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

该项目涉及改进短期数值天气预报和小规模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“对流允许”的决议中已经变得实用。允许对流的模拟使我们的预报能力发生了一个飞跃，因为现在可以直接用数值模式模拟单个对流风暴。许多影响大的天气事件都与这类风暴有关，特别是在山洪暴发和破坏性阵风的情况下，对这类模式的使用和依赖继续稳步增加，其好处也得到了广泛承认。然而，这一经验也揭示了一些重要的问题和局限性。模拟对模型分辨率非常敏感。模型分辨率的提高并不能以任何直接的方式转化为模拟的改进。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，当前模型中的关键指标也不会收敛。模拟对湍流混合处理中的选择非常敏感。模拟对云的形成和微观物理的假设非常敏感，这些问题是密切相关的。实际上，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影响它们。在“模型世界”中，小尺度是不存在的，但我们试图通过引入额外的湍流混合和云方案的假设来模拟它们的效果。正是这些假设造成了问题。在工程流体动力学界，使用“动态”技术，不再规定关键的湍流混合参数，从而实现了作为分辨率函数的模型行为的改进。相反，它们可以在模拟过程中作为演变流的函数“在运行中”计算。计算中的关键原则是要确保尊重湍流特性的尺度依赖性。与目前的技术应用相比，对流风暴的模拟具有额外的热力学和微物理复杂性（或者，我们喜欢这样认为，使大气产生如此丰富和有趣的流动形式的额外物理）。该博士项目将扩展和增强动态技术，以开发一种适用于对流允许尺度下对流风暴模拟的尺度自适应和流量自适应子网格湍流模型。

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.