The Route to Raindrop Formation in a Shallow Cumulus Cloud Simulated by a Lagrangian Cloud Model
The Route to Raindrop Formation in a Shallow Cumulus Cloud Simulated by a Lagrangian Cloud Model
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DOI：
10.1175/jas-d-16-0220.1
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发表时间：
2017-06
影响因子：
3.1
通讯作者：
F. Hoffmann;Y. Noh;S. Raasch
中科院分区：
文献类型：
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作者：
F. Hoffmann;Y. Noh;S. Raasch
摘要利用拉格朗日云模式（LCM）研究了一个浅积云中雨滴的形成机制。分析的重点是如何以及在何种条件下，通过跟踪历史的单个拉格朗日水滴的云滴成长为雨滴。结果表明，当云顶附近出现半径为20 μm的单个雨滴时，会触发雨滴的快速碰撞增长，从而导致雨滴的形成。该区域具有液态水含量高、湍流强、平均雨滴尺寸大、雨滴尺寸分布宽和过饱和度高等特点。雨滴形成很容易发生时，考虑的是有序性诱导的碰撞增强（TICE），有或没有任何额外的展宽的DSD的另一种机制（如夹带和混合）。相反，当TICE不被认为是，雨滴形成严重延迟，如果没有其他的增宽机制是活跃的。通过对理想化边界条件的分析，阐明了这种差异的原因。
AbstractThe mechanism of raindrop formation in a shallow cumulus cloud is investigated using a Lagrangian cloud model (LCM). The analysis is focused on how and under which conditions a cloud droplet grows to a raindrop by tracking the history of individual Lagrangian droplets. It is found that the rapid collisional growth, leading to raindrop formation, is triggered when single droplets with a radius of 20 μm appear in the region near the cloud top, characterized by large liquid water content, strong turbulence, large mean droplet size, broad drop size distribution (DSD), and high supersaturations. Raindrop formation easily occurs when turbulence-induced collision enhancement (TICE) is considered, with or without any extra broadening of the DSD by another mechanism (such as entrainment and mixing). In contrast, when TICE is not considered, raindrop formation is severely delayed if no other broadening mechanism is active. The reason for the difference is clarified by the additional analysis of idealized bo...