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Non-local convergence coupling in a simple stochastic convection model

机译：简单随机对流模型中的非局部收敛耦合

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Observational studies show a strong correlation between large-scale wind convergence and precipitation. However, using this as a convective closure assumption to determine the total precipitation in a numerical model typically leads to deleterious wave-CISK behavior such as grid-scale noise. The quasi-equilibrium (QE) schemes ameliorate this issue and smooth the precipitation field, but still inadequately represent the intermittent and organized nature of tropical convection. However, recent observational evidence highlights that the large-scale convergence field primarily affects precipitation by increasing the overall convective cloud fraction rather than the energetics of individual convective elements. In this article, the dynamical consequences of this diagnostic observation are studied using a simple one baroclinic mode stochastic model for convectively coupled waves. A version of this model is implemented which couples the stochastic formation of convective elements to the wind convergence. Linearized analysis shows that using the local convergence results in a classic wave-CISK standing instability where the growth rate increases with the wavenumber. However, using a large-scale averaged convergence restricts the instability to physically plausible scales. Convergence coupling is interpreted as a surrogate for the non-local effects of gregarious convection. In nonlinear stochastic simulations with a non-uniform imposed sea surface temperature (SST) field, the non-local convergence coupling introduces desirable intermittent variability on intraseasonal time scales. Convergence coupling leads to a circulation with a similar mean but higher variability than the equivalent parameterization without convergence coupling. Finally, the model is shown to retain these features on fine and coarse mesh sizes. (C) 2016 Elsevier B.V. All rights reserved.
机译：观测研究表明，大规模的风辐合与降水之间有很强的相关性。但是，将其用作对流闭合假设来确定数值模型中的总降水量通常会导致有害的波CISK行为，例如网格规模噪声。准平衡（QE）方案改善了这一问题并平滑了降水场，但仍不足以代表热带对流的间歇性和有组织性。但是，最近的观测证据表明，大范围的汇聚场主要通过增加整体对流云量而不是单个对流元素的能量来影响降水。在本文中，使用简单的对流耦合波的一种斜压模式随机模型研究了此诊断观测的动力学后果。实现了该模型的一种版本，该模型将对流元素的随机形式耦合到风收敛。线性化分析显示，使用局部收敛会导致经典的CISK驻波不稳定性，其中增长率随波数增加。但是，使用大规模平均收敛将不稳定性限制在物理上合理的规模上。会聚耦合被解释为群居对流的非局部效应的替代。在具有非均匀强加的海面温度（SST）场的非线性随机模拟中，非局部收敛耦合在季节内时标上引入了所需的间歇性变化。与没有收敛耦合的等效参数化相比，收敛耦合导致的循环具有相似的均值，但变异性更高。最后，该模型显示在细网格和粗网格上保留了这些特征。 （C）2016 Elsevier B.V.保留所有权利。

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