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Unified Entrainment and Detrainment Closures for Extended Eddy‐Diffusivity Mass‐Flux Schemes

机译:扩展涡流偏差质量磁通方案的统一夹带和碎屑闭合

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We demonstrate that an extended eddy‐diffusivity mass‐flux (EDMF) scheme can be used as a unified parameterization of subgrid‐scale turbulence and convection across a range of dynamical regimes, from dry convective boundary layers, through shallow convection, to deep convection. Central to achieving this unified representation of subgrid‐scale motions are entrainment and detrainment closures. We model entrainment and detrainment rates as a combination of turbulent and dynamical processes. Turbulent entrainment/detrainment is represented as downgradient diffusion between plumes and their environment. Dynamical entrainment/detrainment is proportional to a ratio of a relative buoyancy of a plume and a vertical velocity scale, that is modulated by heuristic nondimensional functions which represent their relative magnitudes and the enhanced detrainment due to evaporation from clouds in drier environment. We first evaluate the closures offline against entrainment and detrainment rates diagnosed from large‐eddy simulations (LES) in which tracers are used to identify plumes, their turbulent environment, and mass and tracer exchanges between them. The LES are of canonical test cases of a dry convective boundary layer, shallow convection, and deep convection, thus spanning a broad range of regimes. We then compare the LES with the full EDMF scheme, including the new closures, in a single column model (SCM). The results show good agreement between the SCM and LES in quantities that are key for climate models, including thermodynamic profiles, cloud liquid water profiles, and profiles of higher moments of turbulent statistics. The SCM also captures well the diurnal cycle of convection and the onset of precipitation. Plain Language Summary The dynamics of clouds and turbulence are too small in scale to be resolved in global models of the atmosphere, yet they play a crucial role in controlling weather and climate. These models rely on parameterizations for representing clouds and turbulence. Inadequacies in these parameterizations have hampered especially climate models for decades; they are the largest source of physical uncertainties in climate predictions. It has proven challenging to represent the wide range of cloud and turbulence regimes encountered in nature in a single parameterization. Here we present such a parameterization that does capture a wide range of cloud and turbulence regimes within a single, unified physical framework, with relatively few parameters that can be adjusted to fit data. The framework relies on a decomposition of turbulent flows into coherent updraft and downdraft (i.e., plumes) and random turbulence in their environment. A key contribution of this paper is to show how the exchange of mass and properties between the plumes and their turbulent environment—the so‐called entrainment and detrainment of air into and out of plumes—can be modeled. We show that the resulting parameterization represents well the most important features of dry convective boundary layers, shallow cumulus convection, and deep cumulonimbus convection.
机译:我们证明,扩展涡流沟通量通量(EDMF)方案可以用作来自干扰对流边界层的一系列动态制度,通过浅对流到深度对流的统一速度湍流和对流的统一参数化。实现统一划分型动作的统一代表的核心是夹带和剥离的封闭。我们将夹带和剥离率模拟为湍流和动态过程的组合。湍流夹带/碎屑被认为是羽毛和环境之间的降级扩散。动态夹带/碎屑与羽流的相对浮力和垂直速度比的比例成比例,其通过启发式非尺寸的函数来调节,该功能由代表其相对幅度和增强的碎屑,由于从干燥环境中的云蒸发蒸发。我们首先评估从大型涡旋模拟(LES)诊断的夹带和碎屑率的封闭率,其中示踪剂用于识别它们之间的羽毛,其湍流环境和群众和示踪剂交换。 LES是一种干燥对流边界层的规范测试用例,浅对流和深对流,从而跨越广泛的制度。然后,在单列模型(SCM)中,我们将LES与完整的EDMF方案(包括新闭合)进行比较。结果表明,SCM和LES之间的良好一致性是气候模型的关键,包括热力学型材,云液体水分谱和湍流统计的更高时刻的概况。 SCM还捕获了对流的昼夜循环和降水的发生。普通语言摘要云和湍流的动态在大气的全球模型中,云和湍流的动态太小,但它们在控制天气和气候方面发挥着至关重要的作用。这些模型依赖于代表云和湍流的参数化。这些参数化的不足因素妨碍了几十年来尤其是气候模型;它们是气候预测中最大的身体不确定性来源。它已证明在单个参数化中代表本质上遇到的广泛云和湍流制度挑战。在这里,我们提出了这样的参数化,该参数化确实在单个统一的物理框架内捕获了广泛的云和湍流状态,可以使用相对较少的参数来调整以适合数据。该框架依赖于湍流的分解成相干的上升法和下降(即羽毛)和环境中的随机湍流。本文的一个关键贡献是展示如何建模羽毛与其湍流环境之间的质量和性质交流和流量的所谓夹带和拆卸空气 - 可以模拟羽毛的空气。我们表明所得到的参数化代表了干燥对流边界层,浅层云峰对流和深层积压对流的最重要特征。

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