Thermal Chains and Entrainment in Cumulus Updrafts. Part II: Analysis of Idealized Simulations
Thermal Chains and Entrainment in Cumulus Updrafts. Part II: Analysis of Idealized Simulations
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积云上升气流中的热链和夹带。
DOI：
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发表时间：
2020
影响因子：
3.1
通讯作者：
S. Giangrande
中科院分区：
文献类型：
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作者：
J. Peters;H. Morrison;A. Varble;W. Hannah;S. Giangrande
研究表明，深对流的结构通常由一系列上升的热气流或“热链”组成，这与用于构建积云参数化的现有概念模型形成鲜明对比。本研究第一部分中获得的上升气流特性的简化理论表达式用于提出解释为什么会出现这种结构的假设。在这个假设中，积云上升气流结构受到上升气流垂直速度最大值以下有组织的夹带的强烈影响。在干燥环境中，这种增强的夹带会局部降低冷凝率并增加蒸发，从而削弱浮力。对于中等至较大的初始云半径 R，这会将上升气流分解为一系列离散的上升运动脉冲（即热链）。对于较小的 R，这会导致单一、隔离的上升热结构。相比之下，潮湿环境被假设有利于中等至大 R 的羽状上升气流。在一系列轴对称数值云模拟中，系统地改变 R 和环境相对湿度 (RH) 以检验这一假设。这些运行中的分数夹带率、被动示踪剂浓度、浮力和垂直速度的垂直剖面与根据第一部分中的理论表达式计算的垂直剖面非常吻合。模拟分析支持假设的上升气流结构对 R 和 RH 的依赖性，即它是否由孤立的热气流、热链或羽流组成，以及有组织的夹带在驱动这种依赖性中的作用。分析了额外的三维 (3D) 湍流云模拟，这些 3D 运行的行为在质量上与理论表达式和轴对称模拟一致。
Research has suggested that the structure of deep convection often consists of a series of rising thermals, or “thermal chain,” which contrasts with existing conceptual models that are used to construct cumulus parameterizations. Simplified theoretical expressions for updraft properties obtained in Part I of this study are used to develop a hypothesis explaining why this structure occurs. In this hypothesis, cumulus updraft structure is strongly influenced by organized entrainment below the updraft’s vertical velocity maximum. In a dry environment, this enhanced entrainment can locally reduce condensation rates and increase evaporation, thus eroding buoyancy. For moderate-to-large initial cloud radius R, this breaks up the updraft into a succession of discrete pulses of rising motion (i.e., a thermal chain). For small R, this leads to the structure of a single, isolated rising thermal. In contrast, moist environments are hypothesized to favor plume-like updrafts for moderate-to-large R. In a series of axisymmetric numerical cloud simulations, R and environmental relative humidity (RH) are systematically varied to test this hypothesis. Vertical profiles of fractional entrainment rate, passive tracer concentration, buoyancy, and vertical velocity from these runs agree well with vertical profiles calculated from the theoretical expressions in Part I. Analysis of the simulations supports the hypothesized dependency of updraft structure on R and RH, that is, whether it consists of an isolated thermal, a thermal chain, or a plume, and the role of organized entrainment in driving this dependency. Additional three-dimensional (3D) turbulent cloud simulations are analyzed, and the behavior of these 3D runs is qualitatively consistent with the theoretical expressions and axisymmetric simulations.