超級MJO事件之濕靜能收支分析;A Moist Static Energy Budget Analysis of Super Madden-Julian Oscillation Events

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Title:	超級MJO事件之濕靜能收支分析;A Moist Static Energy Budget Analysis of Super Madden-Julian Oscillation Events
Authors:	黃齡慧;Huang, Ling-Hui
Contributors:	大氣科學學系
Keywords:	溼靜能;Moist Static Energy
Date:	2018-08-14
Issue Date:	2018-08-31 11:25:16 (UTC+8)
Publisher:	國立中央大學
Abstract:	前人研究結果顯示，在全球暖化的情境之下，Madden-Julian oscillation (MJO)會隨之增強。因此本篇利用對於過去極端MJO個案進行能量方程收支分析，探討其水平及垂直結構的合成特性。並且將其與一般MJO比較，歸納出造成MJO得以增強為極端個案之原因。超級MJO事件的定義係利用Wheeler-Hendon所提出的MJO指數以及降雨強度作為篩選條件。當MJO指數於降雨相位(第2, 3, 4, 5相位)時，其振幅大於氣候平均兩倍標準差且持續兩個相位以上，且其降雨峰值位於研究目標區域(5°S-5°N, 55°E-85°E)超過9mm/day以上，則定義此事件為超級MJO事件。根據上述定義條件，於1998年至2014年間，共選出六個超級MJO個案。利用此六個案合成分析結果顯示，超級MJO的週期較一般(氣候平均)MJO短 (42.8 vs. 46天)，表示超級MJO東傳的速度增快。同時，從合成對流垂直結構特性發現，一般MJO事件為波數1.5的結構，但超級MJO事件則呈現波數1的結構，可能與超級MJO事件產生了較強且範圍較廣之對流系統，以及其東傳速度較快有關。此外，在濕靜能(Moist static energy)收支分析中，一般MJO事件以及超級MJO事件間最大差異在於第八相位的表現。在超級MJO事件中，濕靜能水平平流項貢獻(負貢獻)減弱，導致濕靜能得以於氣柱中累積，有利於下個相位中產生較強MJO事件，與粗濕穩定度(Gross moist stability)相位分析結果一致。本研究顯示，監測第負七相位時MJO的風場和濕靜能收支可能是預報強MJO個案的前兆。 ;Previous studies indicated that the MJO intensity increases as the earth’s climate becomes warmer. In this study, the column-integrated moist static energy (MSE) budget is employed to investigate the composited horizontal and vertical characteristics of super MJO events and to explore the differences between mean state (climatologically-mean) and super MJOs. Using the Wheeler-Hendon’s MJO index as a metric, along with precipitation intensity, from the ERA-interim reanalysis data and TRMM satellite-retrieved data during the period from 1998 to 2014, a total of 6 super Madden-Julian Oscillation (MJO) cases is identified. We define a super MJO case when i) the MJO index is over two standard deviations above the mean value for at least two consecutive phases over the Indo-Western-Pacific sector; and ii) the peak precipitation intensity is greater than 9mm/day over the targeted equatorial Indian Ocean domain (5°S-5°N, 55°E-85°E). The column moist static energy (MSE) budget analysis provides a quantitative phase-to-phase comparison between mean state and super MJOs. It is found that, aside from magnitude differences, the averaged period of super MJOs is notably shorter than that of mean state MJOs (42.8 vs. 46 days), implying a faster eastward propagating speed for super MJOs. Moreover, the super MJOs exhibit a wavenumber 1 composited structure in Q1 (apparent heat source) and Q2 (apparent moisture sink); while the mean state MJOs show a wavenumber 1.5 composited structure. Furthermore, the greatest contrast between super and mean state MJOs appears at nagetive phase 7 when the accumulation of column MSE is particular strong in super MJOs due to the very weak lateral export of MSE (i.e., very weak negative contribution from the horizontal advection of MSE), which is consistent with the results from GMS (gross moist stability) plane. This study suggests that monitoring the column MSE budget at nagetive phase 7 could be served as a precursor to predict the occurrence of a strong (or super) MJO case.
Appears in Collections:	[Department of Atmospheric Sciences and Graduate Institute of Atmospheric Physics ] Department of Earth Sciences

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