Kelvin Helmholtz Instability “Tube” & “Knot” Dynamics, Part I: Expanding Observational Evidence of Occurrence and Environmental Influences

Abstract

Multiple recent observations in the mesosphere have revealed large-scale Kelvin-Helmholtz instabilities (KHI) exhibiting diverse spatial features and temporal evolutions. The first event reported by Hecht et al. (2021) exhibited multiple features resembling those seen to arise in early laboratory shear-flow studies described as “Tube” and “Knot” (T&K) dynamics by Thorpe (1985, 1987). The potential importance of T&K dynamics in the atmosphere, and in the oceans and other stratified and sheared fluids, is due to their accelerated turbulence transitions and elevated energy dissipation rates relative to KHI turbulence transitions occurring in their absence. Motivated by these studies, we survey recent observational evidence of multi-scale Kelvin-Helmholtz instabilities throughout the atmosphere, many features of which closely resemble T&K dynamics observed in the laboratory and idealized initial modeling. These efforts will guide further modeling assessing the potential importance of these T&K dynamics in turbulence generation, energy dissipation, and mixing throughout the atmosphere and other fluids. We expect these dynamics to have implications for parameterizing mixing and transport in stratified shear flows in the atmosphere and oceans that have not been considered to date. Companion papers describe results of a multi-scale gravity wave direct numerical simulation (DNS) that serendipitously exhibits a number of KHI T&K events and an idealized multi-scale DNS of KHI T&K dynamics without gravity wave influences.