Multi‐Scale Kelvin‐Helmholtz Instability Dynamics Observed by PMC Turbo on 12 July 2018: 2. DNS Modeling of KHI Dynamics and PMC Responses.

Kjellstrand et al. (2022), https://10.1029/2021JD036232 describes the evolution and dynamics of a strong, large‐scale Kelvin‐Helmholtz instability (KHI) event observed in polar mesospheric clouds (PMCs) on 12 July 2018 by high‐resolution imagers aboard the PMC Turbulence (PMC Turbo) stratospheric long‐duration balloon experiment. The imaging provides evidence of KH billow interactions and instabilities that are strongly influenced by gravity waves at larger scales. Specific features include initially separated regions of KHI, secondary convective and KH instabilities of individual billows, and "tubes" and "knots" that arise where billow cores are mis‐aligned or discontinuous along their axes. This study describes a direct numerical simulation of KH billow interactions in a periodic domain seeded with random initial noise that enables excitation of multiple KH billows exhibiting variable phase structures that capture multiple features of the observed KHI dynamics. Variable KH billow phases along their axes yield initial vortex tubes having diagonal alignments that link adjacent, but mis‐aligned, billow cores. Weak initial vortex tubes and billow cores having nearly orthogonal alignments amplify, interact strongly, and drive intense vortex knots at these sites. These vortex tube and knot (T&K) dynamics excite "twist waves" that unravel the initial vortex tubes, and drive increasingly strong vortex interactions and a cascade of energy and enstrophy to successively smaller scales in the turbulence inertial range. The implications of T&K dynamics are much more rapid and intense breakdown and decay of the KH billows, and significantly enhanced energy dissipation rates, where these interactions occur. Plain Language Summary: Kelvin‐Helmholtz instabilities (KHI) are ubiquitous throughout the atmosphere (and oceans) and have been studied for many years. Interactions between adjacent KH billows seen in early laboratory experiments named "tubes and knots" by Steve Thorpe were only recently recognized in imaging in the mesosphere. These KHI interactions were seen in the laboratory to lead to turbulence faster than secondary instabilities of individual billows. Despite very many papers describing KHI modeling, none have addressed tube and knot dynamics prior to their recent identification in the mesosphere. This paper describes modeling performed to explore the tube and knot dynamics seen during the PMC Turbo experiment and described in the companion paper. Results reveal that tube and knot dynamics yield dramatic increases in energy dissipation that may have important influences in the atmosphere and oceans. Key Points: Multi‐scale Kelvin‐Helmholtz (KH) instability dynamics arise due to natural variations in background flows and initial conditionsInteracting KH billows induce "tubes" and "knots" that form rapidly and are distinct from secondary instabilities of individual billowsTube and knot dynamics evolve more rapidly and yield larger energy dissipation rates than those of individual KH billows [ABSTRACT FROM AUTHOR]