Large Wave Events in a Changing Climate

在急流的高度，中纬度地区的大气环流趋于相当波浪形，高压和低压中心大致沿着急流轴向西移动。但有时波浪会变慢并变得相当大，在喷射水平流中形成一个“块”。阻塞的气流会导致天气系统的停滞，从而导致长时间的寒流、过度降水和其他形式的极端天气。一段时间以来，自然阻塞一直是人们深入研究的主题，但对它的一个令人满意的解释一直难以捉摸。一个原因是，许多相关的波动动力学理论都假设波的振幅很小，而阻塞的情况并非如此。为了弥补这种不匹配，PI和他的同事们开发了一种纬度圈周围波浪度的整体测量方法，称为有限振幅波活动（FAWA），以及纵向变化的对应物，局部波活动（LWA，见AGS-1563307）。FAWA和LWA非常适合于阻塞的研究，因为它们可以应用于块的整个生命周期，即使阻塞模式非常明显。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。该理论的思想是，波活动的增加减慢了急流，从而减慢了波沿着急流的传播。这种减速导致LWA的堆积，增加了波动度，进一步降低了喷气机的速度。由于LWA的积累而形成的街区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积聚，从而进一步减慢高速公路的速度，直到产生堵塞。从某种意义上说，这种类比是准确的，因为LWA堆积形成的街区可以用20世纪30年代和50年代的论文中导出的经典交通流方程来建模。在这里，PI使用他的交通阻塞理论对导致阻塞的波-平均流相互作用进行诊断研究，使用交通流理论中的概念来理解阻塞的形成。例如，射流的“容量”，即沿射流行进而不引起波传播减速的LWA的数量，可以类比于高速公路的交通容量来定义。像容量这样的量可以很容易地从天气图和模型模拟中计算出来。进一步的工作将FAWA/LWA框架应用于平流层突然变暖（SSW）事件，在这种事件中，围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100华氏度。ssw被认为是由于涡旋被向上传播的波破坏而发生的，因此ssw是使用FAWA/LWA框架分析的逻辑目标。由于与积木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，并且从最先进的预测模型模拟通常缺乏阻塞。因此，在这项工作中发展起来的理解可能对改进阻塞的模拟和预测很重要。从预报的角度来看，ssw同样具有挑战性，它们也可以以各种方式影响地面天气。根据气象资料计算FAWA和LWA的代码已通过github提供给研究界，以促进它们的采用和使用。教育和推广是通过两年一次的暑期学校和当地社区科学中心进行的。此外，该项目还资助了两名研究生和一名博士后，从而培养了该研究领域的下一代科学家。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.