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標題: | 對流-輻射在熱帶季內震盪演化中的角色 The Roles of Convection and Radiation in the Evolution of Tropical Intraseasonal Oscillation |
作者: | 戴祥任 Hsiang-Jen Tai |
指導教授: | 隋中興 Chung-Hsiung Sui |
關鍵字: | 馬登-朱利安震盪,大尺度波動動力,垂直模分解,對流耦合之凱爾文波,淺對流,雲輻射效應, Madden-Julian Oscillation,large scale wave dynamics,vertical mode decomposition,convective coupled Kelvin wave,shallow convection,cloud radiative feedback, |
出版年 : | 2023 |
學位: | 碩士 |
摘要: | 本研究透過兩種方式探討馬登-朱利安震盪(Madden-Julian Oscillation, MJO)之東西不對稱特徵。首先,針對位於印度洋上的MJO,我們將其環流、水氣、對流場做了合成分析,分析結果顯示在MJO對流東側,有東風與邊界層輻合從海洋大陸延伸至西太平洋(100°E-160°E),且此區域之水氣正值與上升運動位於低層大氣;反之,在MJO對流西側,水氣正值與上升運動則是位於高層大氣。若將MJO的垂直結構做線性分解,則可看到MJO主要的東西向翻轉環流是由第一斜壓模構成,且其範圍相當寬廣(60°E-170°E),而第二斜壓模中心則是相對第一斜壓模中心向西平移15°。同時,本研究也透過CloudSat衛星資料,觀察到經向翻轉環流中東西不對稱之雲種與輻射加熱率分布,在MJO對流東側,由於受到第一斜壓模下沉運動壓抑,該區域多為淺雲,伴隨著中層大氣的長波輻射冷卻;而在MJO對流中心,則有較多的深對流雲,產生明顯的溫室效應加熱;最後在MJO對流西側,則是以中層大氣層狀雲為主,在層狀雲上方的雲頂冷卻與下方的溫室效應影響下,垂直的輻射加熱率呈現極大對比。
接著我們透過在全球環流模式(Global Circulation Model, GCM)中設置了4組水星球實驗來探討MJO的東西不對稱特徵,實驗中海溫不隨時間改變,東西方向沒有梯度,而南北方向梯度接近於實際觀測,使用預設模式物理參數法之實驗為控制組(CNTL),另2組與CNTL不同的實驗分別關閉了淺對流(NSC)與雲輻射效應(NCRF)。所有實驗皆能在赤道上模擬出東傳的波動,且這些波動能組織成大尺度對流雲簇(Super Cloud Cluster, SCC),從時空頻譜分析上,可認定這些SCC為對流耦合之凱爾文波 (Convective Coupled Kelvin Wave),且分析結果顯示此波動是頻散的。實驗中SCC的結構與觀測的MJO相似,但其東西向尺度(70°-80°)小於MJO,而SCC的相速度在不同實驗中也有所不同,在CNTL中,SCC的相速度區間為每秒8-15公尺,在NSC與NCRF中,SCC的相速度區間則為每秒12-18公尺。在CNTL中,速度較慢的SCC通常相對速度較快的SCC更具組織性,且伴隨較寬廣、較強的淺對流區,同時也會有耦合的羅士培波(Rossby Wave)出現。淺對流對SCC的影響則是透過NSC與CNTL的比較來討論,當淺對流混合作用被關閉後,水氣會被限制在邊界層內,使得低層大氣較少的水氣與較大的逸入率使得整體對流被減弱,而缺乏淺對流加熱下,也導致SCC相速度加快。另一方面,NCRF與CNTL的比較指出深對流雲與中層層狀雲的溫室效應能提高高層大氣的浮力與水氣含量,使得對流變得更強、更具組織性。此外,長波輻射也能加強淺對流活動,使得SCC的相速度變慢,但長波輻射對SCC的東西方向尺度則無影響。 In this thesis, we investigate the zonally asymmetric features of the Madden-Julian Oscillation (MJO) by two approaches. First, we perform a composite analysis of circulation, moisture, convection (OLR and rainfall) of the MJO over Central Indian Ocean. The analyses show that, to the east of the MJO convection, easterly and boundary-layer convergence span over the Maritime Continent and Western Pacific (100°E-160°E) where the moisture and vertical velocity anomalies exhibit a bottom-heavy structure. In contrast, the moisture and vertical velocity anomalies show a top-heavy structure to the west of the MJO convection. A vertical mode decomposition of the MJO circulation shows that the equatorial zonal overturning circulation consists of the first baroclinic mode with a broad scale (60°E-170°E), and the second baroclinic mode with a 15° longitudes westward shift relative to the first baroclinic mode. The corresponding composite cloud types and radiative heating rates of CloudSat data show asymmetric distribution from shallow cloud and LW cooling anomaly in the suppressed descending branch of the first baroclinic circulation, through deep cloud and greenhouse warming in the MJO convection core, to mid-level stratiform cloud with distinct vertical heating-cooling contrast in the western branch of circulation. We then perform four aqua-planet experiments in a global GCM with prescribed sea surface temperature varying only in latitude similar to the current climate to study the dynamics giving rise to the above zonal asymmetric MJO features. The experiment with default full physics is control (CNTL). The other two differ from CNTL, one with No Shallow Cumulus parameterization (NSC) and one with No Cloud Radiative Feedback in LW (NCRF). Eastward-moving disturbances are prevalent in all experiments with convection organized as super cloud clusters (SCCs) that are identified as convectively coupled equatorial Kelvin waves by their space-time spectra. The space-time spectra indicate that these waves are dispersive. The structure of the simulated SCCs is similar to that of the observed MJO, but its zonal scale is smaller (70°-80°), and the phase speed varies from 8-15 ms-1 in CNTL (Control experiment), and 12-18 ms-1 in NSC and NCRF. Relative to the fast-moving SCCs (phase speed 12-15 ms-1) in CNTL, the slower-moving SCCs (8-12 ms-1) in CNTL are more organized with broader shallow cloud region associated with stronger shallow convective heating and emerging coupled Rossby waves. The effect of shallow convection is further examined by comparing NSC with CNTL. Without convection mixing by shallow cumulus, moisture is trapped in the planetary boundary layer (PBL). The weaker moisture and stronger entrainment in lower troposphere together weaken the overall convection. The lack of shallow convective heating also causes an increase in the phase speed of the SCC. On the other hands, the differences between CNTL and NCRF indicate that greenhouse warming in the deep and mid-level stratiform cloud region enhances moisture and buoyancy in the upper troposphere that favors stronger and more organized convection. Furthermore, the LW cloud radiative feedback tends to strengthen shallow cloud activity, and leads to slower SCC phase speed. But the cloud-radiative forcing does not affect the zonal scale of SCC. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/87580 |
DOI: | 10.6342/NTU202300530 |
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顯示於系所單位: | 大氣科學系 |
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