以區域氣象模式模擬氣膠對冷雲過程的影響

Abstract

雲在大氣中的水循環以及輻射收支平衡中佔有很重要的角色。但目前對於氣膠如何影響冷雲的研究著墨卻不多。Li et al. (2005)比較Microwave Lamb Sounder (MLS)衛星觀測資料以及ECMWF再分析資料發現，陸地上MLS所測量到的冰態水含量(IWC)值比ECMWF模擬的高，推測是因為ECMWF的模式中，沒有考慮到氣膠數量濃度對雲冰的影響，模式無法反映出污染物對雲冰的改變。 本研究使用中尺度氣象模式MM5，配合CLR雲微物理參數法，分別模擬平均背景型氣膠濃度及10倍平均背景型氣膠濃度情況，探討不同氣膠數量濃度對高層卷雲所造成的影響，並與CloudSat衛星資料比較。模擬個案為2007年4月25日及5月11日中南半島地區。研究的推論為：氣膠從近地面藉由深對流過程被帶到空中；較多的氣膠使雲滴粒子變小、雲的生命期變長，抑制暖雲降水。當雲滴粒子較小，在空中停留時間較久，雲滴的碰撞減弱，雲滴容易被帶到高層經由同質核化過程凍結形成雲冰。 模擬結果顯示，模式中加入10倍的平均背景型氣膠數量，會比模式中加入平均背景型氣膠數量所產生的雲滴數量多約2倍且粒徑變小。但氣膠數量的不同會改變系統的動力結構，因此，除了研究假設外，上升速度與雲頂發展的高度也是影響雲冰數量的多寡的原因。同時，較多、較小的雲滴也使雲滴蒸發速率加快，因而增強白吉龍─芬代生轉換過程，加速雲冰成長為雪，而雪花藉由撞併形成雪團的產生率變高，甚至影響到冰雹的形成。不過，雲滴變小也使雪花與冰雹的淞化成長受到限制。研究中所模擬的兩個個案，同樣在加入10倍氣膠數量濃度時，增加了雲的光學厚度以及反照率。

Aerosol plays an important role in the radiation field of the earth-atmosphere system, but not much attention was paid to a similar effect on cirrus clouds. In fact, the role of aerosols in the formation of ice clouds is still an open issue (Seifert et al., 2004). Li et al. (2005) compared the observed data by Microwave Lamb Sounder (MLS) with ECMWF reanalysis data and found that the sampled ECMWF ice water content (IWC) values are significantly smaller than the MLS estimates over nearly all the tropical landmasses. We presume that is mainly due to the fact that ECMWF analysis does not consider the aerosol effect on the formation of cloud ice. In this study, we examined two cases: 2007.4.25 and 2007.5.11 in Indochina, using the Fifth-Generation NCAR/PSU Mesoscale Model (MM5), with implementation of the CLR cloud microphysical scheme. Sensitivity tests were conducted using average background aerosol by multiplying the number concentration by ten times to discuss aerosol influence on cloud ice. Model results were compared with CloudSat data. The hypothesis to be examined is that increasing aerosol in the boundary layer affects deep convection by increasing cloud drop number concentration and reflectance, as well as the lifetime of the cloud; smaller cloud drop with lower collision efficiency suppresses the rainfall formation, allowing more cloud drops to reach the upper level and freeze into ice particles via homogeneous nucleation, which then influence the subsequent mixed-phase processes. The simulated results indicate that using 10 times the aerosol number concentration doubles the number concentration and reduces the size of cloud drops. Aerosols may also affect the dynamic of the convection system, resulting in changes in the updraft velocity and cloud top temperature, which in turn affects cloud ice number concentration. Furthermore, the growth of ice particles via the Wagner-Bergeron- Findeisen process is also enhanced due to faster evaporation of the smaller cloud drops. However, smaller cloud drops restrict the riming of snow and graupel. In these two cases, more aerosol number concentration increase the cloud optical depth and the albedo.