Large Wave Events in a Changing Climate

在急流的高度，中纬度的大气环流往往是波浪形的，高、低压中心大致沿着急流轴向西移动。但有时波动会减慢并变得很大，在急流中形成一个“障碍”。阻塞的水流导致天气系统停滞，这可能导致长时间的寒流、过度降水和其他形式的极端天气。一段时间以来，自然阻断一直是一个深入研究的主题，但对此一直难以找到令人满意的解释。一个原因是，许多相关的波浪动力学理论都假定波的振幅很小，但对于阻塞来说，情况并非如此。为了纠正这种不匹配，Pi和他的同事们开发了一种测量纬度圆周波度的整体指标，称为有限幅度波活动(FAWA)，以及它的经向变化的局域波活动(LWA，见AGS-1563307)。Fawa和LWA非常适合于阻塞的研究，因为他们可以应用于阻塞的整个生命周期，即使在阻塞模式非常明显的情况下也是如此。最近，Pi和他的团队基于平均射流速度和LWA之间的关系发展了一个简单的阻塞理论。该理论的想法是，波活动的增加会减缓喷流，从而减缓波沿喷流的传播。减速导致LWA堆积，这增加了波浪度，进一步减慢了喷气式飞机的速度。由于LWA的积累而形成的阻塞类似于交通堵塞的形成，在这种情况下，高速公路上的减速导致汽车的聚集，从而进一步减慢高速公路的速度，直到产生拥堵。这个类比是准确的，因为LWA的堆积形成的阻塞可以用20世纪30年代和50年代的论文中推导的经典交通流方程来模拟。在这里，PI使用他的阻塞的交通堵塞理论来执行导致阻塞的波-平均流相互作用的诊断性研究，使用交通流理论中的概念来理解阻塞的形成。例如，喷流的“能力”，即在不引起波传播减速的情况下，可以沿着喷流行进的LWA的量，可以类似于高速公路的交通能力来定义。从天气图和模式模拟中可以很容易地计算出像容量这样的量。进一步的工作将Fawa/LWA框架应用于平流层突然变暖(SSW)事件，在SSW事件中，环绕北极的平流层涡旋破裂，平流层温度急剧上升，有时在几天内超过100F。SSW的发生被认为是因为涡旋被向上传播的波破坏，因此SSW是使用Fawa/LWA框架进行分析的合理目标。由于与块体相关的极端天气及其对人类活动的影响，这项工作具有更广泛的影响。阻塞事件很难预测，最先进的预测模型的模拟通常在阻塞方面存在缺陷。因此，这项工作中发展起来的理解对于改进阻塞的模拟和预测可能是重要的。从预报的角度来看，SSW同样具有挑战性，它们还可以通过各种方式影响地面天气。通过GitHub向研究界提供从气象数据计算Fawa和LWA的代码，以促进采用和使用这些代码。教育和外展通过两年一次的暑期学校和当地社区科学中心进行。此外，该项目支持两名研究生和一名博士后研究员，从而在该研究领域培养下一代科学家。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

At the altitude of the jet streams the atmospheric circulation over the middle latitudes tends to be quite wavy, with high and low pressure centers moving westward roughly along the jet axes. But at times the waves slow down and grow quite large, forming a "block" in the jet-level flow. The blocked flow results in a stagnation of weather systems, which can lead to prolonged cold spells, excessive precipitation, and other forms of extreme weather. Naturally blocking has been a topic of intensive research for some time, but a satisfactory explanation for it has been elusive. One reason is that that much of the relevant wave dynamics theory assumes that the waves have small amplitudes, which is not the case for blocking. To remedy this mismatch the PI and his colleagues have developed a bulk measure of the waviness around a latitude circle called Finite Amplitude Wave Activity (FAWA), along with its longitudinally varying counterpart, Local Wave Activity (LWA, see AGS-1563307). FAWA and LWA are well suited to the study of blocking because they can be applied throughout the lifecycle of a block, even when the blocking pattern is quite pronounced.Recently the PI and his group developed a simple theory of blocking based on the relationship between mean jet speed and LWA. The idea of the theory is that an increase in wave activity slows down the jet stream, which slows down the propagation of waves along the jet. The slowdown leads to a pile-up of LWA which increases waviness and further slows down the jet. The formation of blocks by accumulation of LWA is analogous to the formation of a traffic jam, in which a slow-down on a highway leads to accumulation of cars which further slows highway speed until a jam is produced. The analogy is exact in the sense that the formation of a block by the piling up of LWA can be modeled using the classical traffic flow equation derived in papers from the the 1930s and 1950s.Here the PI uses his traffic jam theory of blocking to perform a diagnostic study of wave-mean flow interactions leading to blocks, using concepts from traffic flow theory to understand block formation. For instance the "capacity" of a jet stream, meaning the amount of LWA that can travel along a jet without initiating a slowdown in wave propagation, can be defined in analogy with the capacity of a highway for traffic. Quantities like capacity can be easily calculated from weather maps and from model simulations.Further work applies the FAWA/LWA framework to Sudden Stratospheric Warming (SSW) events, in which the stratospheric vortex circling the North Pole breaks down and stratospheric temperatures rise dramatically, sometimes over 100F within a few days. SSWs are thought to occur because the vortex is disrupted by upward-propagating waves, thus SSWs are a logical target for analysis using the FAWA/LWA framework.The work has broader impacts due to the extreme weather associated with blocks and the impact it has on human activities. Blocking events are difficult to predict, and simulations from state-of-the-art forecast models are generally deficient in blocking. The understanding developed in this work may thus be important for improved simulation and prediction of blocking. SSWs are equally challenging from a forecast perspective, and they can also affect surface weather in various ways. Codes for the calculation of FAWA and LWA from meteorological data are made available to the research community via github to promote their adoption and use. Education and outreach are conducted through a biennial summer school and a local community science center. In addition, the project supports two graduate students and a postdoctoral fellow, thereby fostering the next generation of scientists in this research area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.