| 摘要: | 氣膠為影響氣候的重要因子，透過輻射和微物理效應改變雲的生命週期和降水分布。在定量的水氣條件下，增加雲凝結核的數目會導致雲滴數量濃度增加、雲滴體積變小、抑制暖雲底部的降水，使雲的生命週期變長，進一步改變降水型態。然而目前的全球模式中輻射驅動力在雲與氣膠交互作用(Aerosol-Cloud Interactions, ACI)的估計中仍有較高的不確定性，且近年來的研究顯示，全球模式明顯地高估毛雨(Drizzle)發生的頻率，因此本篇研究透過整合2005年至2017年10/15至11/30期間的地面觀測資料和衛星資料，對氣膠、雲的光學特性和降水特性進行長期分析，以探討氣膠、雲與降水的交互作用(Aerosol-Cloud-Precipitation Interactions, ACP) ，試圖了解當氣膠含量增加時，是否會抑制降水，以及對毛雨發生頻率的影響為何。
研究結果顯示，臺灣北部人為活動較頻繁的區域(桃園地區)，其暖雲的特徵大多屬於薄且破碎的雲，且在定量的雲水光程(Cloud Water Path, CWP)下，當氣膠含量增加，會導致雲有效半徑(Cloud Effective Radius, CER)減少、雲光學厚度(Cloud Optical Thickness, COT)增加以及雲頂溫度(Cloud Top Temperature, CTT)降低，此結果與氣膠間接效應一致。利用Feingold et al. (2001)所提出計算氣膠對雲微物理影響程度的公式，並以ACI index (ACI值)做為表示，比較污染較低的海洋及污染較高的陸地區域，結果顯示在150 ≤ CWP < 297 (g m-2)的雲水光程中，乾淨區域的ACI值為0.09、污染區域的ACI值為0.06，說明氣膠間接效應在乾淨的地區更為敏感，可能因較高的氣膠濃度使雲與氣膠交互作用趨於飽和，導致對污染區域的影響程度降低。在氣膠、雲與降水的交互作用中，氣膠含量增加會改變雲的生命週期，更多的雲凝結核將重新分配雲中的水含量，使雲滴數量增加、雲滴有效半徑減少，降低碰撞結合率進而抑制降水，並可能造成降水時間的延遲，且改變降水型態。搭配分析雨滴粒徑觀測資料指出，在高氣膠濃度的條件下，會降低毛雨的發生頻率，但在小於等於1 mm hr -1的降水情境下，高氣膠濃度驅使雨滴往小的雨滴粒徑(雨滴粒徑0.359 mm)移動，增加毛雨滴的出現。本研究利用長期地面及衛星資料分析，有助於初步了解北臺灣在氣膠環境的變異下，潛在對於環境及水循環的衝擊影響，並應用於未來氣膠-雲-降水交互作用之觀測策略規劃。
;Aerosols are an important factor influencing the climate. They can alter cloud lifecycles and precipitation distributions through radiative and microphysical effects. An increase in cloud condensation nuclei number causes an increase in cloud droplet concentration and a decrease in cloud droplet size for fixed cloud water content. Whereby the reduction in cloud droplet size suppress the precipitation at the bottom of the warm cloud, leads to the increase of the cloud lifetimes, and further changing the precipitation pattern. However, the estimate of the radiative forcing in aerosol-cloud interactions (ACI) still has high uncertainty in the current global models. Moreover, recent studies showed that global models significantly overestimate the occurrence of drizzle events. In order to understand such aerosol-cloud-precipitation interactions (ACP), we through integrate the data from surface observations and satellite data and analyze aerosol, cloud optical and precipitation properties between 10/15 and 11/30 from 2005 to 2017. This study attempts to understand whether an increase in aerosol loading will suppress the precipitation, and how does an increase in aerosol loading impact on the frequency of drizzle events.
The results indicate that in northern Taiwan where human activities are more frequent (Taoyuan area), the characteristics of warm clouds are mostly thin and broken. In the fixed cloud water path (CWP), an increase in aerosol loading leads into cloud effective radius (CER) decrease, cloud optical thickness (COT) increase, cloud fraction (CF) increase, cloud top temperature (CTT) decrease, and this result shows in agreement to the aerosol indirect effects. Using the formula for calculating the ACI index of the influence of aerosol on cloud microphysical properties proposed by Feingold et al. (2001). Comparing with the less polluted oceans and the more polluted land areas, the result demonstrates that for cloud water path in the group 9 (150 ≤ CWP < 297), the ACI index of the clean area is 0.09, and the polluted area is 0.06, indicating that aerosol indirect effects are more sensitive in the clean area. It might be that the ACI is saturated under large aerosol concentrations, leading to lower ACI index in the polluted area lower. In the aerosol-cloud-precipitation interactions, an increase in aerosol loading could change cloud lifetimes. Greater NCCN redistributes cloud water to more numerous and smaller droplets, reducing collision-coalescence rates, which will suppress the precipitation and may delay the precipitation time. It also could change the precipitation pattern. Combining the analysis of raindrop size observations indicates that when aerosol loading is higher, it results in a decrease in the frequency of drizzle events. However, in the scenario of precipitation less than or equal to 1 mm hr -1, high aerosol concentration drives raindrops to move toward smaller drop sizes and increase the appearance of the drizzle drops. This study uses the analysis of long-term surface and satellite observation data to understand how the variation in the aerosol environment in northern Taiwan can potentially impact the environment and the water cycle. These preliminary results can be applied to observational strategy planning in the future about aerosol-cloud-precipitation interactions. |
