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Title: | 雲微物理對水同位素分餾的影響 The Effect of Cloud Microphysics on Water Isotope Fractionation |
Authors: | Wan-Yu Chen 陳婉瑜 |
Advisor: | 陳正平 |
Keyword: | 氫同位素,同位素分餾,雲微物理機制,熱力平衡法,雙流馬克斯威爾氣體動力法, isotope fractionation,deuterium,thermal equilibrium method,two stream Maxwellian kinetic method,cloud microphysics, |
Publication Year : | 2014 |
Degree: | 碩士 |
Abstract: | 氫和氧有相同質子數但中子數不同的同位素,結合成水、半重水、重氧水等水同位素。水同位素的物理性質的差異導致相變時,重的同位素傾向留在液態(或固態),而輕的同位素傾向於氣態,此現象稱之同位素分餾。同位素分餾情況大致受兩項因素影響:同位素的補充與傳送和與雲微物理過程,前者決定空氣中同位素從下邊界的移入和傳送,後者影響相變時同位素分餾的情形和透過降水移除同位素。經由同位素分餾的情形可回溯大氣水循環,提供更多降水效率、水氣源區、相變環境的訊息。先前相關的同位素研究主要著墨在氣候尺度下同位素分餾與各種氣象要素的關係,如:降水量、地表溫度;本研究則利用短時間的天氣個案來討論雲微物理處理的差異對同位素分餾的影響,並透過敏感性測試比較模式和觀測資料,藉以討論同位素的補充與傳送和與雲微物理過程的影響。
研究方法使用耦合於中尺度天氣預報模式WRF(Weather Research and Forecasting)中的CLR雲微物理參數法來模擬造成氫同位素分餾的雲微物理過程。將模式結果配合觀測比較,可推斷所分析的鋒面降水事件中重水同位素比例主要受水氣源區主宰,其次為雲微物理導致的相變與降水。水氣源區與傳送過程對同位素分餾率δD的影響可達100左右。雲微物理與降水機制使得近地表氣態同位素分餾率下降50。冰相過程分餾程度較液相為高,也會造成同位素垂直分布差異,使高層氣態同位素含量減少4%左右。此外並比較以熱力平衡法和雙流馬克斯威爾氣體動力法兩種方式計算方法的差異,其影響主要在於半重水在垂直方向的分布。以氣體動力法所做的計算約落後熱力平衡法,導致較多的半重水留在氣態被帶至高空,顯著影響重同位素水氣分佈。 欲透過同位素了解水循環,取決於模式中同位素的模擬,而熱力平衡法和氣體動力法垂直分布上的差異,將使回推的降水效率、相變環境、水氣源區與實際情形有所偏差。 The mass difference between isotopes causes their fractionation during phase changes, as heavier isotopes prefer to stay in the condensed phases more than the lighter isotopes. The measurement data of isotope fractionation in precipitation shows a significant temporal and spatial variation which would provide the information about water cycle. This variation depends on the water vapor source and cloud microphysics. The former includes the supplement of water vapor from lower boundary and atmospheric transport, while the latter includes the process inside cloud and the remove from precipitation. Most of the earlier modeling studies on isotope fraction in precipitation applied global scale models with simple cloud physics and with the assumption that isotope exchange between liquid and gas phase is under an equilibrium state. Such a treatment tends to be inaccurate in describing the highly kinetic phase transformation processes in clouds. This study focuses on detailed cloud microphysical processes that govern the isotope fractionation without assuming an equilibrium state. To achieve this objective, we added both thermal equilibrium and two stream Maxwellian kinetic methods into the CLR cloud microphysical scheme that has been coupled into WRF (Weather Research and Forecasting) V3.2.1 meteorological model. We conducted a simulation for a precipitation case from June 11th to 17th, 2012. Results showed that the isotope fractionation was dominated by water vapor source in this frontal precipitation case. The influence of water vapor source on surface isotope fractionation factor δD, was about 100. Whereas the cloud microphysics and precipitation removal contributed about 50 in δD. Results also showed that the two stream Maxwellian kinetic method would bring vapor containing more deuterium into higher altitude than the thermal equilibrium method and causes redistribution of isotopic vapor. Dealing phase change by assuming isotopic thermal equilibrium as done in many past studies would lead to discrepancy in the estimation of precipitation efficiency and water vapor source. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/58456 |
Fulltext Rights: | 有償授權 |
Appears in Collections: | 大氣科學系 |
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ntu-103-1.pdf Restricted Access | 7.8 MB | Adobe PDF |
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