Scale Dependence of Cloud Microphysical Response to Turbulent Entrainment and Mixing
Scale Dependence of Cloud Microphysical Response to Turbulent Entrainment and Mixing
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DOI：
10.1029/2018ms001487
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发表时间：
2018-11-01
影响因子：
6.8
通讯作者：
Shaw, Raymond A.
中科院分区：
文献类型：
--
作者：
Kumar, Bipin;Goetzfried, Paul;Shaw, Raymond A.
大气云的动力学和寿命与卷吸和湍流混合密切相关。本文提出了直接的湍流混合的数值模拟，然后在一个米大小的体积在云清晰的空气界面的液滴蒸发，高达近5亿个单独的云水液滴合奏。混合过程对畴尺寸的依赖性表明，随着畴尺寸的增加，非均匀混合变得越来越重要。液滴尺寸分布的形状随空间尺度变化很大，出现明显的负指数尾。在瞬态混合过程中，相对色散的增加强烈依赖于混合的尺度，因此也依赖于Damkohler数，Damkohler数的定义是湍流大涡时间尺度除以云的过饱和弛豫时间。简明语言摘要大气中的云仍然是更可靠的天气和气候预报的最大不确定因素之一。它们的动力学和寿命作为一个整体与卷吸过程和随后在它们的界面处的湍流混合紧密耦合。我们的工作提出了高分辨率的直接数值模拟，在一个米大小的体积在云清晰的空气界面，研究湍流运动到耗散尺度，并描述了液态水含量高达5亿个单独的液滴的人口。本数值模拟的目的是提供一个系统的分析的依赖性的混合过程中的模拟尺寸。我们发现，不均匀的混合，这主要是由大规模的旋涡，通过云界面扫过，变得越来越重要的域的大小被放大。非均匀混合过程增强了云滴谱分布的宽度，这种增强作用随着模拟尺度的增大而增强。我们的工作揭示了云动力学的多重反馈，耦合湍流与多尺度相变的高度非线性热力学。
The dynamics and lifetime of atmospheric clouds are tightly coupled to entrainment and turbulent mixing. This paper presents direct numerical simulations of turbulent mixing followed by droplet evaporation at the cloud-clear air interface in a meter-sized volume, with an ensemble of up to almost half a billion individual cloud water droplets. The dependence of the mixing process on domain size reveals that inhomogeneous mixing becomes increasingly important as the domain size is increased. The shape of the droplet size distribution varies strongly with spatial scale, with the appearance of a pronounced negative exponential tail. The increase of relative dispersion during the transient mixing process is strongly dependent on the scale of the mixing and therefore on the Damkohler number, defined as the turbulence large-eddy time scale divided by the cloud supersaturation relaxation time.Plain Language Summary Clouds in the atmosphere are still one of the biggest uncertainty factors for more reliable weather and climate prognoses. Their dynamics and lifetime as a whole is tightly coupled to entrainment processes and subsequent turbulent mixing at their interface. Our work presents high-resolution direct numerical simulations in a meter-sized volume at the cloud-clear air interface that study the turbulent motion down to the dissipative scales and describe the liquid water content as a population of up to half a billion individual droplets. The goal of the present numerical simulations is to provide a systematic analysis of the dependence of the mixing process on the simulation size. We show that inhomogeneous mixing, which is primarily caused by large-scale vortices that sweep through the cloud interface, becomes increasingly important as the domain size is enlarged. The inhomogeneous mixing process enhances the width of the cloud droplet size distribution, with the enhancement getting stronger with increasing simulation size. Our work sheds new light on the multiple feedbacks in cloud dynamics that couple turbulence with the highly nonlinear thermodynamics of phase changes on multiple scales.