Generation of unbalanced motion at horizontal boundaries in the atmosphere and the oceans

中纬度地区大气和海洋的动力学可以很好地描述为以接近流体静力学和地转平衡的缓慢、大规模运动为重点。然而，重力波形式的快速小尺度运动及其不稳定性在许多过程（如动量输运、湍流、垂直混合和耗散）中起着至关重要的作用，这些过程尤其影响大尺度环流、大气中层环流和海洋热盐环流。因此，产生快速不平衡运动的机制近年来受到了极大的关注。然而，所谓的自发产生机制，即不断发展的流体静力和地转平衡运动直接激发不平衡运动，尽管它们很重要，但仍然知之甚少。一个困难是，这些机制只有在平衡运动发展得足够快时才有效，而这种快速的发展在大气和海洋的大部分中是相当不典型的。然而，在水平边界附近，快速进化是典型的。通过水平边界，我们不仅指海洋表面和固体地球，还指对流层顶，它起着类似的作用。并非巧合的是，这些恰恰是观察到强烈不平衡活动的区域。这些区域的不平衡运动也会产生重要的后果，湍流对飞机的影响即使不是最重要的，也是最明显的。提出的研究将检查如何产生不平衡运动在水平边界发生。这将通过分析简单的模型来完成，这些模型分离了导致不平衡运动产生的边界动力学的关键特征，即表面温度中非常活跃结构的形成，具有小空间尺度和短时间尺度。这一特征已经在（平衡）表面准地转模型的背景下得到了很好的证明，但其对不平衡运动的影响仍有待描述。本课题将首次完成这一任务。它将不仅考虑锋面崩塌的情况，而且考虑更复杂的小规模不稳定级联，这些不稳定是地表演化的特征，并导致某种形式的湍流。目的是将不平衡运动的特征与表面场的特征联系起来，作为最终目标，参数化其对混合和耗散的影响。

The dynamics of the atmosphere and oceans at mid-latitudes is well described by focusing on the slow, large-scale motion, in nearly hydrostatic and geostrophic balance. However, fast small-scale motion in the form of gravity waves and associated instabilities plays a crucial role in a number of processes (such as momentum transport, turbulence, vertical mixing, and dissipation), which impact on large-scale circulations, middle-atmospheric circulation and ocean thermohaline circulation in particular. The mechanisms of generation of the fast, unbalanced motion have therefore received a great deal of attention in recent years. However, the so-called spontaneous-generation mechanisms, whereby the evolving hydrostatically and geostrophically balanced motion directly excites unbalanced motion, remain poorly understood, in spite of their importance. One difficutly is that these mechanisms are only efficient when the balanced motion evolves sufficiently rapidly, and that such a rapid evolution is rather atypical in the bulk of the atmosphere and oceans. Near horizontal boundaries, however, a rapid evolution is typical. By horizontal boundaries, we refer not only to the ocean surface and solid earth, but also to the tropopause, which plays a similar role. Not coincidentally, these are precisely the regions where strong unbalanced activity is observed. These also are regions where unbalanced motion has crucial consequences, the effect of turbulence on airplanes being the most obvious, if not the most important. The proposed research will examine how the generation of unbalanced motion at horizontal boundaries takes place. This will be done by analysing simple models which isolate the key feature of the boundary dynamics that is reponsible for unbalanced-motion generation, namely the formation of very active structures in the surface temperature, with both small spatial scales and short temporal scales. This feature has been well demonstrated in the context of the (balanced) surface quasi-geostrophic model, but its impact on unbalanced motion remains to be described. The proposed research will carry out this task for the first time. It will consider not only situations of frontal collapse but also the more complex cascade of small-scale instabilities which characterise surface evolution and leads to a form of turbulence. The aim is to relate the characteristics of the unbalanced motion to those of the surface fields with, as ultimate goal, the parameterisation of its impact on mixing and dissipation.