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標題: | 冰相微物理過程與環境不穩定度對晚間對流系統維持的影響 The impact of Ice Microphysics and Ambient Instabilities on Nocturnal Convective System Maintenance |
作者: | Iat-Hin Tam 譚日軒 |
指導教授: | 楊明仁(Ming-Jen Yang) |
共同指導教授: | 李文兆(Wen-Chau Lee) |
關鍵字: | 降水過程,雲微物理,中尺度動力機制,後方入流,終端落速, Precipitation Process,Cloud Microphysics,Mesoscale Dynamics,Rear-inflow jets,Terminal velocity, |
出版年 : | 2019 |
學位: | 碩士 |
摘要: | 本研究利用於2015年7月15日於美國堪薩斯州進行之PECAN (Plains Elevated Convection at Night) 觀測實驗相關資料及WRF敏感度實驗研究美國中西部中尺度對流系統在晚間強度維持的原因。觀測資料顯示於晚上減弱的系統有較多對流於系統後方形成(System rear CIs),而於晚上增強的系統則有更多對流於系統邊緣形成(System edge CIs)。利用觀測資料,我們提出兩個不同的假設來解釋這種差異:(一)System edge CIs附近大氣邊界層上方水氣含量較多和(二)兩種對流系統的運動場結構與微物理過程有一定差異,而這些差異有利於有System edge CIs 的系統增強。
雙偏極雷達觀測結果顯示有較多System edge CIs系統有較強並水平延伸之後方入流(RIJ)及較明顯的軟雹/冰雹淞化成長過程(Riming growth),顯示(二)可能扮演較重要的角色。我們可以利用WRF微物理敏感度實驗對假設 (二) 作進一步的探討。實驗結果顯示,當實驗容許軟雹或冰雹可沿大小不同以不一終端速度下降的時候,後方入流會有增強並於水平方向能有所延伸。同時,RIJ的水平延伸容許更多對流於系統邊緣產生。 上述結果出現的原因與對流系統內熱力場的變化有關:密度較大(較重)的軟雹粒子較大的終端落速使這些粒子能於較靠近對流上升區(Updraft region)的位置降到0oC線以下。軟雹溶解過程中的潛熱消耗可增大對流上升區溶解層上下的浮力差並產生較明顯的負浮力氣壓擾動(negative buoyancy pressure perturbation),此相對低壓對RIJ增強有主導影響。實驗結果同時顯示較強的RIJ可將部分下降至0oC線附近的軟雹重新帶到上升氣流中並可以增強淞化成長,而令更多軟雹能在上升區中形成。這過程中所產生額外的潛熱釋放可使對流系統強度增加並有更大維持能力。 In this study, the sustenance of the nocturnal CIs within mesoscale convective systems (MCSs) developed on 15 July 2015 during the Plains Elevated Convection at Night (PECAN) field campaign were investigated with a combination of in-situ observations and a set of Weather Research and Forecasting (WRF) experiments. Observational analyses revealed that systems with a greater percentage of CIs near the system edge had greater maintainability than system where CIs tended to cluster in system rear. Two hypotheses were proposed to explain this phenomenon: (a) environmental instability near the system edge CIs were greater due to enhanced moisture above the boundary layer and (b) the kinematic-microphysical structures of systems with system edge CIs evolved in a manner that was favorable for system maintenance. Specifically, dual-polarimetric observations indicate stronger, more extended rear-inflow jet (RIJ) and increased riming growth within the convective updrafts for these systems. A set of microphysical sensitivity experiments were performed to evaluate the two hypotheses. Since the ambient environmental instabilities were similar between the experiments, internal processes would play a dominate role if significant inter-model differences in updraft strength were found. Statistical analyses suggest that simulated systems were stronger when rimed particles can sediment at different terminal velocities with regard to their sizes. RIJs in these systems tended to the stronger and more horizontally expanded, allowing more system edge CIs. In these experiments, preferential sedimentation of melting graupel increased the buoyancy gradient near system edge and created stronger negative buoyancy pressure perturbation, which enhanced the system RIJs. Stronger and more horizontally extended RIJs could subsequently strengthen the system by extra riming and deposition when the RIJs transported the graupel back to the updrafts. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/71578 |
DOI: | 10.6342/NTU201900175 |
全文授權: | 有償授權 |
顯示於系所單位: | 大氣科學系 |
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