Stochastic coalescence in Lagrangian cloud microphysics
Stochastic coalescence in Lagrangian cloud microphysics
复制标题
拉格朗日云微物理中的随机合并
DOI：
10.5194/acp-17-13509-2017
复制
发表时间：
2017
影响因子：
6.3
通讯作者：
H. Pawłowska
中科院分区：
文献类型：
--
作者：
P. Dziekan;H. Pawłowska
摘要。采用Shima et al.（2009）的超液滴法（super-droplet method， SDM）研究了云滴碰撞生长的随机性。统计数据是通过在单个混合良好的细胞中碰撞-合并模拟的集合来计算的。将SDM与直接数值模拟和主方程进行了比较。本文认为，一个计算液滴代表一个实际液滴的SDM模拟与主方程的精度水平相同。利用这种模拟研究了自转化时间、溶胶-凝胶转变和幸运液滴生长速率的波动，并与理论预测进行了比较。发现聚结细胞的大小对系统的行为有很大的影响。在小细胞中，液滴大小和液滴耗竭的相关性减慢了雨的形成。在大的细胞中，雨滴之间的碰撞更频繁，这也可以减缓降雨的形成。雨滴间碰撞率的增加可能是由于假设混合体积过大而造成的假象。在中等大小的细胞中，雨水与云水的比例最高。接下来，我们使用这些精确的模拟来确定更近似方法的有效性：Smoluchowski方程和多重度大于1的SDM。在后者中，我们确定需要多少计算液滴才能正确地模拟期望数和自动转换时间的标准偏差。湍流混合良好的体积的最大尺寸估计为Vmix = 1.5 × 10−2 cm3。斯摩鲁霍夫斯基方程在如此小的体积中是无效的。有人认为，更大的体积可以被认为是近似混合的，但这种近似需要与解决液滴运动的细网格模拟的比较来支持。
Abstract. Stochasticity of the collisional growth of cloud droplets is studied using the super-droplet method (SDM) of Shima et al.(2009). Statistics are calculated from ensembles of simulations of collision–coalescence in a single well-mixed cell. The SDM is compared with direct numerical simulations and the master equation. It is argued that SDM simulations in which one computational droplet represents one real droplet are at the same level of precision as the master equation. Such simulations are used to study fluctuations in the autoconversion time, the sol–gel transition and the growth rate of lucky droplets, which is compared with a theoretical prediction. The size of the coalescence cell is found to strongly affect system behavior. In small cells, correlations in droplet sizes and droplet depletion slow down rain formation. In large cells, collisions between raindrops are more frequent and this can also slow down rain formation. The increase in the rate of collision between raindrops may be an artifact caused by assuming an overly large well-mixed volume. The highest ratio of rain water to cloud water is found in cells of intermediate sizes. Next, we use these precise simulations to determine the validity of more approximate methods: the Smoluchowski equation and the SDM with multiplicities greater than 1. In the latter, we determine how many computational droplets are necessary to correctly model the expected number and the standard deviation of the autoconversion time. The maximal size of a volume that is turbulently well mixed with respect to coalescence is estimated at Vmix = 1.5 × 10−2 cm3. The Smoluchowski equation is not valid in such small volumes. It is argued that larger volumes can be considered approximately well mixed, but such approximation needs to be supported by a comparison with fine-grid simulations that resolve droplet motion.
影响因子：
4.4
作者：
Gillespie, Daniel T.;Petzold, Linda R.;Seitaridou, Effrosyni
通讯作者：
Seitaridou, Effrosyni
影响因子：
5.1
作者：
S. Unterstrasser;F. Hoffmann;Marion Lerch
通讯作者：
S. Unterstrasser;F. Hoffmann;Marion Lerch