On the Energetics of Convectively Coupled Kelvin Waves

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题名:	On the Energetics of Convectively Coupled Kelvin Waves
作者:	洪翊彬;Horng, Yi-Bin
贡献者:	大氣科學學系
关键词:	對流耦合之凱爾文波;濕靜能收支;convectively coupled Kelvin waves;moist static energy budget
日期:	2023-07-04
上传时间:	2024-09-19 14:11:13 (UTC+8)
出版者:	國立中央大學
摘要:	對流耦合之凱爾文波（Convectively coupled Kelvin wave; CCKW）是一種存在於赤道地區的東傳大尺度波動，會造成風場與壓力場的變化，因此通常伴隨著對流，並為熱帶地區的國家帶來許多降雨事件，對於熱帶的天氣與氣候調節扮演相當重要的角色。由於沒有地形的阻礙，CCKW 在印度洋和太平洋海盆相對活躍，但這兩個區域的CCKW在強度、對稱性與傳播速度上皆顯示出不同的特性。本研究的前半部分針對印度洋和太平洋的CCKW結構進行分析，在水平空間結構上，印度洋的CCKW與理論的凱爾文波相似，而太平洋的CCKW除了有較顯著的經向環流，其伴隨的對流訊號也有明顯向北半球偏移的特性；在垂直結構部分，兩者皆表現出隨著高度向西傾斜的特性，但太平洋的CCKW除了有較弱的對流前緣邊界層濕化，在對流最強的階段也較為不傾斜。我們進一步分析兩者在對流發展過程的垂直速度變化，結果顯示太平洋的CCKW具有較大的強度，但對流發展的過程轉換更為快速，可能與太平洋較有利於對流發展的環境條件有關。為了解釋CCKW在印度洋和太平洋海盆的差異，我們在本研究的後半部分利用濕靜能收支方程來分析各個貢獻項，以揭示驅動 CCKW 能量充放電過程的主導因素。結果表明，關鍵因素可能在於水平平流、垂直平流與視水汽源匯（apparent moisture source and sink）之間的相互影響，水平平流項的差異可能是由於兩個洋區的背景風向不同所造成；而垂直平流與視水汽源匯在太平洋明顯有較快的充放電相位轉換，暗示太平洋CCKW較強的上下層耦合，我們推測太平洋地區在基本狀態下的濕靜能較大，較有利於對流發展，CCKW自身造成的震盪即能輕易激發對流生成，這個推測與太平洋CCKW垂直結構中較弱的邊界層濕化吻合。;Convectively coupled Kelvin wave (CCKW) is one of the important large-scale atmospheric oscillations in equatorial areas. It is an eastward-propagating mode that constantly causes fluctuations in pressure, rainfall and winds, and thus, CCKW often takes part in shaping the weather and climate by bringing countless rainfall events to countries in tropics. CCKW is relatively active in the Pacific and the Indian ocean basins due to the absence of obstruction from topography, and yet it shows distinct properties over these two regions, including the intensity, symmetry, and propagation speed. In this study, we first compare the structures of CCKW between the Indian and the Pacific basins. In terms of horizontal structure, the Indian CCKW bears a resemblance to the theoretical Kelvin wave, while the Pacific CCKW exhibits significant meridional winds and a noticeable northward deflection of its convective part. The vertical profiles of CCKW in both the Indian and the Pacific basins present a westward tilt with height; however, the Pacific CCKW shows a weaker pre-moistening at the leading edge of the convection and a less tilted pattern during the peak of convection. A further examination of vertical velocity suggests that the Pacific CCKW has a larger amplitude but experiences more rapid transitions between convective stages, which is possibly related to the differences in environmental conditions. To explain the structural differences between the Indian and the Pacific CCKW, we conduct a detailed moist static energy (MSE) budget analysis to uncover the dominant factors in driving the energy recharge-discharge process of CCKW. Even though the MSE tendency term in both basins resembles each other, the contributing terms in the MSE budget indicate that the energy comes from diverse sources. The key factors might lie in the interplay between horizontal advection, vertical advection and the apparent moisture source and sink profiles, especially in the low troposphere. The distinction in horizontal advection term is possibly related to the background wind direction, while the more upright structures of vertical advection and the apparent moisture sink reveal an instantaneous recharge-discharge phases change, indicating a stronger coupling between lower and upper troposphere in the Pacific CCKW. We speculate that convection associated with CCKW is more easily triggered due to the larger background state of MSE in the Pacific basin, which is probably the reason for the weaker boundary layer pre-moistening in the Pacific CCKW.
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