Improving convective storm simulations through scale-adaptive and flow-adaptive sub-grid methods

该项目旨在改进短期数值天气预报和小尺度区域气候预报所需的数值模拟。在过去的十年中，这种模拟在“允许对流”的分辨率下变得实用起来。允许对流的模拟使我们的预报能力发生了一步变化，因为单个对流风暴现在可以直接用数值模式模拟。许多高影响天气事件都与这种风暴有关，特别是在山洪暴发和破坏性阵风的情况下。对这些模式的使用和依赖继续稳步增长，它们的好处是公认的。然而，这一经验也暴露出一些重要的问题和局限性。模拟结果对模型分辨率非常敏感。模型分辨率的提高并不能直接转化为模拟效果的改善。事实上，我们最近发现的证据表明，即使在非常高的分辨率下，关键指标也不会在当前模型中收敛。模拟对湍流混合处理的选择非常敏感。模拟对云的形成和微物理的假设非常敏感。这些问题密切相关。在现实中，发生在比模型网格更小尺度上的过程与发生在更大尺度上的过程相互作用并产生影响。在“模型世界”中，小尺度根本不存在，但我们试图通过引入关于湍流混合和云方案的额外假设来模拟它们的影响。正是这些假设造成了问题。在工程流体动力学领域，使用不再规定关键湍流混合参数的“动态”技术，模型行为作为分辨率的函数得到了改进。相反，它们可以在模拟过程中作为演化流的函数“动态地”计算出来。在进行计算时引用的关键原则是确保尊重流动的湍流特性的适当的尺度依赖性。与目前该技术的应用相比，对流风暴的模拟有额外的热力学和微物理复杂性（或者，正如我们所喜欢的那样，额外的物理使大气产生如此丰富和有趣的流动形式）。该博士项目将扩展和加强动态技术，以开发适合于在对流允许的尺度上模拟对流风暴的尺度自适应和流动自适应亚网格湍流模型。

The project is concerned with improving the numerical simulations required for short-range numerical weather prediction and small-scale regional climate prediction. Over the past decade, such simulations have become practical at "convection-permitting" resolutions. Convection-permitting simulations have enabled a step change in our forecasting capabilities because individual convective storms can now be simulated directly with the numerical model. Many high-impact weather events are associated with such storms, particularly in cases of flash flooding and damaging wind gusts.There continues to be a steady increase in the use and reliance on such models, and their benefits are well recognized. However, that experience has also revealed some important issues and limitations.a. The simulations are strongly sensitive to the model resolution. Increases in model resolution do not translate into improvements in simulation in any straightforward way. Indeed, we have recently found evidence that key metrics do not converge in current models even at very high resolutions.b. The simulations are strongly sensitive to choices made in the treatment of turbulent mixing.c. The simulations are strongly sensitive to assumptions about cloud formation and microphysics.These issues are strongly related. In reality processes that happen on smaller scales than the model grid interact with and influence the processes that take place on larger scales. In "model world" the smaller scales simply do not exist, but we try to mimic their effects by introducing extra assumptions about turbulent mixing and cloud schemes. It is those assumptions that create problems.In the engineering fluid dynamics community, improved model behaviour as a function of resolution has been achieved using "dynamic" techniques in which key turbulent mixing parameters are no longer prescribed. Rather, they can be computed "on the fly" during the simulation as a function of the evolving flow. The key principle invoked in making the computation is to ensure respect for the proper scale-dependence of the turbulent characteristics of the flow.Compared to the current applications of the technique, simulations of convective storms have additional thermodynamic and microphysical complications (or, as we like to think of it, additional physics that makes the atmosphere produce such rich and interesting forms of flow). This PhD project will extend and enhance the dynamic technique to develop a scale-adaptive and flow-adaptive sub-grid turbulence model that is suitable for simulations of convective storms at convection-permitting scales.