Multi‐Scale Kelvin‐Helmholtz Instability Dynamics Observed by PMC Turbo on 12 July 2018: 1. Secondary Instabilities and Billow Interactions

Abstract

The Polar Mesospheric Cloud (PMC) Turbulence experiment performed optical imaging and Rayleigh lidar PMC profiling during a 6‐day flight in July 2018. A mosaic of seven imagers provided sensitivity to spatial scales from ∼20 m to 100 km at a ∼2‐s cadence. Lidar backscatter measurements provided PMC brightness profiles and enabled definition of vertical displacements of larger‐scale gravity waves (GWs) and smaller‐scale instabilities of various types. These measurements captured an interval of strong, widespread Kelvin‐Helmholtz instabilities (KHI) occurring over northeastern Canada on July 12, 2018 during a period of significant GW activity. This paper addresses the evolution of the KHI field and the characteristics and roles of secondary instabilities within the KHI. Results include the imaging of secondary KHI in the middle atmosphere and multiple examples of KHI “tube and knot” (T&K) dynamics where two or more KH billows interact. Such dynamics have been identified clearly only once in the atmosphere previously. Results reveal that KHI T&K arise earlier and evolve more quickly than secondary instabilities of uniform KH billows. A companion paper by Fritts et al. (2022), https://doi.org/10.1029/2021JD035834R reveals that they also induce significantly larger energy dissipation rates than secondary instabilities of individual KH billows. The expected widespread occurrence of KHI T&K events may have important implications for enhanced turbulence and mixing influencing atmospheric structure and variability.

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

“…Smaller‐amplitude GWs instead modulate such large‐scale shears, and thus the scales and intensities of KHI that arise. Additionally, GW superpositions can yield local KHI seen to occur in numerical simulations of multi‐scale dynamics (MSD) that appear to account for local KHI events in some observations (Baumgarten & Fritts, 2014, Fritts et al., 2013, 2014; Kjellstrand et al., 2022; hereafter K22).…”

Gravity Waves Emitted From Kelvin‐Helmholtz Instabilities

“…Relative to the more widely recognized GW sources, such KHI sources have been recognized for many years, but remain one of the least quantified GW sources. KHI has been observed in polar mesospheric clouds and in airglow layers in the mesosphere (Fritts et al, 2019;Hecht et al, 2021;Kjellstrand et al, 2022). They often arise where vertical gradients of horizontal winds are enhanced due to high-frequency GWs with periods of several minutes and IGWs attaining enhanced local shears at large amplitudes.…”