A comprehensive investigation is carried out to establish best practice guidelines for the modelling of statistically steady-state non-neutral urban boundary layers (UBL) using large-eddy simulation (LES). These steady-state simulations enable targeted studies under realistic non-neutral conditions without the complications associated with the inherently transient nature of the UBL. An extensive set of simulations of convective and stable conditions is carried out to determine which simplifications, volumetric forcings, and boundary conditions can be applied to replicate the mean and turbulent (variance and covariance) statistics of this intrinsically transient problem most faithfully. In addition, a new method is introduced in which a transient simulation can be ‘frozen’ into a steady state. It is found that non-neutral simulations have different requirements to their neutral counterparts. In convective conditions, capping the boundary-layer height h with the top of the modelled domain to h /5 and h /10 (which is common practice in neutral simulations) reduces the turbulent kinetic energy by as much as 61% and 44%, respectively. Consistent with the literature, we find that domain heights $$l_z \ge 5 |L|$$ l z ≥ 5 | L | are necessary to reproduce the convective-boundary-layer dynamics, where L is the Obukhov length. In stably stratified situations, the use of a uniform momentum forcing systematically underestimates the mechanical generation of turbulence over the urban canopy layer, and therefore leads to misrepresentations of both the inner- and outer-layer dynamics. The new ‘frozen-transient’ method that is able to maintain a prescribed flow state (including entrainment at the boundary-layer top) is shown to work well in both stable and convective conditions. Guidelines are provided for future studies of the capped and uncapped convective and stable UBL.

中文翻译：

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对流稳定城市边界层的稳态大涡模拟

进行了一项综合调查，以建立使用大涡模拟 (LES) 对统计稳态非中性城市边界层 (UBL) 进行建模的最佳实践指南。这些稳态模拟能够在现实的非中性条件下进行有针对性的研究，而不会出现与 UBL 固有的瞬态性质相关的并发症。对对流和稳定条件进行了大量模拟，以确定可以应用哪些简化、体积强迫和边界条件来最忠实地复制这个固有瞬态问题的均值和湍流（方差和协方差）统计数据。此外，还引入了一种新方法，可以将瞬态模拟“冻结”到稳定状态。发现非中性模拟与其中性对应物具有不同的要求。在对流条件下，将建模域顶部的边界层高度 h 限制为 h /5 和 h /10（这是中性模拟中的常见做法），可将湍流动能降低多达 61% 和 44%，分别。与文献一致，我们发现域高度 $$l_z \ge 5 |L|$$ lz ≥ 5 | 升| 是重现对流边界层动力学所必需的，其中 L 是奥布霍夫长度。在稳定分层的情况下，使用均匀的动量强迫系统地低估了城市冠层上湍流的机械产生，因此导致对内层和外层动力学的误解。新的“冻结瞬态”方法能够保持规定的流动状态（包括边界层顶部的夹带），在稳定和对流条件下都能很好地工作。为未来研究有上限和无上限的对流稳定 UBL 提供了指南。