Mixed-phase clouds are globally omnipresent and play a major role in the Earth's radiation budget and precipitation formation. The existence of liquid droplets in the presence of ice particles is microphysically unstable and depends on a delicate balance of several competing processes. Understanding mechanisms that govern ice initiation and moisture supply are important to understand the life cycle of such clouds. This study presents observations that reveal the onset of drizzle inside a ∼ 600 m deep mixed-phase layer embedded in a stratiform precipitation system. Using Doppler spectral analysis, we show how large supercooled liquid droplets are generated in Kelvin–Helmholtz (K–H) instability despite ice particles falling from upper cloud layers. The spectral width of the supercooled liquid water mode in the radar Doppler spectrum is used to identify a region of increased turbulence. The observations show that large liquid droplets, characterized by reflectivity values larger than −20 dBZ, are generated in this region. In addition to cloud droplets, Doppler spectral analysis reveals the production of columnar ice crystals in the K–H billows. The modeling study estimates that the concentration of these ice crystals is 3–8 L⁻¹, which is at least 1 order of magnitude higher than that of primary ice-nucleating particles. Given the detail of the observations, we show that multiple populations of secondary ice particles are generated in regions where larger cloud droplets are produced and not at some constant level within the cloud. It is, therefore, hypothesized that K–H instability provides conditions favorable for enhanced droplet growth and formation of secondary ice particles.

中文翻译：

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雷达多普勒光谱观测揭示的开尔文-亥姆霍兹不稳定性中的过冷液态水和二次冰的产生

混相云在全球范围内无处不在，在地球辐射收支和降水形成中发挥着重要作用。在冰粒存在的情况下液滴的存在在微观物理上是不稳定的，并且取决于几个相互竞争的过程的微妙平衡。了解控制冰形成和水分供应的机制对于了解此类云的生命周期很重要。这项研究提出了一些观察结果，揭示了一个～ 600 m 深的混合相层嵌入层状降水系统中。使用多普勒频谱分析，我们展示了尽管冰粒从上层云层落下，但在开尔文-亥姆霍兹 (K-H) 不稳定性中会产生多大的过冷液滴。雷达多普勒频谱中过冷液态水模式的频谱宽度用于识别湍流增加的区域。观察结果表明，在该区域产生了反射率值大于− 20 dBZ 的大液滴。除了云滴之外，多普勒光谱分析还揭示了 K-H 巨浪中柱状冰晶的产生。建模研究估计这些冰晶的浓度为 3-8 L ^-1，这比初级冰核粒子高至少 1 个数量级。鉴于观察的细节，我们表明在产生较大云滴的区域中产生了多个次级冰粒子群，而不是在云内的某个恒定水平上。因此，假设 K-H 不稳定性提供了有利于增强液滴生长和二次冰粒子形成的条件。