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系列的波，南北交替的气流叠加在向东流动的平均急流上是有用的。基于这一波平均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流动的影响。Prpincipal Investigator(PI)为这一理论做出了贡献，他开发了一种波度测量方法，它捕捉到了一种权衡，即波的活动以牺牲平均射流速度为代价，反之亦然，这是被称为开尔文循环定理的角动量守恒的流体模拟的结果。这种测量被称为有限幅度波活动(Fawa)，其中有限幅度是指即使在波浪扭曲流动足以产生与主流分离的大环流特征的情况下，也可以使用该测量。Fawa代表纬度圈周围的整体波动活动，PI发展了一个相关的量，称为局地波活动(LWA)，以表示纬度圈上每个经度的波度。PI团队以前的工作使用了Fawa/LWA框架来解释平流层突然警告(SSW)期间极涡的破裂和阻塞反气旋的形成(见AGS-1909522)。Fawa和LWA在解决SSW和阻塞等问题上的应用依赖于它们捕捉波与东向平均气流之间相互作用的能力，这可以在不知道波是如何产生或最终发生在它们身上的情况下被理解。但Fawa和LWA的演化方程也描述了波活动的产生和消散，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，波活动通过扩散混合物质的消散与低纬经向翻转单体(哈德利单体)的强度有关，因此，对汇项的分析可以为气候变化模拟中看到的单体扩张和相关的干旱提供洞察力。源和汇的分析是有希望的，但也具有挑战性，因为需要进行复杂的计算，涉及很难观察到的小空间尺度和数量。该奖项下的工作接受了这一挑战，使用了应用于理想化模型、存档的气候模型输出和观测数据集(特别是ERA5再分析)的各种技术。通过源/汇分析将讨论的主题包括波活动的季节周期、大气边界层在产生和抑制波活动中的作用，以及对流云中凝结加热产生的波活动。由于对流层上层波度与地面天气之间的联系，这项工作具有社会价值，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/汇术语的软件将通过GitHub向公众开放，以及它们的使用文档。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。

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.