使用衛星觀測分析東亞冬春季海洋暖雲對氣膠之敏感性

Abstract

氣膠做為雲凝結核,可以透過改變雲的微觀與巨觀物理性質進而影響雲輻射力。過去研究指出，雲凝結核數量增加會導致雲滴有效半徑減少，從而抑制碰撞—合併成長，這可能會延長雲的壽命，增加雲中液態水含量、雲量與雲反照率，並產生冷卻效應。最近的研究著重於淺雲的氣膠—雲交互作用，認為雲與氣膠交互作用最重要的物理過程取決於雲的降水狀態以及大氣環境狀態。東亞冬、春季時，冬季季風環流和與黑潮是主導大氣環境的兩大因素，本研究旨在探討上述環境參數對雲的型態乃至雲-氣膠交互作用的影響。 本研究使用 2006-2010 年冬季與春季來自 A-Train 衛星序列的共同定位資料。首先，根據 CALIPSO 的雲層數資料和 Aqua-MODIS 的雲頂溫度和雲頂壓力，篩選出海洋上的單層暖雲，並應用 CloudSat 2C-PRECIP-COLUM precipitation flag 分類降水與不降水的海洋暖雲樣本。最後根據近地表穩定度(NSS)和估計逆溫強度(EIS)進一步分類雲的樣本，以探討不同環境條件下雲敏感性的變化。 當寒冷的大陸空氣移動到溫暖的海洋上時,強烈的海氣溫差會使海洋邊界層不穩定，同時雲水含量、雲量、雲光學厚度與雲頂高度均增加，顯示淺雲對流因為海洋邊界層趨於不穩定而增強。冷高壓出海時，大尺度沉降增強，在邊界層頂形成逆溫層，限制了雲頂高度的發展，整體而言，雲水含量呈現減少趨勢，雲量與雲光學厚度則是隨著逆溫強度增強而增加，表示低雲的形態則從較破碎的雲轉變成連續分布的雲。 雲敏感性分析的結果顯示，降水雲與不降水雲對氣膠的反應會受到近地面穩定度與逆溫強度的影響：在穩定的海洋邊界層中，不降水雲當中的蒸發-逸入回饋因為氣膠增加而增強，降低雲水含量,而降水雲則因為氣膠抑制了碰撞-合併成長過程，使得毛毛雨降水的形成被抑制，雲中的液態水含量因此增加。降水雲的敏感性比不降水雲更強,連帶影響雲光學厚度和雲反照率的敏感性。比較環境狀態對敏感性的影響，在不穩定的海洋邊界層以及強沉降逆溫的環境下，雲的敏感性較強。 未來擬使用高解析度模式與新一代的衛星觀測資料分析海洋暖雲降水的敏感性，以了解不同形態或發展階段的雲之間獨特的氣膠-雲交互作用，並估計東亞冬季和春季雲輻射力對氣膠的敏感性。

Aerosols can affect the cloud radiative forcing by acting as the cloud condensation nuclei and subsequently changing the micro- and macro-physical properties of clouds. The classical theory states that the increase in CCN concentration results in the decrease of cloud droplet effective radius (Re), thereby inhibiting the coalescence process, which in turn may extend the cloud lifetime, enhance cloud water contents and increase the clod fraction, producing negative radiative forcing. More recent studies focusing on the aerosol-cloud interactions of shallow clouds have suggested that the cloud liquid water path does not necessarily increase with increasing aerosol loading, and the response depends on the precipitation state of the cloud and the thermodynamic conditions of the environment. It is found that the environmental condition in East Asian winter and spring is largely controlled by the monsoonal circulation and the warm sea surface temperature associated with the Kuroshio current. The environmental factors not only affect the types of low clouds but also the aerosol-cloud responses. The current study applies the co-located aerosol and cloud retrievals from the A- train satellites during 2006-2010 to investigate the aerosol-cloud interactions of marine warm clouds in East Asia in winter and spring, the seasons in which the low-level marine clouds and high aerosol pollution coexist most frequently. First, the single-layer warm clouds over the open ocean are identified based on the number of cloud layers detected by Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and the cloud-top temperature and pressure retrieved from Moderate Resolution Imaging Spectroradiometer (MODIS). The precipitation flag retrieved from the Cloud Profiling Radar on CloudSat are also applied to separate the state of precipitation of the clouds. The clouds are then further classified based on the near-surface stability (NSS) and estimated inversion strength (EIS) to explore variation of the susceptibility of cloud properties under different environmental conditions. As cold continental air moves onto the warm ocean, it destabilizes the marine boundary layer, resulting in the formation of more shallow convective clouds. As the large-scale subsidence associated with the southeastward movement of Siberian high strengthens, the type of clouds transfer from broken clouds into continuous clouds. The cloud susceptibility is stronger for precipitating clouds and under unstable conditions. For non-precipitating clouds, LWP decreases slightly under the stable condition and increases in unstable condition with increasing aerosols. For precipitating clouds, LWP increases with increasing aerosols and increases more under unstable condition. Note that the susceptibility of precipitating clouds is stronger than non- precipitating ones, it results in larger cloud optical depth and cloud albedo. In the future, the susceptibility of precipitation of marine warm clouds will be analyzed by high resolution model in purpose of understanding the unique aerosol-cloud interaction among different cloud types. In addition, the aerosol-cloud radiative forcing during winter and spring in East Asia will also be estimated.