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標題: | 以雲解析模式探討中尺度對流渦漩對熱帶旋生之作用 The Study on the Impact of Mesoscale Convective Vortices on Tropical Cyclogenesis using Cloud Resolving Model |
作者: | Hsin-Yu Chu 朱心宇 |
指導教授: | 吳健銘 |
關鍵字: | 熱帶旋生,中尺度對流渦旋,垂直質量通量,氣柱飽和比,渦度熱塔, Tropical Cyclogenesis,Mesoscale Convective Vortex,Vertical Mass Flux,Saturation Fraction,Vortical Hot Tower, |
出版年 : | 2019 |
學位: | 碩士 |
摘要: | 此研究中,我們於三維雲解析向量渦度模式中(VVM)植入六組最大風速位於不同高度的雷坎渦旋 (Rankine Vortex),分別代表位在不同高度的理想化中尺度對流渦漩 (Mesoscale Convective Vortex, MCV),藉此評估不同MCV造成的垂直擾動位溫結構對旋生過程的影響。前人之研究中指出,由於平衡熱力的結果,在潛在位渦(PV)最大值高度以上,會有正位溫距平,而在其下會有負位溫距平,造成垂直位溫結構趨於穩定。這個距平容易激發垂直質量通量(MF)最大值在中低對流層之 “Bottom-Heavy” 對流系統。此種對流結構會增強底部水氣輻合,增加整體氣柱的飽和比(SF),或者透過海表焓通量作用,激發更多對流。而底部輻合也會透過渦度垂直拉伸項造成底層渦旋加強。
我們發現初始雷坎渦旋最大風速(vmax)高度位在4.5公里的實驗最早發生,領先其他組實驗7小時以上。由此可知,MCV最大風速高度位在4.5公里為最有利旋生之高度。另外,當渦旋初始最大風速高度在地表或7.5公里以上,模式積分144小時之後仍不會發生旋生,顯示MCV造成的熱力調整可能影響到旋生的過程。由實驗發生旋生與否,我們能進一步將實驗分成發展組(DS)跟不發展(NDS)組。為了進一步分析和濕熵及濕熵的時序變化,我們將高層及低層大氣的飽和濕熵差值定義穩定度指數(SI),其數值越小代表穩定度越大。DS組的SI在旋生前 48 小時開始即有系統性的減少,而在旋生前 0-12 小時區間DS 的 SI 中位數比NDS低5 JK^-1kg^-1左右。同時DS的氣柱飽和比NDS的氣柱飽和比高,而且隨著時間接近旋生,DS 的 SF 同樣的會有系統性的增加。SI與SF之聯合分布的結果,SI 大致上與SF呈現反比,顯示熱力環境的穩定伴隨氣柱的飽和。 根據前人雲解析模擬的結果,在環境處於高氣柱飽和比時,較容易產生極端降水及大型組織性對流系統。於旋生前環境中具有高渦度量值的對流系統,即所謂的渦度熱塔(Vortical Hot Tower, VHT),而VHT的增加及合併在旋生時增加環境渦度及加熱,扮演重要的角色。本研究透過六向連通元件標記法,先將三維空間中的雲元件標記出來,再將雲厚大於10公里的雲元件定義為 VHT。我們發現,DS 在旋生前 36 小時前,靠近渦旋中心 100×100 km^2之區域出現體積大於 10^4 km^3之渦度熱塔之機率密度即超過NDS。最後透過定量分析垂直質量通量在低SI與高SI之機率分布,發現在低SI的環境中較容易產生Bottom-Heavy 的對流。對流更有效率的集結及產生Bottom-Heavy 的垂直質量通量,有助於初始渦旋透過加熱及渦度拉伸等機制,讓低層渦度增強。 In this study, Rankine vortices that represent idealized Mesoscale Convective Vortex (MCV) with maximum wind speeds at different levels embedded in a quiescent tropical environment are studied using Vector Vorticity Model (VVM), a three-dimensional, cloud resolving model. We aim to evaluate how different extent of the potential temperature profile modulates the evolution of the initial vortex. Under balanced thermodynamics, positive (negative) potential temperature anomaly will be present above (below) maximum level of potential vorticity. If such profile is optimally placed, the corresponding stabilization is theorized to enhance “bottom heavy” mass flux profile, inducing convergence at lower level, further promoting column saturation and precipitation, contributing to the spin-up of the low-level vortex. The experiment with a Rankine vortex where maximum wind (vmax) located at z = 4.5 km, is the earliest to undergo cyclogenesis, leading other runs by approximately 7 hours or more. The vortex with vmax at sea level or above 7.5km, does not develop after 144 hours. The time required to reach cyclogenesis is proportional to the difference between the level in which vmax is located below or above 4.5 km, where z = 4.5 km seems to be the optimal height to promote genesis. Based on cyclogenesis occurs in 144 hours or not, we can categorize them into developing sets (DS) and non-developing sets (NDS). To analyze the stabilization of thermodynamic environment, we define stability index (SI) based on the difference of saturated moist entropy (s*) between upper and lower troposphere. Smaller (larger) values of SI indicates higher (lower) environmental stability. 48 hours prior to the genesis, the SI of DS systematically decreases, and during the period 0-12 hours prior to the genesis, the median SI of DS is around 5JK^-1kg^-1 lower than that of NDS. The saturation fraction (SF) in DS also shows a systematic increase prior genesis. Joint-PDF of SI and SF confirms the fact that SF is inversely proportional to SI, demonstrating stabilization is accompanied by column saturation. In an environment with higher saturation fraction is more likely to produce large, organized convection. Convective structures that are highly rotational in cyclogenetic environments are phrased as “Vortical Hot Towers (VHTs)” and the increase in number and merger of VHTs plays a notable role as a source of vorticity convergence and heating during genesis. Here we identify the size, height and other characteristics of clouds by connecting cloud grids together as cloud objects using a six-connected segmentation algorithm. After cloud objects are labeled, VHTs are then filtered using cloud thickness exceeding 10 km as the criteria. We then compare the size distribution of these VHTs between the DS and NDS. In the DS, probability density of large VHTs with volumes over 10^4 km3 within a 100×100 km^2 square box around the vortex center steadily overpasses NDS, which is a sign of aggregation and upscale growth 24-36 hours prior to the genesis. Finally, we show that the generation of “bottom-heavy” mass flux profile is more likely in low SI environments by comparing the probability distribution of a bottom-heavy index in low SI and high SI environments. The tendency to generate large VHTs and bottom-heavy mass flux profiles, promotes processes such as organized heating and stretching, which intensifies the incipient vortex. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/7193 |
DOI: | 10.6342/NTU201901531 |
全文授權: | 同意授權(全球公開) |
顯示於系所單位: | 大氣科學系 |
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