Stochastic and dynamical models of sudden stratospheric warmings

研究领域：连续介质力学该项目的目的是通过开发一系列简化的模型来研究平流层突然变暖的基本流体动力学过程。平流层突然变暖(SSW)是通常在北半球冬季每两到三年发生一次的大气现象。它们涉及极地平流层温度在短短1-2天内迅速上升20-30K。南半球也有一次记录在案的事件，这是2002年发生的近70年有记录的观测结果。SSW对气候很重要，因为SSW被认为是温带外平流层年际变化的主要来源，在过去的20年里一直是研究的主题，因为它们被发现对北半球地表气候产生重大影响。SSW被认为主要是流体动力学现象。极地温度的快速上升是因为被困在被称为极地平流层涡旋的柱状结构中的极地冷空气在行星尺度波(Rossby波)的作用下从极点移位。本项目的目的是从流体动力学，特别是关于Rossby波在涡旋上的传播的理解，来理解SSW的发生。我们将关注的关键问题是SSW的频率在不断变化的气候中可能会如何变化。该项目将通过开发SSW简化模型的层次结构来进行，每个模型都能捕捉到流体动力学行为的更多细节。所考虑的最简单的模型将是随机应变流动中的椭圆形旋涡。令人感兴趣的问题是，涡旋变得细长的概率如何取决于应变流的统计数据。这里的随机应变流代表了动力活跃的对流层对平流层的影响，其统计数据将随着气候的变化而变化。有趣的是，同样的模型也可以用来分析二维湍流中的涡流寿命。该层次结构中的下一个模型将是球体上的干燥原始方程模型，该模型将被设置为在更真实的设置中捕捉对流层和平流层之间的基本流体动力相互作用。将分析长积分的统计数据，以确定来自简单椭圆涡旋模式的见解是否有助于预测模式SSW发生的变化。我们将从模型和观察到的行为之间的比较中得出结论，以期解释(A)SSW频率的预测变化，以及(B)不同未来情景之间的行为变化。

Research Area: Continuum MechanicsThe aim of the project is to investigate the fundamental fluid dynamics of sudden stratospheric warmings, by developing a range of simplified models capturing the key fluid dynamical processes involved.Sudden stratospheric warmings (SSWs) are atmospheric phenomena that typically occur every two to three years during the Northern hemisphere winter. They involve a rapid increase in polar stratospheric temperatures of 20-30K over a period of just 1-2 days. There has also been a single recorded event in the Southern hemisphere, from nearly 70 years of recorded observations, which took place in 2002. SSWs are important for climate because considered to be the main source of interannual variability in the extratropical stratosphere, and over the last two decades have been the subject of much research interest, because they have been discovered to significantly influence Northern Hemisphere surface climate.SSWs are understood to be primarily fluid dynamical phenomena. The rapid increase in polar temperatures occurs because cold polar air, which is trapped within a columnar structure known as the polar stratospheric vortex, is displaced from the pole by the action of planetary-scale waves (Rossby waves). The aim of the current project is to bring understanding from fluid dynamics, particularly concerning the propagation of Rossby waves on vortices, to bear on the problem of understanding the occurrence of SSWs. The key question we will be concerned with is how the frequency of SSWs might change in a changing climate.The project will proceed by developing a hierarchy of simplified models of SSWs, each of which captures more detail of the fluid dynamical behaviour. The simplest model considered will be that of an elliptical vortex in a random straining flow. The question of interest is how the probability that the vortex becomes elongated (`strained out') depends upon the statistics of the straining flow. The random straining flow here represents the influence of the dynamically active troposphere on the stratosphere, and its statistics will be subject to change in a changing climate. Interestingly, the same model can be used to analyse vortex lifetimes in two-dimensional turbulent flows. The next model in the hierarchy will be a dry primitive equation model on the sphere, which will be set up to capture the fundamental fluid dynamical interactions between troposphere and stratosphere in a more realistic setting. The statistics of long integrations will be analysed, to determine whether insights from the simple elliptical vortex model are useful for predicting changes in model SSW occurrence. Conclusions will be drawn from a comparison between the models and the observed behaviour, with a view to explaining (a) predicted changes in SSW frequency, and (b) variations in behaviour between different future scenarios.