Numerics and subgrid-scale modeling in large eddy simulations of stratocumulus clouds.
Numerics and subgrid-scale modeling in large eddy simulations of stratocumulus clouds.
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DOI：
10.1002/2016ms000778
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发表时间：
2017-06
影响因子：
6.8
通讯作者：
Tan Z
中科院分区：
文献类型：
--
作者：
Pressel KG;Mishra S;Schneider T;Kaul CM;Tan Z
层积云是边界层云最常见的类型；它们的辐射效应强烈地调节着气候。层积云的大涡模拟（LES）常常难以与观测结果保持一致，因为在云顶的夹卷界面层存在着急剧的梯度。层积云给LES带来的挑战在基于DYCOMS - II外场试验的LES相互比较中发现的多种解决方案中显而易见，在相同的初始条件和边界条件下，模拟的液态水路径差异近12倍。在此，我们重新审视DYCOMS - II RF01案例，并表明通过仅改变运动方程的数值处理方法和次网格尺度（SGS）闭合的性质，在单个LES代码中就可以实现先前LES结果的广泛差异。确定了与DYCOMS - II观测结果保持最高一致性的模拟。结果表明，对所有可解的平流项使用加权本质无振荡（WENO）数值方法且无显式SGS闭合，始终能产生最高保真度的模拟。这表明WENO格式固有的数值耗散在此层积云案例中起到了高质量的隐式SGS闭合的作用。相反，对动量平流使用振荡中心差分数值格式，对标量使用WENO数值方法，并显式模拟SGS通量，则始终产生最低保真度的模拟。我们将此归因于动量场中的数值误差与标量SGS模型相互作用，在云顶附近产生了异常大的SGS通量。
层积云LES模拟的保真度取决于平流格式和SGS闭合
无显式SGS闭合的WENO格式能很好地模拟层积云
一个简单的模型展示了平流格式和SGS闭合如何相互作用并影响云模拟
Stratocumulus clouds are the most common type of boundary layer cloud; their radiative effects strongly modulate climate. Large eddy simulations (LES) of stratocumulus clouds often struggle to maintain fidelity to observations because of the sharp gradients occurring at the entrainment interfacial layer at the cloud top. The challenge posed to LES by stratocumulus clouds is evident in the wide range of solutions found in the LES intercomparison based on the DYCOMS‐II field campaign, where simulated liquid water paths for identical initial and boundary conditions varied by a factor of nearly 12. Here we revisit the DYCOMS‐II RF01 case and show that the wide range of previous LES results can be realized in a single LES code by varying only the numerical treatment of the equations of motion and the nature of subgrid‐scale (SGS) closures. The simulations that maintain the greatest fidelity to DYCOMS‐II observations are identified. The results show that using weighted essentially non‐oscillatory (WENO) numerics for all resolved advective terms and no explicit SGS closure consistently produces the highest‐fidelity simulations. This suggests that the numerical dissipation inherent in WENO schemes functions as a high‐quality, implicit SGS closure for this stratocumulus case. Conversely, using oscillatory centered difference numerical schemes for momentum advection, WENO numerics for scalars, and explicitly modeled SGS fluxes consistently produces the lowest‐fidelity simulations. We attribute this to the production of anomalously large SGS fluxes near the cloud tops through the interaction of numerical error in the momentum field with the scalar SGS model. Fidelity of LES simulations of stratocumulus clouds depends on advection schemes and SGS closures WENO schemes without explicit SGS closures simulate stratocumulus well A simple model shows how advection schemes and SGS closures interact and affect cloud simulations