSD parameters (NW and D0), the precipitation systems were dominated by relatively less drops number and smaller drops size at the initial stage of a convective system.
Nevertheless, the DSD parameters: Nw and D0 increase notably at the mature stage of convective system. In this stage, the precipitation system was composed by two different characteristic of the DSD. The leading edge (southern part) of the convective cells accompanied with less drops number and larger drops size. However, the relatively higher concentration of raindrops and relatively smaller drop size at rear part (northern part) of convective cell was observed from NCU CPOL.
As a result we concluded that the stronger upper motion associated with the front to rear air flow will cooperate with the downward motion. Consequently, the rain droplets will be carried toward the rear part of the flow, and only relatively big drop can reach the surface. Since the rear part of the system has less upper movement, the droplets will have the tendency to descent. Therefore, the results agree with the wind field and the polarimetric measurements. It also reflects that the dynamical and microphysical DSD consistent which were retrieved from the original polarization parameter.

並列關鍵字

dual-polarization radar ； DSD

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