Stratified Turbulence in the Atmosphere

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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.