Stratified Turbulence in the Atmosphere

大气的湍流流体动力学跨越了一个惊人的长度尺度，从数千公里宽的天气系统，到飓风、雷暴和小漩涡。中尺度是数百公里到公里的中间尺度，在大气动力学中起着重要作用。密度分层——较重的空气在较轻的空气下分层——对中尺度动力学有着深远的影响，此外还有其他一些现象，包括地球自转、云层和地表强迫。中尺度就像一个湍流级联，将能量从大尺度（由太阳加热注入）转移到越来越小的尺度，在那里它最终被空气的粘性消散。然而，这个级联的细节仍然没有被完全理解。此外，在这个级联的小尺度末端，中尺度湍流在亚公里尺度上分解成较小尺度的湍流，在大气模式中往往不能很好地表现出来。这里提出的工作将使用精心设计的实验和各种大气数值模型来回答有关中尺度级联的重要开放问题。引力波的作用已经争论了好几年，将通过改进诊断来识别模型输出中的波。小尺度中尺度湍流破裂的性质将通过大型高分辨率模拟进行研究。此外，还将开发一些方法，以更好地表示小规模湍流的影响，而这种影响并不总是被大气模式明确地捕捉到。这个研究项目的结果将帮助建模者评估他们的模拟，比较模型，更准确地代表未解决的湍流，除了推进我们对大气的基本理解。

The turbulent fluid dynamics of the atmosphere span an amazing range of length scales, from weather systems of thousands of kilometres across, down to hurricanes, thunderstorms, and small eddies. The mesoscale is an intermediate range of length scales from hundreds of kilometres to kilometres, which plays an important role in the dynamics of the atmosphere. Density stratification — the layering of heavier air under lighter air — has a profound effect on  mesoscale dynamics, in addition to several other phenomena, including the Earth's rotation, clouds, and surface forcing. The mesoscale acts as a turbulent cascade, transferring energy from large scales, where it is injected by solar heating, down to smaller and smaller scales, where it is eventually dissipated by the viscosity of air. However, the details of this cascade are still not fully understood. In addition, the small-scale end of this cascade, where mesoscale turbulence breaks down into smaller-scale turbulence at sub-kilometre scales, is often not well represented in atmospheric models. The work proposed here will use carefully designed experiments with a variety of numerical models of the atmosphere to answer important open questions about the mesoscale cascade. The role of gravity waves, which has been debated for several years, will be addressed with improved diagnostics for identifying waves in model output. The nature of the breakdown of mesoscale turbulence at small scales will be investigated with large, high-resolution simulations. In addition, methods for better representing the effects of small-scale turbulence, which are not always explicitly captured by atmospheric models, will be developed. The outcomes of this research program will assist modellers in assessing their simulations, comparing models, and more accurately representing unresolved turbulence, in addition to advancing our fundamental understanding of the atmosphere.