Composited structure of non-precipitating shallow cumulus clouds
Composited structure of non-precipitating shallow cumulus clouds
复制标题
非降水浅层积云的复合结构
DOI：
10.1002/qj.4101
复制
发表时间：
2021
影响因子：
8.9
通讯作者：
Gu J
中科院分区：
文献类型：
--
作者：
Gu J
利用海洋和大陆的大涡模拟，通过一种复合算法研究了浅层云内外热力学和动力变量的归一化分布。标准化星等在云中心附近最大，向外减小。相对湿度（RH）和云液态水（）随环境的变化而平稳下降，而垂直速度、虚位温（）和位温（）的扰动在云边界处表现出更为复杂的行为。在逆温层以下，在垂直速度从上升气流转变为云外下沉壳之前变为负值，表明上升气流为负浮力的过渡区存在。由于云外的下沉气流和边缘水平湍流混合增强，标准化湍流动能（TKE）和水平湍流动能（HTKE）从云中心向外的降低速度要慢于热力学变量。垂直风切变的分布均呈现非对称结构，负浮力空气较多，下沉气流较强，下切变侧TKE较大。我们讨论了理论模型和参数化中分布的几个含义。云底附近的正浮力主要是由于水蒸气的虚效应，强调了湿气在触发中的作用。平均垂直速度约为每个云内最大垂直速度的一半，这为某些模型实现可能的幂律分布提供了约束。最后，利用不同变量的归一化分布估计了云内的垂直热湿通量。结果表明，云边附近的分布和最大扰动的变异性需要仔细处理。逆温层的通量被低估了，因为云顶下降气流不能被很好地捕获。
The normalized distributions of thermodynamic and dynamical variables both within and outside shallow clouds are investigated through a composite algorithm using large‐eddy simulations of oceanic and continental cases. The normalized magnitude is maximum near the cloud centre and decreases outwards. While relative humidity (RH) and cloud liquid water () decrease smoothly to match the environment, the vertical velocity, virtual potential temperature (), and potential temperature () perturbations have more complicated behaviour towards the cloud boundary. Below the inversion layer,becomes negative before the vertical velocity has turned from an updraft to a subsiding shell outside the cloud, indicating the presence of a transition zone where the updraft is negatively buoyant. Due to the downdraft outside the cloud and enhanced horizontal turbulent mixing across the edge, the normalized turbulent kinetic energy (TKE) and horizontal turbulent kinetic energy (HTKE) decrease more slowly from the cloud centre outwards than the thermodynamic variables. The distributions all present asymmetric structures in response to the vertical wind shear, with more negatively buoyant air, stronger downdrafts, and larger TKE on the downshear side. We discuss several implications of the distributions for theoretical models and parameterizations. Positive buoyancy near the cloud base is mostly due to the virtual effect of water vapour, emphasizing the role of moisture in triggering. The mean vertical velocity is found to be approximately half the maximum vertical velocity within each cloud, providing a constraint to achieve possible power‐law distributions for some models. Finally, the normalized distributions for different variables are used to estimate the vertical heat and moisture fluxes within clouds. The results suggest that distributions near the cloud edge and variability of maximum perturbations need careful treatment. The fluxes are underestimated in the inversion layer because cloud‐top downdrafts cannot be captured well.