Stratified Turbulence in the Atmosphere

大气的湍流动力学跨越了令人惊讶的长度范围，从数千公里宽的天气系统到飓风、雷暴和小漩涡。中尺度是从数百公里到公里的长度尺度的中间范围，在大气动力学中发挥着重要作用。密度分层-较重空气在较轻空气下的分层-除了其他几种现象，包括地球自转、云和地表强迫外，还对中尺度动力学有深远影响。中尺度就像一个湍动的级联，将能量从大尺度转移到越来越小的尺度，在那里它被太阳加热注入，最终被空气的粘性消散。然而，这个级联的细节仍然没有完全被理解。此外，这种级联的小尺度末端，即中尺度湍流分解为亚千米尺度的较小尺度湍流的地方，在大气模型中往往没有得到很好的描述。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公开问题。重力波的作用已经争论了几年，将通过改进的诊断方法来解决，以识别模型输出中的波。小尺度中尺度湍流破裂的本质将用大的、高分辨率的模拟来研究。此外，还将开发更好地表示小尺度湍流影响的方法，而大气模型并不总是明确地捕捉到这些影响。这项研究计划的结果将帮助模型师评估他们的模拟，比较模型，并更准确地表示未解决的湍流，此外，还将促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.