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通过着重于在近流体静力学和地转平衡中的缓慢、大尺度运动而得到很好的描述。然而，重力波形式的快速小尺度运动和相关的不稳定性在许多过程（如动量输送、湍流、垂直混合和耗散）中起着至关重要的作用，这些过程特别影响大尺度环流、中层大气环流和海洋温盐环流。因此，近年来，产生快速不平衡运动的机制受到了极大的关注。然而，所谓的自发生成机制，即不断发展的流体静力学和地转平衡运动直接激发不平衡运动，仍然知之甚少，尽管他们的重要性。一个困难是，只有当平衡运动发展得足够快时，这些机制才有效，而这种快速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，快速演化是典型的。水平边界不仅指海洋表面和固体地球，也指对流层顶，它起着类似的作用。并非巧合的是，这些地区正是观察到强烈不平衡活动的地区。这些区域也是不平衡运动具有关键后果的区域，湍流对飞机的影响是最明显的，如果不是最重要的话。拟议的研究将探讨如何在水平边界的不平衡运动的产生发生。这将通过分析简单的模型来完成，这些模型隔离了边界动力学的关键特征，该特征是不平衡运动产生的原因，即在表面温度中形成非常活跃的结构，具有小的空间尺度和短的时间尺度。这一特征在（平衡）地面准地转模式中得到了很好的证明，但它对不平衡运动的影响仍有待描述。这项研究将首次完成这项任务。它不仅要考虑锋面崩溃的情况，而且要考虑更复杂的小尺度不稳定性的级联，这些不稳定性阻碍表面演变并导致一种湍流形式。其目的是将不平衡运动的特征与表面场的特征相关联，并将其对混合和耗散的影响参数化。

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.