Microscopic approach to cloud droplet growth by condensation
Microscopic approach to cloud droplet growth by condensation
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凝结云滴生长的微观方法
DOI：
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发表时间：
2001
期刊：
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通讯作者：
P. Vaillancourt
中科院分区：
文献类型：
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作者：
P. Vaillancourt
这项工作的目的是回答以下问题：云液滴的大小和位置的空间分布和/或湍流介质中可变垂直速度的非均匀性是否有助于扩大液滴尺寸分布。在湍流环境中，使用了数万个云液滴的数值方法，用于模拟湍流环境中的数千滴液滴的生长和轨迹，其性质从液滴到液滴不等。湍流中颗粒的有限惯性会导致颗粒与高涡度区域不同，并优先在低涡度区域内收敛，从而在粒子浓度上产生较强的偏差。第一步，在非散布，沉积或非评估液滴的背景下检查了惯性效应。已经发现，在积云云中典型的云滴的条件下，有可能具有统计学意义的优先浓度。在没有沉积物的情况下，优先浓度随Stokes数字街的函数而增加，使液滴降低了降低的优先浓度，从而随着速度比SY的增加而增加。然后进行了一系列实验，包括液滴的冷凝生长。发现尽管涡流耗散率的提高，液滴的优先浓度的增加确实会导致过饱和扰动分布的瞬时分散体的增加，这是液滴尺寸分布的宽度，这是液滴的宽度，这是该液滴分布的宽度过饱和扰动的时间内分散量减少。该结果是由于湍流强度增加而过饱和扰动的去相关时间减少的结果。将结果与准绝热云核中的观察结果进行比较，得出的结论是，即使在没有湍流的最有利条件下，微观方法也会产生太少的扩展，无法解释观察结果。
The goal of this work is to answer the question of whether nonuniformity in the spatial distribution of sizes and positions of cloud droplets and/or variable vertical velocity in a turbulent medium can contribute to the broadening of the droplet size distribution. A numerical approach to simulate the growth and trajectory of several tens of thousands of cloud droplets in a turbulent environment whose properties vary from droplet to droplet is used. The finite inertia of particles in a turbulent fluid causes particles to diverge from regions of high vorticity and to converge preferentially in regions of low vorticity, thus creating strong deviations in particle concentration. As a first step, the inertia effect was examined in the context of nongrowing, sedimenting, or nonsedimenting droplets. It was found that statistically significant preferential concentration is possible in conditions typical of cloud droplets in cumulus clouds. In the absence of sedimentation, preferential concentration increases as a function of the Stokes number St. Allowing the droplets to sediment decreases preferential concentration to a degree that increases with the velocity ratio Sy. A series of experiments including condensational growth of droplets was then performed. It was found that while the increasing preferential concentration of droplets, as a result of increasing eddy dissipation rate, does result in increases in the instantaneous dispersion of the supersaturation perturbation distribution, the width of the size distribution of droplets, which is a function of the dispersion in the time integral of the supersaturation perturbations, decreases. This result is a consequence of the decrease in decorrelation time of the supersaturation perturbations as the turbulence intensity increases. Comparison of the results herein with the observations made in quasi-adiabatic cloud cores leads one to the conclusion that the microscopic approach, even under the most favorable condition of no turbulence, produces too little broadening to explain the observations.