Quantifying Sources and Sinks of Rossby Wave Activity in the Atmosphere

把中纬度以上的大气环流看作是一种波的级数是有用的，这种波交替着向北和向南的气流，叠加在向东流动的平均急流上。基于波浪-平均气流框架的理论已经成功地解释了大气环流的许多方面，包括锋面天气系统的演变和山脉对对流层上层气流的影响。首席研究员（PI）对这一理论做出了贡献，他开发了一种波浪度的测量方法，这种方法捕捉到了波浪活动增加以牺牲平均射流速度为代价的权衡，反之亦然，这是角动量守恒的流体模拟的结果，被称为开尔文循环定理。这种测量方法被称为有限振幅波浪活度（FAWA），其中有限振幅指的是，即使波浪扭曲水流到足以产生与主流水流分离的大循环特征时，也可以使用这种测量方法。FAWA代表纬度圈周围的整体波活动，而PI已经开发了一个相关的量，称为局部波活动（LWA），以表示沿纬度圈每个经度的波浪度。PI小组以前的工作已经使用FAWA/LWA框架来解释平流层突然变暖（SSWs）期间极地涡旋的破裂和阻塞反气旋的形成（见AGS-1909522）。FAWA和LWA在ssw和阻塞等问题上的应用依赖于它们捕捉波浪和向东平均流之间相互作用的能力，而这些相互作用可以在不知道波浪如何产生或最终发生什么的情况下被理解。但是，在FAWA和LWA的演化方程中也表现了波活动的产生和耗散，这些方程可以用来研究波源和波汇及其在大气环流中的作用。例如，通过扩散混合的波活动耗散关系到低纬度经向翻转单元（哈德利单元）的强度，因此对汇项的分析可以深入了解气候变化模拟中看到的单元的扩展和相关的干旱化。源和汇的分析是有希望的，但也具有挑战性，因为需要涉及难以观察的小空间尺度和数量的复杂计算。该奖项下的工作接受了这一挑战，使用了应用于理想化模式、存档气候模式输出和观测数据集（特别是ERA5再分析）的各种技术。源汇分析的主题包括波活动的季节循环、大气边界层在产生和抑制波活动中的作用以及对流云中冷凝加热对波活动的产生。这项工作具有社会价值，因为对流层上层波浪和地面天气之间存在联系，例如，阻塞事件与极端降水有关，而ssw可能导致严重的冷空气爆发。用于生成FAWA、LWA和源/接收术语的软件将通过github公开提供给公众，并提供有关其使用的文档。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。

It is useful to think of the atmospheric circulation above the middle latitudes as a progression of waves, with alternating northward and southward air flow, superimposed on an eastward-flowing mean jet stream. The theory based on this wave-mean flow framework has been successful in explaining many aspects of the general circulation including the evolution of frontal weather systems and the effect of mountains on the upper-tropospheric flow. The Prpincipal Investigator (PI) has contributed to this theory by developing a measure of waviness which captures a trade-off in which wave activity increases at the expense of mean jet speed and vice versa, a consequence of the fluid analog of angular momentum conservation known as Kelvin's circulation theorem. The measure is called Finite Amplitude Wave Activity (FAWA), where finite amplitude refers to the fact that the measure can be used even when the waves distort the flow enough to create large circulation features which break off from the prevailing flow. FAWA represents the bulk wave activity around a latitude circle, and the PI has developed a related quantity called Local Wave Activity (LWA) to express waviness at each longitude along a latitude circle. Previous work by the PI's team has used the FAWA/LWA framework to explain the breakdown of the polar vortex during Sudden Stratospheric Warmings (SSWs) and the formation of blocking anticyclones (see AGS-1909522).The application of FAWA and LWA to problems like SSWs and blocking relies on their ability to capture interactions between waves and the eastward mean flow, which can be understood without knowing how the waves are generated or what eventually happens to them. But the generation and dissipation of wave activity are also represented in the evolution equations for FAWA and LWA, and the equations can be used to study wave sources and sinks and their role in atmospheric circulation. For example the dissipation of wave activity through diffusive mixing matters for the strength of the low-latitude meridional overturning cells (the Hadley cells), thus analysis of the sink term could offer insights into the expansion of the cells and the associated aridification seen in climate change simulations. Analysis of sources and sinks is promising but also challenging due to the need for intricate calculations involving small spatial scales and quantities which are hard to observe. Work under this award takes up the challenge, using a variety of techniques applied to idealized models, archived climate model output, and observational datasets (the ERA5 reanalysis in particular). Topics to be addressed through source/sink analysis include the seasonal cycle of wave activity, the role of the atmospheric boundary layer in generating and damping wave activity, and the generation of wave activity by condensational heating in convective clouds.The work has societal value due to the connection between upper-tropospheric waviness and surface weather, for instance blocking events are associated with extreme precipitation and SSWs can lead to severe cold air outbreaks. The software used to generate FAWA, LWA, and source/sink terms will be made openly available to the public via github, along with documentation on their 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.