帶狀化時間診斷

Filamentation Time Diagnosis

蔡禹明(Yu-Ming Tsai)

指導教授 ： 郭鴻基

國立臺灣大學/理學院/大氣科學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.01091

摘要

本論文利用球座標等熵面上的帶狀化時間，診斷中緯度槽線窄化與割離低壓形成的過程、南極極渦波動破碎，以及梅雨鋒生的個案。流體細絲帶狀化的概念源自亂流，帶狀化時間的定義爲渦度梯度成長率的指數折疊時間。研究結果顯示，帶狀化時間可以清楚定量地提供槽線窄化及切斷的位置與時間尺度。比較不同緯度槽線窄化的過程，顯示較高緯度槽線窄化的過程較低緯度快，所對應的帶狀化時間亦較短。由梅雨鋒生個案的研究，顯示帶狀化時間的分析結果與變形部分的鋒生函數十分吻合。而在計算帶狀化時間的過程中，我們可忽略地球曲率項以達到簡化的目的；但爲保持精確度需保留等熵面上的輻散項。我們亦進一步推導包含非絕熱過程的帶狀化時間，比較結果發現，包含非絕熱過程在槽線割離的過程中，會診斷出更多的快速帶狀化區域；而在梅雨鋒生的個案中，使帶狀化時間的診斷與鋒生函數的診斷更爲吻合。我們亦利用正壓模式模擬中心渦旋與外圍對流的交互作用，比較強迫作用的時間尺度與帶狀化時間的長短對於渦旋模擬的影響。同時，爲了使高解析度的數值計算能更有效率，我們也發展出了切比雪夫波譜法搭配區域切割的平行計算方法。本論文研究結果顯示，帶狀化時間的診斷有助於分析綜觀尺度帶狀的流體。

關鍵字

帶狀化時間 ； 割離低壓 ； 南極極渦 ； 鋒生 ； 非絕熱加熱 ； 正壓模擬 ； 切比雪夫波譜法

並列摘要

In this dissertation, the filamentation time in spherical coordinates on an isentropic surface has been derived and used as a diagnostic tool to analyze both midlatitude and tropical trough thinning processes, wave breaking of Antarctic polar vortex, and mei-yu frontogenesis events. Filamentation time is defined as the e-folding time for the vorticity gradient growth rate. The diagnostic is an accurate measure of stirring processes under the condition of slowly varying velocity gradients. For the trough events, the results indicate that the filamentation time for the tropical trough thinning event is generally longer than those for the midlatitude trough events. From the mei-yu frontogenesis study, the results suggest filamentation time diagnosis matches well with the deformation part of the frontogenetical function. In addition to the effects of stretching and shearing deformation, the filamentation time diagnostic contains the effects of divergence. For the calculation of filamentation time on isentropic surfaces in spherical coordinates, it is acceptable to ignore the curvature effects in the tropics, but in both the midlatitudes and the tropics, isentropic divergence effects should be retained for improved accuracy. The modified filamentation time formula which includes the diabatic heating effect is derived and analyzed with trough thinning events and mei-yu frontogenesis event. In the trough thinning events, the results suggest the filamentation time with diabatic heating effect could capture additional local filamentation zones caused by the local convection, but for the whole cutoff process, the diabatic heating effect seems to play secondary role in the filamentation time diagnosis. In the mei-yu frontogenesis event, including the diabatic heating effect is beneficial to match better with the frontogenetical function analysis. Barotropic simulation is presented for comparing the forcing time scale and the filamentation time scale. For high resolution simulation, Chebyshev spectral method with parallel domain decomposition method is developed. In conclusion of this dissertation, the results suggest that the filamentation time diagnostic can serve as a useful aid in the analysis of observed synoptic banded flows.

並列關鍵字

filamentation time ； cutoff low ； Antarctic polar vortex ； frontogenesis ； diabatic heating ； barotropic simulation ； Chebyshev spectral method

參考文獻

Appenzeller, C., H. C. Davies, and W. Norton, 1996: Fragmentation of stratospheric intrusion. J. Geophys. Res., 101, 1435-1456.

Bowman, K. P., 1993: Barotropic simulation of large-scale mixing in the Antarctic polar vortex. J. Atmos. Sci., 50, 2901-2914.

Bowman, K. P., and N. J. Mangus, 1993: Observations of deformation and mixing of the total ozone field in the Antarctic polar vortex. J. Atmos. Sci., 50, 2915-2921.

Chen, G. T.-J., C.-C. Wang, and A.-H. Wang, 2007: A case study of subtropical frontogenesis during a blocking event. Mon. Wea. Rev., 135, 2588-2609.

Ferreira, R. N., and W. H. Schubert, 1999: The role of tropical cyclones in the formation of tropical upper-tropospheric troughs. J. Atmos. Sci., 56, 2891-2907.