頁籤選單縮合
題名 | 低層噴流和中尺度對流系統間的相關性研究=A Study of the Relationship betwen in Low-Level Jet and Mesoscale Convective System |
---|---|
作者 | 宋偉國; 陳泰然; 郭英華; | 書刊名 | 大氣科學 |
卷期 | 25:2 1997.06[民86.06] |
頁次 | 頁211-234 |
分類號 | 328.57 |
關鍵詞 | 低層噴流; 中尺度對流系統; Low-level jet; LLJ; Mesoscale convective system; MCS; |
語文 | 中文(Chinese) |
中文摘要 | 1987年TAMEX期間,於5月15日至16日在華南地區存在一梅雨鋒,其鋒前有低層噴 流 (LLJ) 與中尺度對流系統 (MCS) 發展。 本文目的即在分析此個案觀測資料並利用 Penn/NCAR MM4 中尺度模式進行模擬,以探討 LLJ 與 MCS 的相關性。觀測結果顯示,本個 案雲帶沿著鋒面呈東北 - 西南走向,隨時間往南移動,雲帶內具有深對流之 MCS 系統,生 命期長,原位於 LLJ 北側,之後往南發展,並位於 LLJ 主軸之上。 鋒前 LLJ 之 10ms 以 上強風區範圍,主軸長度於 24 小時期間由 1000 公里擴展至 2000 公里左右,LLJ 出區的 大尺度氣壓梯度力也於此期間加強。在 MM4 中尺度模式物理過程完整的控制實驗 (CNR) 裡 ,對於 850hPa 鋒面位置與 LLJ 分佈之模擬結果, 均和觀測接近,亦即模式能合理的掌握 梅雨鋒面之主要特徵。降水的模擬,在初期落後衛星觀測的 MCS,此種現象主要是在模式預 報初期需一段物理調整時間,唯大致上強降水系統分佈均和衛星觀測的 MCS 接近。 在無對 流潛熱釋放的模式實驗 (FAK) 裡,降水強度與 LLJ 均較 CNR 弱許多, 顯示對流潛熱釋放 對於 MCS 之自我加強與 LLJ 的維持有顯著貢獻。 CNR 模擬的 MCS 伴隨低層輻合、高層輻 散及上升運動。MCS 發展後其潛熱釋放則加強低層輻合與高層輻散,其伴隨之強上升運動, 形成自我加強過程, 使 MCS 繼續發展。 MCS 發展後,對於 LLJ 的加強可能有三種途徑: (1) 在 25 ° N 以北之 MCS 所伴隨的低層非地轉風為南風或東南風,可透過柯氏加速效應 以加強 MCS 南側的 LLJ。 渦度收支方程顯示,對流區低層氣旋式渦度增加主要由輻合而來 ,此種輻合之渦度生成顯然部份由於柯氏作用。 (2) 25 ° N 以南 MCS 伴隨的非地轉風為 西南風,可加強原西南氣流之 LLJ。(3) MCS 潛熱釋放造成低層氣壓的降低,可促使大範圍 暖濕南來氣流往對流區平流,此暖濕氣流可進一步使大範圍氣壓下降,以增加大尺度氣壓梯 度力,經由地轉調整過程加強 LLJ。 |
英文摘要 | A Mei-Yu front existed over southeastern China in 15-16 May 1987 during TAMEX period. Ahead of the Mei-Yu front, a low-level jet (LLJ) and mesoscale convective system (MCS) developed with time. The purpose of this paper is to analyze observational data and to use Penn/NCAR MM4 mesoscale model to simulate this case in order to study the relationship between LLJ and MCS. Observations showed that the along-front cloud band and the embedded MCS moved southward with time. The long-lived MCS originally located to the north side of the LLJ, developed and moved southward to coinside with the LLJ. The length of LLj (≧ 10ms) extended from 1000 km to 2000 km in 24 hours. At the same time, the large-scale pressure gradient force intensified in the exit area of the LLJ. Results of the control run (CNR) in the MM4 model simulation show that the general characteristics of 850 hPa front position and LLj are similar to the observations. It indicates that the model handled the characteristics of the Mei-Yu frontal system reasonably well. In the very early phase of the model run, the time of prectipitation lagged behind GMS observed MCSs. This is probably due to the adjustments time needed for model to form precipitation. Nevertheless overall precipitation pattern is similar to the MCS from satellite imageries. Results of fake run (FAK) without latent heat show that the intensity of the precipitation and the LLJ is much weaker than that in CNR. It indicates that the latent heat released by MCS has significant contribution to the development MCS and LLJ. The CNR simulation shows that MCS is associated with low-high-level convergence/divergence and a significant upward motion. As the MCS developed, the low-high-level convergence/ divergence and accompanied upward motion intensified due to latent heating and led to the MCS self-intensify process. At the same time, the LLJ was enhanced by three different ways: (1) To the north of 25 ° N, the LLJ tended to the enhanced by the Coriolis acceleration in the area of southerly or southeasterly ageostrophic flow to the south of MCS. Vorticity budget computations showed that the low-level cycloic vorticity increase over the MCS area was mainly due to the horizontal convergence. The vorticity generation was apparently partially due to the Coriolis effect. (2) To the south of 25 ° N, the southwesterly LLJ tended to the enhanced by the ageostrophic southwesterlies which were associated with MCS. (3) The low-level pressure was decreased due to latent heating of MCS. This led to the flows of warm and moist air from the south into the MCS area and produced additional pressure decrease. The increase of pressure gradient force then intensified the LLJ through the geostrophic adjustment process. |
本系統之摘要資訊系依該期刊論文摘要之資訊為主。