Collaborative Research: Improving the representation of the Quasi-biennial Oscillation and its surface impacts in NCAR climate models

该奖项的全部或部分资金来自《2021年美国救援计划法案》(公法117-2)。赤道平流层的风(比方说地表上方20至50公里)在赤道附近稳定而一致地吹着，但它们的方向每隔28个月左右就会从东风转向西风。这种被称为准两年一度的振荡(QBO)的风向逆转被认为有一系列后果，包括影响马登-朱利安振荡(MJO)，这是热带地区的一种大范围风和降雨模式；北大西洋振荡(NAO)，一种影响美国东部和西欧天气的环流模式；以及沿北太平洋急流移动的风暴路径。因此，QBO的缓慢进展可能会给全球天气带来一些长期预测。但天气和气候模型不擅长模拟QBO，它们无法再现QBO，这可能阻碍了更好的预测。QBO主要由大气重力波驱动，大气重力波与海浪类似，只是它们既可以垂直传播，也可以水平传播，因此可以向上传输动量来驱动QBO。重力波是由深对流云产生的，通常被描绘成活塞，通过将周围的空气向上和向下泵送来产生波浪，同时它们的上升和下降运动。这样的波浪产生确实存在，但它往往会产生传播速度相对较快的波，而观测表明，对QBO至关重要的大部分动量通量来自缓慢传播的波。这里所做的工作探索了一种替代的波浪产生机制，即深对流云通过阻挡云顶附近的水平风来产生波浪。高空的风通常比表面的风更强，所以当空气在云中上升时，它可能会比周围的空气移动得更慢。因此，上升的空气可能会对高层风构成障碍，高层风在其上方或周围流动时会产生波浪，就像水流在溪流中的岩石上流动一样。这样的波相对于产生它们的对流云是静止的，对流云相对于地面移动得很慢。因此，这一机制可以解释活塞式垂直运动产生的波与从相速较慢的波获得动量通量的观测结果之间的相速差异。该项目的主要活动是在整个大气社区气候模式(WACCM)中添加云作为障碍生成机制的表示。在模拟的波(特别是动量通量)的性质与卫星和平流层气球的观测之间进行了一些比较，并进一步检查了波驱动的QBO模拟。WACCM目前只能使用活塞机制来模拟QBO，但它是通过人为地将波的相速度降低四倍来做到这一点的。进一步研究QBO对MJO和其他环流型的影响。这项工作具有社会意义，因为如上所述，更好的QBO模拟对长期天气预报具有潜在价值。这项工作还与空间天气有关，因为对流产生的重力波可以传播到电离层，并造成通信和导航中断。WACCM的一个扩展版本WACCM-X被用来研究这种影响，因此这项工作有一条直接的途径来造福于空间气象研究社区。这一奖项反映了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。

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).The winds of the equatorial stratosphere (say 20 to 50 kilometers above the surface) blow steadily and consistently around the equator, but their direction somehow reverses from easterly to westerly every 28 months or so. This wind reversal, called the Quasi-biennial Oscillation (QBO), is thought to have a number of consequences including influences on the Madden-Julian Oscillation (MJO), a large-scale pattern of winds and rainfall in the tropics; the North Atlantic Oscillation (NAO), a circulation pattern that influences weather in the eastern US and western Europe; and the paths of storms that move along the jet stream in the North Pacific. The slow progression of the QBO could thus impart some long-range predictability to worldwide weather. But weather and climate models are not good at simulating the QBO and their inability to reproduce it may be standing in the way of better forecasts.The QBO is largely driven by atmospheric gravity waves, waves similar to ocean waves except that they can propagate vertically as well as horizontally and thus can transport momentum upward to drive the QBO. The gravity waves are generated by deep convective clouds, commonly pictured as pistons that make waves by pumping the ambient air up and down with their rising and sinking motions. Such wave generation does occur but it tends to make waves which have relatively fast propagation speeds, while observations suggest that much of the momentum flux that matters for the QBO comes from slowly propagating waves.Work performed here explores an alternative wave generation mechanism in which deep convective clouds generate waves by blocking the horizontal wind near the cloud tops. Winds aloft are commonly stronger than winds at the surface, so air rising in a cloud is likely to be moving more slowly than ambient air when it reaches the top of the cloud. The rising air can thus present an obstacle to the upper-level wind, which flows over or around it generating waves in the same way as water flowing over rocks in a stream. Such waves will be stationary relative to the convective clouds that generate them, which move slowly relative to the ground. This mechanism could therefore explain the discrepancy in phase speed between waves generated by piston-like vertical motion and observations that find momentum flux from waves with slower phase speeds.The primary activity in the project is adding a representation of the cloud-as-obstacle generation mechanism to the Whole Atmosphere Community Climate Model (WACCM). A number of comparisons are performed between the properties of simulated waves (momentum flux in particular) and observations from satellites and stratospheric balloons, and further work examines QBO simulations driven by the waves. WACCM is currently able to simulate the QBO using only the piston mechanism but it does so by artificially reducing the phase speed of the waves by a factor of four. Further work examines the impact of the QBO on the MJO and other circulation patterns.The work has societal relevance due to the potential value of better QBO simulation for long-range weather forecasting, as noted above. The work also has relevance for space weather since convectively-generated gravity waves can propagate into the ionosphere and cause disruptions in communications and navigation. An extended version of WACCM known as WACCM-X is used to study such effects, thus there is a direct pathway for the work to benefit the space weather research community.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.