Microscale characteristics of homogeneous freezing events in cirrus clouds
Microscale characteristics of homogeneous freezing events in cirrus clouds
复制标题
卷云中均匀冻结事件的微尺度特征
DOI：
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发表时间：
2017
期刊：
影响因子：
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通讯作者：
Eric J. Jensen
中科院分区：
文献类型：
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作者：
B. Kärcher;Eric J. Jensen
我们研究水气溶胶颗粒的均匀冻结，这是卷云中的基本冰形成过程。我们估计了冻结时间尺度和冻结层的垂直延伸，证明这种冻结事件是高度短暂的和局部的。虽然时间尺度随着驱动冰成核的垂直速度的增加而减小，但层深是垂直速度的弱函数。我们的结果用于讨论湍流扩散和夹带混合对卷云均匀冻结的可能影响。大的湍流扩散率可以扩大水蒸气耗尽的冻结层，并促进新成核的冰晶从其中沉积到冰过饱和的空气中。均匀冻结事件可能会受到强烈湍流消散率事件中微尺度湍流的影响，尽管这种事件很少见。我们推测，在异质冰成核的情况下，冻结层更宽，并且沉积对成核的影响变得更加重要。我们的研究结果指出了从测量中推断有核卷云冰晶数量的困难，并对卷云模型在空间和时间分辨率方面施加了严格的限制。
We investigate homogeneous freezing of aqueous aerosol particles, a fundamental ice formation process in cirrus clouds. We estimate freezing time scales and vertical extensions of freezing layers, demonstrating that such freezing events are highly transient and localized. While time scales decrease with increasing vertical velocity driving ice nucleation, layer depths are weak functions of the vertical velocity. Our results are used to discuss possible effects of turbulent diffusion and entrainment‐mixing on homogeneous freezing in cirrus. Large turbulent diffusivity acts to broaden water vapor‐depleted freezing layers and facilitate sedimentation of freshly nucleated ice crystals out of them into ice‐supersaturated air. Homogeneous freezing events could be affected by microscale turbulence in episodes of intense turbulence dissipation rates, although such episodes are rare. We conjecture that freezing layers are broader in the case of heterogeneous ice nucleation and effects of sedimentation on nucleation increase in importance. Our findings point to the difficulty of inferring nucleated cirrus ice crystal numbers from measurements and place tight constraints on cirrus models with regard to spatial and temporal resolution.