TAMEX IOP13鋒面雨帶之個案研究（一）：低層噴流和垂直風切在激發、組織與維持雨帶的角色

Case Study of TAMEX IOP13 Frontal Rainband. Part I: The Role of Low-Level Jet and Vertical Wind Shear in Triggering, Organizing, and Maintaining the Rainband

周仲島(Ben Jong-Dao Jou) ； 洪景山(Jing-Shan Hong) ； 鄧秀明(Shiung-Ming Deng)

《大氣科學》 23卷2期 (1995/06) Pp. 179-207

摘要

本研究利用雙都卜勒雷達合成三維風場及反求擾動壓力場、溫度場，配合綜觀環境條件，針對TAMEX IOP13（1987年6月24－25日）鋒面雨帶個案，探討低層噴流和垂直風切在激發、維持與組織這個雨帶系統所扮演的角色。個案顯示伴隨鋒面的對流雨帶主要發生在地面鋒面的前緣，低層鋒面的界面舉升是對流的激發機制，而中低對流層存在的對流不穩定度則是自由對流的能量來源。鋒前2公里以下有強烈的低層噴流，但噴流的大部分分量均與鋒面平行。高層氣流顯著的北風分量使對流向鋒前傾斜，似乎不利於對流的發展。本研究從力平衡的觀點出發，試圖為此長生命期對流系統的動力行為尋求一個新的定位。就低層噴流而言，由於氣流方向和鋒面大略平行，因此無法期待在低層產生強烈的輻合。但從另一個角度來看，也正因為氣流方向和鋒面大略平行而使得低層噴流可在鋒面帶上“有效地累積”風切不穩定度。當不穩定度釋放時，透過水平流場的擾動可在局部地方產生集中且有效的輻合。本個案顯示鋒面帶在空間上具有週期性斷裂的現象，而其流場的演化與風切不穩定釋放時所伴隨的現象具有高度的相似性。此結果顯示本個案中風切不穩定度在激發和組織對流行為上可能扮演重要的角色。在本個案中對流受垂直風切影響向鋒前傾斜，並且透過對流尺度下沖流之質量堆積及蒸發冷卻在鋒前近地面產生一擾動冷高壓。此擾動高壓將沿鋒方向富含能量的西南氣流向鋒面帶加速以產生有效的輻合。因此高層風切在此反為維持對流發展的有利因子。然而隨著對流的持續發展，擴大範圍的降雨區將阻斷新鮮空氣的內流，強大的氣壓梯度力也會破壞鋒面的結構，這時對流的發展受到抑制，是對流系統自我破壞的階段。待對流系統消散後，meso-α尺度鋒面所擁有的修補能力將重新建立meso-β尺度的鋒面界面，如此形成鋒面－對流系統演化的一個循環週期。總的來說，長生命期對流系統除了需要熱力上的支援外，動力上的配合亦為重要的因子。我們認為本個案中對流雨帶系統之所以能長時間維持，主要是因為高、低層風切適當的配置；而低層噴流透過風切不穩定過程，在對流的激發與組織上扮演重要的角色。

關鍵字

台灣地區中尺度實驗 ； 梅雨鋒面雨帶 ； 低層噴流 ； 垂直風切 ； 風切不穩定度 ； 中尺度冷高壓

並列摘要

The role of low-level jet (LLJ) and vertical wind shear in triggering, organizing and maintaining a convective rainband along the Mei-Yu front was investigated by using synthesized dual-Doppler measurements from TAMEX IOP13 (June 24-25, 1987). Eight consecutive dual-Doppler horizontal winds associated with the rainband were derived from CAA and CP4 radars with time interval of 10 minutes. The perturbation temperature and pressure fields were also retrieved.The rainband moved southeastward with a speed of 4 ms^(-1). The rainband was composed of several long-lived convective cells. These cells occurred over southwest portion of the rainband and then moved eastward. It was observed that a pronounced LLJ (＞15 ms^(-1)) existed in front of the surface front below 2 km height. The angle between LLJ and the front was very small and the major component of LLJ was almost parallel to the front. Speed convergence associated with LLJ along the front was not significant. Most of the convective cells embedded within the rainband developed over the cyclonic shear side of the prefrontal LLJ and dissipated after moved over the jet axis. From 1 km synthesized horizontal wind fields, concentrated vertical component of relative vorticity in a belt shape with intensity larger than 1×10^(-3)s^(-1) was found over the cyclonic shear side of LLJ. In the time period of study, some parts of the concentrated relative vorticity belt broke up and locally-enhanced horizontal convergence was observed. The enhanced low-level horizontal convergence coincided well with the strong upward motion at 5 km height. It is suggested, from the evolutoin of the low-level relative vorticity field, the release of shear instability along the frontal surface at cyclonic shear side of LLJ was the dynamical process responsible for break-up of the concentrated relative vorticity belt. The redistribution of horizontal wind fields produced locally-enhanced horizontal convergence and was responsible for triggering the convective cells along the surface front.The evolution of the long-lived convective cells associated with the rainband revealed different structures at different life stages. At the developing stage, strong updraft (＞ 5 ms^(-1)) reached 10 km height at top of the surface front and slightly tilted to the prefrontal region. The downdraft was weak and not organized and mainly at upper levels. The major precipitation echo was located at frontal region and showed upright feature and slightly downshear tilt. At the mature stage, the updraft was highly tilted and with a much weaker intensity. The major precipitation echo fell flat over the lower troposphere at the prefrontal region. The fall-flat precipitation echo colocated with strong and organized downdraft. In terms of perturbation pressure and temperature, the upshear high and downshear low in the middle-to-high troposphere in the developing stage suggested the interaction between the convective updraft and the environmental vertical wind shear. This pressure pattern provided a horizontal pressure gradient force directed toward prefront and was consistent with the downshear tilt of the convective updraft. The observed high pressure and cold temperature perturbations at the lower troposphere colocated well with the high precipitation echoes at the prefrontal region in the mature stage. This observation suggested the effect of precipitation through evaporation cooling on producing localized mesoscale cold high. The existence of this prefrontal cold high deflected the impinging southwesterly LLJ and increased low-level horizontal convergence over the frontal region. The increased horizontal convergence maintained the highly tilted updraft over the frontal region and, consequently, maintained the long-lived convective cells along the Mei-Yu front.

並列關鍵字

TAMEX ； Mei-Yu frontal rainband ； Low-level jet ； Vertical wind shear ； Shear instability ； Mesoscale cold high