NSF Award
2109996
This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).<br/><br/>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.<br/><br/>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.<br/><br/>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.<br/><br/>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.<br/><br/>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.<br/><br/>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.
