台灣中部山區組織性對流系統渦度之分析

Analysis on Dipole Vortex of an Organized Convective System over the Mountain Range in the Central Part of Taiwan

張茂興(Mou-Hsiang Chang) ； 周仲島(Ben Jong-Dao Jou) ； 何台華(Tai-Hwa Hor)

《大氣科學》 28卷3期 (2000/09) Pp. 177-204

摘要

本文是利用台灣地區中尺度實驗(TAMEX,1987)期間密集的（每4-5分鐘完成一次扇形掃描）雙都卜勒氣象雷達觀測資料，探討台灣中部山區近似滯留之中尺度對流系統內，降水與運動場結構之特徵。1987年6月20日(TAMEX IOP11)午後，在垂直風切微弱與明顯對流不穩定的大氣條件下，於台中與苗栗山區產生由孤立雷雨胞發展至具組織性的中尺度對流系統，生命週期近6小時。此系統發展初期(1316L-1412L)較無組織，主要以個別雷雨胞或多胞狀之雷雨系統發展爲主，降水回波強度大於10dBZ的高度可發展至12公里，最大垂直上升值7m/s，且位於高度7-8公里處。系統發展後期(1622L-1655L)爲本文探討重點，此時已形成具組織性的中尺度對流系統，降水回波強度大於10dBZ的高度可達18公里，最大垂直上升速度增強至18m/s，高度在8-10公里處。另外，發現於此近似滯留之中尺度對流系統內，明顯具有組織性之垂直渦度偶極型態。
經由垂直渦度收支的分析，此組織性中尺度對流系統發展初期，低層（高度2km以下）垂直渦度的主要貢獻，來自輻散項與傾斜項，其餘各高度以傾斜項爲主；在最成熟階段，整層均以傾斜項爲主；於成熟後期，在高度7km以下以傾斜項爲主，但高於7km的高度，顯示輻散項與垂直平流項的貢獻較爲重要。綜合上述的結果，顯示垂直運動場、水平運動場及渦度偶極場先後組織及加強的可能機制爲：對流系統中的潛熱釋放首先增加垂直上升運動速度，而垂直運動場在水平面上產生梯度，且形成局部低壓，使得水平風場在各高度層有不同的加速與傾斜效應，同時垂直渦度偶極主要也藉由傾斜作用而產生。

關鍵字

組織性中尺度對流系統 ； 渦度偶極 ； 渦度收支

並列摘要

The precipitation and kinematic structures of an organized convective system over the mountain range in Taiwan were analyzed by using the high spatial and temporal resolution Doppler data of NCAR CP4 and NOAA TOGA radars. In the early afternoon of 20 June 1987 (TAMEX IOP 11), several convective storms occurred at the sloping area over the central part of Taiwan. These storms were merged from several individual thunderstorms and developed into an organized mesoscale convective system in the late afternoon. This mesoscale convective system lasted for about six hours. Its environment had weak vertical wind shear and pronounced convective instability. The results featured that the growth of the isolated thunderstorm cells appeared disorganized, and moved roughly with the mean wind. The merged cells had the maximum updraft intensity of 7ms^(-1) at the 7-8km height. The height of precipitation echoes greater than 10 dBZ could reach12km.The organized mesoscale convective system in the later stage showed the system was quasi-stationany. The height of precipitation echoes greater than 10 dBZ was more than 18km,and the maximum intensity of vertical velocity was 18ms^(-1) with altitude between 8-10km.It also demonstrated that the horizontal wind field, vertical motion and relative vorticity in the strong precipitation center became more intense and well organized, especially accompanying the feature of a well-defined vorticity dipole.
In order to understand the possible triggering mechanisms of the relative vorticity in the quasi-steady, organized convective system, a vorticity budget was formulated by evaluating various terms ir the vorticity equation using three-dimensional wind fields derived from the dual-Doppler synthesis. In the first stage, below 2km, both divergence and tilting were dominant, while above 2km it was only due to tilting. In the mature stage, tilting had the primary contribution to maintain the positive vorticity in the entire system. The net vorticity tendency with positive value was dominated by tilting below 7km, when the significant precipitation occurred during the mature-to-decaying stage. But stretching and vertical advection were the most important effects above 7km. Conclusively, it is suggested that the latent heat release inside the convective system intensified the vertical velocity first. Then the vertical velocity gradient was produced horizontally and a local mesolow was formed, initiating the acceleration and tilting of horizontal winds at different levels. Also, the vorticity dipole was triggered simultaneously, primarily due to the tilting effect.

並列關鍵字

Organized mesoscale convective system ； Vorticity dipole ； Vorticity budget