利用EMD研究莫拉克颱風背景環流之季內震盪

Use EMD to Study the ISO in the Background Circulation of the Typhoon Morakot

呂昆育(Kun-yu Lu)

指導教授 ： 陳瑞興

國立中央大學/理學院/數學系/碩士(2011年)

摘要

藉由濾波方式可擷取出大氣數據中之10~20天與30~60天季內震盪，為影響颱風運動的重要背景環流。Morlet Wavelet Transform可用來濾波，但是當歷史資料不夠長時，如128天訊號，所產生的邊界效應會使30~60天頻段可信度降低；而直接用Empirical Mode Deconposition(EMD)濾波，則會有隨機的mode-mixing現象產生，造成分析上的不便。本研究先用Morlet Wavelet Transform預處理，再以EMD濾波，可有效率地萃取出10~20天與30~60天波動。

濾波結果顯示：2009年莫拉克颱風侵台前後，風場與外逸長波輻射(OLR)場10~20天與30~60天波動皆為有利引進西南氣流的OLR低相位與西南風，交互作用後，使得與強度相同且路徑相似的2008年鳳凰颱風相比之下，降雨期較長。

從OLR的時間延遲相關分析來看，有利10~20天頻段移動的路徑，大致上與前人研究一致，所以綜合Wavelet和EMD的確可以自即時預報資料128天訊號中萃取出10~20天與30~60天波動。

關鍵字

濾波 ； 季內震盪 ； 預處理

並列摘要

In this project we extract the 10~20 day and 30~60 day introseasonal oscillations from he atmospheric data and then study and analyze these signals. One way to do that is by Wavelet Transform, and the other is by the method of Empirical Mode Decomposition (EMD). However, there are shortcomings in these methods when each is used alone. In our work we use Morlet Wavelet Transform as a pre-filter to filter out the high frequency signal from the data and then use EMD to further filter the remaing signals. As a result we are able to obtain the desired signals with contentment.

Our results show that in the period when typhoon Morakot invaded Taiwan in 2009, the 10~20 day and 30~60 day oscillations were in phase with each other which in turn enhanced Southwest monsoon circulation and moisture transportation and caused more rainfall in Southern Taiwan than that by typhoon Fung-wong in 2008.

Using the time-lagged correlation analysis of the Outgoing Longwave Radiation (OLR), we show that the path along
which the 10~20 day signal traveled is similar to that shown by others. This implies that the filter method we developed in this study can successfully extract the intraseasonal oscillations.

並列關鍵字

pre-filtering ； Wavelet ； Empirical Mode Decomposition ； Intra-seasonal Oscillation ； Filtering

參考文獻

[1] 鄒治華、柯文雄、張卜仁，1999：利用Wavelet分析南海地區季內震盪與東亞季風之研究，大氣科學，28，27-46。

[3] 許晃雄、洪志誠、翁春雄、李明營、羅資婷、郭芮伶、柯重、周佳，2010：莫拉克颱風的多重尺度背景環流，大氣科學，38，1-20。

[4] Torrence, C. and Compo, G. P. (1998): A Practical Guide to Wavelet Analysis, Bulletin of the American Meteorological Society, Vol.79, No.1.

[5] Huang, et al (1998): The Empirical Mode Decomposition and Hilbert Spectrum for Nonlinear and Non-stationary Time Series Analysis, Proc. R. Soc. Lond. A 454,903-995.

[6] Flandrin, et al (2004): Empirical Mode Decomposition as a Filter Bank, IEEE SIGNAL PROCESSING LETTERS. Vol. 11 2, 112-114.