Mixed‐phase clouds in a turbulent environment. Part 1: Large‐eddy simulation experiments
Mixed‐phase clouds in a turbulent environment. Part 1: Large‐eddy simulation experiments
复制标题
湍流环境中的混合相云第 1 部分：大涡模拟实验。
DOI：
10.1002/qj.2177
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发表时间：
2014
影响因子：
8.9
通讯作者：
B. Shipway
中科院分区：
文献类型：
--
作者：
A. Hill;P. Field;K. Furtado;A. Korolev;B. Shipway
混合相云在热力学上是不稳定的，也就是说，在没有其他外力作用的情况下，冰会以过冷液态水为代价生长，最终导致云完全冻结。在存在动力强迫的情况下，例如有规律的运动或湍流波动，在冰云中可以产生液态水。早期的理论思考已经确定了在已有冰云中产生液态水必须满足的两个必要条件：（i）冰云气块的垂直速度必须超过一个阈值速度；（ii）冰云气块的垂直位移必须高于一个阈值高度以达到水汽饱和。本文使用大涡模拟（LES）模型来研究仅满足这些条件是否可以作为在湍流环境中混合相云出现的预测工具。研究表明，一般对于一系列微物理假设、冰浓度和热力学条件，确定满足这两个动力条件的点能够很好地估计LES中的区域液态云比例以及液态云比例随时间的演变。当存在相对较大的液态水含量时，理论低估了液态云比例。此外，当允许冰沉降时，理论高估了液态云比例。文中提出了对该理论的两种修正，并展示了这些修正如何减少预测液态云比例与模拟云比例之间的偏差。
Mixed‐phase clouds are thermodynamically unstable, i.e. with no other forcing ice will grow at the expense of supercooled liquid water, eventually leading to complete glaciation of the cloud. In the presence of dynamic forcing, e.g. regular motions or turbulent fluctuations, liquid water can be generated in an ice cloud. Earlier theoretical considerations have identified two necessary conditions that had to be satisfied to produce liquid water in a pre‐existing ice cloud: (i) the vertical velocity of an ice cloud parcel must exceed a threshold velocity and (ii) the vertical displacement of an ice cloud parcel must be above a threshold altitude to achieve water saturation. This article uses a large‐eddy simulation (LES) model to investigate whether satisfying these conditions alone can be used as a predictive tool for the occurrence of mixed‐phase clouds in a turbulent environment. It is shown that, in general for a range of microphysical assumptions, ice concentrations and thermodynamic conditions, identifying points that satisfy these two dynamic conditions results in a good estimate of the domain liquid cloud fraction and the evolution of the liquid cloud fraction over time from the LES. When relatively large liquid water contents are present, theory underpredicts liquid cloud fraction. Further, when ice is permitted to sediment, theory overpredicts liquid cloud fraction. Two modifications to the theory are suggested, and it is demonstrated how these reduce the deviation of predicted liquid cloud fraction from simulated cloud fraction.