凝結核與冰核數量濃度對降水影響之數值研究

Abstract

水是人類生活不可或缺的生活要素之一，人類可用水的來源主要來自降水，而探制降水效率的重要關鍵之一為大氣中CCN(Cloud Condensation Nuclei)和IN (Ice Nuclei)的數量濃度。CCN和IN的來源主要為空氣中的懸浮微粒，即是所謂的氣膠。近年來有許多研究著重於CCN和IN對降水的影響，有的研究指出增加CCN或IN有助於降水的形成;但有的些研究結果則否。由於氣膠與雲內動力效應的交互作用的影響相當複雜，再加上對冰相過程的作用所知有限，因此目前對於CCN和IN對降水的影響為何仍未有確切的定論。 故本研究重點將著重在CCN、IN數量濃度的變化對降水的影響。使用的模式為加入C&L-Reisner 2參數法(CLR參數法)的MM5(The Fifth-Generation NCAR/PSU Mesoscale Model)。CLR參數法是結合Chen and Liu (2004)之C&L暖雲參數法與MM5中的Reisner 2冰相過程的參數法，可反映凝結核的數量濃度與氣膠粒徑分佈對暖雲與冷雲過程的影響。 目前選定兩個鋒面個案進行模擬，透過一系列的模擬測試可知，增加或減少氣膠的數量濃度對不同的天氣型態降雨會有不同的影響，在深對流系統的個案裡增加氣膠數量濃度會增加降雨，但對於對流系統較淺的個案而言則是相反的結果，顯示氣膠的數量濃度對降水過程有其重要但非線性的影響。

Humans cannot live without water. Due to the influence of increasing populations and degree of industrialization on global climate and environment, human influence on precipitation efficiency became a very important issue. Precipitation is one of the most basic commodities on earth sustaining human life. Its efficiency is strongly influence by aerosol particles that serve as cloud condensation nuclei (CCN) and ice nuclei (IN). Some past studies indicated that more aerosols may result in more but smaller cloud drops. Smaller cloud drops would suppresses drop coalescence thus inhibit precipitation formation. But there are also studies that shown otherwise. The aerosol effect on clouds remains an unsolved issue and the processes involved are complicated and nonlinear. The main motivation of this study is to gain a better understanding of the mechanisms involved in this aerosol-cloud interaction, using a non-hydrostatic mesoscale cloud model (MM5) as the main tool. A C&L Reisner 2 scheme that considers the effects of CCN on precipitation is incorporated into this model. The simulation results indicated that different concentration of aerosols has different effect on precipitation depending on the types of cloud system. More aerosols generally produce more and smaller cloud drops and inhibit warm rain formation; it also enhances the deposition growth of ice particles but limits their growth by riming. In the deep convective cloud system that simulated here, increasing aerosol can enhance surface rainfall; but for the shallow convective cloud system the surface precipitation is reduced. Increasing IN not necessary increase or decrease precipitation, and the effect actually depends on the strength of homogeneous nucleation and other nucleation processes. Overall, aerosols play complicated and nonlinear roles in precipitation process.