Modeling Kelvin Helmholtz Instability Tube and Knot Dynamics and Their Impact on Mixing in the Lower Thermosphere

Abstract

We present modeling results of Kelvin Helmholtz Instability (KHI) tube and knot (T&K) dynamics accompanying a thermospheric KHI event captured by the 2018 Super Soaker campaign (R. L. Mesquita et al., 2020). Chemical tracers released by a rocketsonde on 26 January 2018 showed a coherent KHI in the lower thermosphere that rapidly deteriorated within 45‐90 s. Using wind and temperature data from the event, we conducted high resolution direct numerical simulations (DNS) employing both wide and narrow spanwise domains to facilitate (wide domain case) and prohibit (narrow domain case) the axial deformation of KH billows that allows tubes and knots to form. KHI T&K dynamics are shown to produce accelerated instability evolution consistent with the observations, achieving peak dissipation rates nearly 2 times larger and 1.8 buoyancy periods faster than axially uniform KHI generated by the same initial conditions. Rapidly evolving twist waves are revealed to drive the transition to turbulence; their evolution precludes formation of secondary convective instabilities (CI) and secondary KHI seen to dominate the turbulence evolution in artificially constrained laboratory and simulation environments. T&K dynamics extract more kinetic energy from the background environment and yield greater irreversible energy exchange and entropy production, yet they do so with weaker mixing efficiency due to greater energy dissipation. The results suggest that enhanced mixing from thermospheric KHI T&K events could account for the discrepancy between modeled and observed mixing in the lower thermosphere (Liu, 2021; Garcia et al., 2014) and merits further study.