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Numerics and subgrid‐scale modeling in large eddy simulations of stratocumulus clouds

机译：层积云大涡模拟中的数值和亚网格尺度建模

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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.
机译：层积云是边界层云最常见的类型。它们的辐射作用强烈地调节了气候。层积云的大涡模拟（LES）通常难以维持观测的保真度，因为在云层顶部夹带界面层出现了急剧的梯度。层积云对LES构成的挑战在基于DYCOMS-II野战的LES比对中发现的解决方案范围很广，在相同的初始条件和边界条件下模拟的液态水路径变化了将近12倍。这里我们回顾了DYCOMS-II RF01的情况，并表明，仅通过改变运动方程的数值处理方式和子网格规模（SGS）闭包的性质，就可以在单个LES代码中实现以前的LES结果的广泛范围。确定了保持DYCOMS-II观测最大保真度的模拟。结果表明，对所有已解析的对流项使用加权的基本非振荡（WENO）数值，并且没有明确的SGS闭合始终可产生最高保真度的模拟。这表明在这种平积层情况下，WENO方案固有的数值耗散起着高质量隐式SGS封闭的作用。相反，对动量对流使用振荡中心差分数值方案，对标量使用WENO数值以及显式建模的SGS通量始终能产生最低保真度的模拟。我们将其归因于通过动量场中的数值误差与标量SGS模型的相互作用，在云顶附近产生了异常大的SGS通量。

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