Finite-amplitude Eddy-mean Flow Interaction in the Extratropical Atmosphere

涡旋(如与中纬度天气系统有关的涡旋)和平均气流(特别是中纬度急流)之间的相互作用是大气环流中一个长期存在的问题。很明显，急流和涡旋相互作用很强，但解释它们相互作用的传统理论是有限的，因为出于数学原因，它们假设涡流是平均流的小幅度扰动。首席研究人员(PI)发展了一种适用于大幅度涡旋的涡流-平均流相互作用理论，在该奖项下开展的工作试图探索这一新理论的含义。新理论将被应用于1)评估气候状态的非保守驱动(辐射强迫、摩擦、混合等)。通过对缓慢变化的参考态进行细致的记录和分析，2)通过比较观测到的纬向平均时平均态的线性稳定性和由理论得到的无涡参考态的线性稳定性来量化斜压涡(“斜压调整”)的稳定作用，3)将环模变率表征为纬向平均纬向风和波浪活动密度的共同变化，进而将其与涡旋的纬向相速度和能量的变化联系起来；4)推广Rossby波破裂的开始判据，并用数值模拟和气象再分析检验这一理论预测；5)进一步推广了纬向变化平均流的波浪破碎判据。这一活动的更广泛的影响是，涡旋平均流理论和由此导出的对角线工具可以应用于包括平流层、对流层和海洋在内的更广泛的流动类别。此外，该项目将支持和培训两名研究生，从而发展这一领域的科学队伍。研究结果还将被纳入PI在他的流体动力学实验室为研究生、本科生和高中水平的观众表演的演示中。

The mutual interaction between eddies (such as those associated with midlatitude weather systems) and the mean flow (midlatitude jet streams, in particular) is a longstanding topic in atmospheric general circulation. It is clear that jet streams and eddies interact strongly, but traditional theories to account for their interaction are limited because, for mathematical reasons, they assume that the eddies are small-amplitude perturbations of the mean flow. The Principal Investigator (PI) has developed a theory of eddy-mean flow interaction which works for large-amplitude eddies, and work conducted under the award seeks to explore the implications of this new theory. The new theory will be applied to 1) evaluate nonconservative driving of the climate state (radiative forcing, friction, mixing, etc.) by carefully documenting and analyzing the slowly varying reference state; 2) quantify the stabilizing effects of baroclinic eddies ("baroclinic adjustment") by comparing the linear stability of the observed zonal-mean time-mean state and that of an eddy-free reference state derived from the theory; 3) characterize annular mode variability as co-variation of the zonal-mean zonal-wind and wave activity density, and in turn associate it with the variation in the zonal phase speed and energy of the eddies; 4) generalize the criterion for the onset of Rossby wave breaking and testing this theoretical prediction with numerical simulation and meteorological reanalysis; and 5) further generalize the wave breaking criterion for zonally varying mean flows.The broader impacts of this activity are that the eddy-mean flow theory and the diagonstic tools derived from it can be applied to a wide class of flows, including the stratosphere, troposphere, and the oceans. In addition, the project will support and train two graduate students, thereby developing the scientific workforce in this area. Results of the research will also be incorporated into demonstrations performed by the PI in his fluid dynamics laboratory, for audiences at the graduate, undergraduate, and high school levels.