Collaborative Research: New Pathways to Enhanced Turbulence and Mixing via Kelvin-Helmholtz Instability Tube and Knot Dynamics

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

The project seeks funding to investigate atmospheric turbulence generation. Shear layers in the atmosphere, where a layer slides over another, can result in instabilities that are commonly seen in thin cloud layers and resemble a series of ocean waves breaking on a beach. These instabilities, called Kelvin-Helmholtz instabilities, cause turbulence and mixing throughout the atmosphere and the oceans where shears are strong. Spacing between these “billows” can vary in the atmosphere from a few meters near the ground to 10 km or larger at altitudes as high as 100 km. Those seen in clouds usually have spacings (or wavelengths) from a few hundred meters to a km or so. The turbulence and mixing when these billows “break” influence the atmospheric structure and weather, especially near the ground, but their effects are not described well in weather prediction models. This research will explore a new type of instability causing breaking and turbulence that was recently discovered in thin clouds at very high altitudes that the research team expects to occur at all altitudes, and to significantly increase the turbulence and mixing due to these processes. If shown to occur for a wider range of conditions, this would significantly influence our ability to model the atmosphere near the ground and improve weather prediction that impacts all of us. The same instabilities occur in the oceans and are expected to also improve prediction of ocean circulations and structure when these processes are more fully understood. The project will involve a graduate student and a postdoctoral researcher experience in state-of-the-art modeling and super-computing. New observations of thin Polar Mesospheric Clouds and airglow layers at high altitudes (~80-90 km) have revealed the occurrence of a new type of instability leading to turbulence arising from mis-aligned Kelvin-Helmholtz (KH) billows accompanying variable geophysical forcing. These instabilities arise due to interactions among adjacent KH billow cores, rather than within single billows, and initial modeling of these dynamics have shown them to be much stronger, and to lead to much more intense turbulence, than occur in their absence. These dynamics arise from interactions between KH billow cores and large-scale vortex tubes that are excited where KH billows are mis-aligned or discontinuous due to initial conditions. Initial modeling employing direct numerical simulations that enable quantitative assessments of these dynamics, their stronger instabilities, and their more intense turbulence suggest that they may also cause enhanced turbulence and mixing in regions, and for conditions, in which turbulence was not previously expected. The research team believes that these enhanced KH billow dynamics are likely to be widespread and that they will allow us to update how these dynamics are modeled, enabling improved weather prediction, and of similar responses in the oceans. Because KH instabilities also play significant roles of other fields of physics, specifically magnetospheric physics and astrophysics, the benefits of this research may prove to be very broad. 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.