西南氣流實驗期間之副熱帶海洋性中尺度氣旋個案研究

Abstract

2008年臺灣和美國聯合的西南氣流觀測實驗期間，一副熱帶海洋性中尺度氣渦旋生成於台灣海峽上。本個案研究利用第六次加強觀測期間（6月4日1800 UTC至6月6日1200 UTC）所收集的資料，包括飛機投落送在海面上的量測、地基探空和雷達網連等高空間解析度的觀測，清楚地呈現此中尺度渦旋的運動場及熱動力場結構，並使用新一代中尺度數值天氣預報模式（Advanced Research Weather Research and Forecasting Model，ARW）之高時空解析度的數值模擬，探討其旋生機制及相關的中尺度對流系統發展過程。 2008年6月4-5日，伴隨梅雨鋒面有一明顯氣漩式渦度帶存在，由台灣中部經台灣海峽延伸至香港附近，此一水平風切長帶是由南北兩側具低層噴流的西南氣流以及東風沉降氣流所組成，環境的中低層垂直風切弱（500-925hPa間約為1.5 m s-1），然而深層風切將近18 m s-1（200-850 hPa間）以上，高層有分流。6月4日1800 UTC，衛星觀測到一中尺度對流系統迅速發展，形成逗點狀雲系，在中尺度對流系統的北側發現螺旋狀雲帶，投落送分析顯示伴隨之渦漩的水平尺度約200公里，發展高度約為6公里，呈現向東傾斜的特徵；渦漩中心附近的大氣具有條件性不穩度及潛在不穩度，並顯示出渦漩前側潮濕、後側逐漸乾化的強烈對比，降雨回波演變成不對稱型態，強降水的低層及其前側有冷池，最強暖心出現在300-400 hPa，中低層伴隨有中尺度低壓。在較強的線狀對流雨帶中，雷達徑向風場亦觀測到一個尺度較小的次渦漩，水平尺度大約25-30公里，最大的風切渦度可達3x10-3 s-1。 模擬渦漩顯出與觀測相似的中尺度結構。模擬之對流初始生成於環境渦度帶中，附近之低邊界層較冷，大氣中層潮濕，具中等對流可用位能，低自由對流高度，個別對流胞皆伴隨深而筆直的渦度熱塔，伴隨潛熱加熱的中高層虛位溫擾動可達5K，模擬之中尺度低壓較觀測稍強。渦度分析顯示渦度由約750 hPa向上下增強，最大渦度的高度隨時間下降至近地層，拉伸作用是渦漩加強的主要來源，扭轉項在750 hPa以上為正貢獻。 然而，觀測及模擬都證明乾燥的中層後側入流被渦漩環流加強，甚至有沉降加熱，渦漩後側及中心的乾化不只抑制層狀降水的生成，更使對流系統逐漸遠離渦漩中心。除此之外，在中對流層以上環境的深層垂直風切逐漸增強，限制了渦漩的垂直伸展（<6公里），最終此副熱帶海洋性中尺度渦漩並沒有進一步形成颱風。

This study investigates a subtropical oceanic mesoscale convective vortex (MCV) that occurred from 1800 UTC 4 June to 1200 UTC 6 June 2008 during Intensive Observing Period (IOP) 6 of the Southwest Monsoon Experiment (SoWMEX) and the Terrain-influenced Monsoon Rainfall Experiment (TiMREX). The dense observations over Taiwan, the Taiwan Strait and South China Sea are using to examine the kinematics and thermodynamic structure of the MCV. High resolution simulation through the Advanced Research Weather Research and Forecasting Model (ARW) are adopted to reveal the cyclogenesis and the overall evolution. During 4-5 June 2008, a Meiyu front accompanied with a nearly barotropic vorticity strip across the middle Taiwan through the southern Taiwan Strait which formed as a southwesterly low-level jet developed to the south of subsiding easterly flow. The vertical wind shear between 500-925 hPa was weak, and yet the deep wind shear reached 18 m s-1 between 200-850 hPa. A mesoscale convective system (MCS) developed rapidly after 1800 UTC 4 June. The high-tropospheric difffluence was presented. An eastward tilted MCV was revealed on the northern edge of the MCS with a horizontal scale of 200 km and the depth of 6 km. In the vicinity of the MCV center, the atmosphere possessed conditional and potential instability. The moist front flank and gradually drying rear flank resulted in the asymmetric precipitation. The cool pool was observed beneath the heavy precipitation. The mid-tropospheric mesolow cohered with higher-level warn core. An inner sub-vortex, on a scale of 25–30 km with maximum shear vorticity of 3x10-3 s-1, embedded in the stronger convection line. The simulated vortex has similar structure compared with the observation. The incipient convection initialed within the ambient vorticity. In the vicinity area, there were colder boundary layer, moist mid-tropospheric air, medium convective available potential energy and low level of free convection. The vortical hot towels were cohered with individual convective cells. The convective updraft maintain by latent heating. The vorticity analyses show the vorticity strengthened upward and downward from 750 hPa. The altitude of maximum vorticity decreased to the near boundary layer. Stretching effect was the principal contribution of cyclogensis, tilting effect provided the positive contribution in the mid-troposphere. Nevertheless, the dry rear inflow penetrated into the MCV not only suppressed staitifrom precipitation but also inhibited convection in the upshear direction. Beside, the intensifying vertical wind shear in the higher troposphere limited the vortex vertical extent to about 6 km. The subtropical oceanic MCV did not further grow into a Typhoon.