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范围，从数千公里的天气系统到飓风，雷暴和小漩涡。中尺度是长度尺度从几百公里到几公里的中间范围，在大气动力学中起着重要作用。密度分层-较轻的空气下的较重的空气分层-对中尺度动力学有深远的影响，除了其他几种现象，包括地球自转，云和表面强迫。中尺度的作用就像湍流级联，将能量从大尺度转移到越来越小的尺度上，在大尺度上，能量是通过太阳能加热注入的，在小尺度上，能量最终被空气的粘性耗散。然而，这一级联的细节仍不完全清楚。此外，这种级联的小尺度端，即中尺度湍流分解成千米以下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体现。这里提出的工作将使用精心设计的实验与各种数值模式的大气，以回答重要的中尺度叶栅开放的问题。重力波的作用已经争论了好几年，将通过改进诊断来解决，以识别模型输出中的波。将用大型高分辨率模拟研究小尺度中尺度湍流破裂的性质。此外，还将制定更好地表示小尺度湍流影响的方法，因为大气模型并不总是明确地捕捉到这种影响。这项研究计划的成果将有助于建模者评估他们的模拟，比较模型，更准确地表示未解决的湍流，以及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.