David C. Fritts

Energy and momentum exchange among tides, gravity waves (GWs), and turbulence shapes the mesosphere and lower thermosphere, yet it is commonly treated as a one‐way cascade from large‐scale waves to small‐scale dissipation. Using coordinated sodium resonance and Rayleigh LiDARs at Poker Flat Research Range, Chatanika, Alaska, on 12 April 2022, we resolve three Kelvin–Helmholtz billow events near 87–90 km and simultaneously detect short‐period gravity‐wave packets (10–30 min; vertical wavelengths 10–20 km) propagating away from these instabilities. Wavelet analyses show that wave power increases following the peak of the instability, consistent with GWs radiated during billow breakdown. To test causality and quantify the coupling, we perform high‐resolution three‐dimensional compressible simulations constrained by the observed winds and temperatures; the model reproduces the observed billow evolution, the characteristic fishbone pattern in altitude–time, and the lagged growth of wave energy. Momentum‐ and heat‐flux divergences diagnose two linked pathways: tidal shear drives Kelvin–Helmholtz instability and turbulent mixing (downscale transfer), while the instability radiates GWs that export energy and momentum to larger scales (upscale transfer) and measurably modifies the semidiurnal tide. These results provide observationally constrained evidence for a wave–instability feedback loop in the middle atmosphere, motivating parameterizations that allow small‐scale instability and turbulence to generate, as well as dissipate, atmospheric waves.

High frequency gravity waves (GWs) promote mixing in the mesosphere and lower thermosphere (MLT) via energy transport and momentum deposition. Kelvin‐Helmholtz instabilities (KHI) accompany strong shear throughout the atmosphere and influence GW breaking and mixing processes. Recent observations of GW‐KHI interactions in the MLT revealed the formation of KHI tube and knot dynamics (T&K) that were reproduced in simulations of unperturbed shear layers. However, simulation studies to‐date have not explored the dominant parameters of GW‐KHI interactions and their mixing implications. We present observations and modeling results of KHI advecting through a stationary GW in the MLT. On 18 July 2018, the advanced mesospheric temperature mapper (AMTM) at Tierra del Fuego captured stationary 30–50 km GWs and orthogonal 8–10 km KHI advecting through the wave field. We conduct numerical simulations to assess the influence of shear layer depth and Richardson number (Ri) on GW‐KHI interactions observed in the OH layer. Simulations reveal the intensification and rapid evolution of KHI triggered by shear layer advection through successive GW phases. Shear layers initialized with yield KHI localized to one GW wavelength, whereas KHI from layers with form faster and extend across multiple GW wavelengths. For , T&K control the transition to turbulence for deeper shear layers, whereas shallower layers result in localized KHI billow core breakdowns at earlier times. We assess the impact of these different KHI scales and shear intensities to determine how KHI‐GW interactions promote T&K in different atmospheric environments.

The NASA Atmospheric Waves Experiment (AWE) employs an OH imager that began measurements aboard the International Space Station (ISS) on 22 November 2023. The motivation for AWE was to quantify gravity wave (GW) responses to diverse sources and their influences extending into the mesosphere and higher altitudes. AWE measures temperatures at 2‐km cross‐track resolution in a 600‐km wide imaging swath at ∼87 km during nighttime. AWE swaths reveal a remarkable range of nighttime GW and instability responses from small to very large spatial scales over diverse terrain, deep convection, and oceans where no sources are apparent. Mountain wave responses over small islands and large‐scale orography exhibit horizontal wavelengths of λ _h ∼ 10–300 km or longer. Convective GWs likewise reveal λ _h ∼ 10–300 km or larger over land and ocean sources. AWE also provides clear evidence of secondary GWs having λ _h ∼ 50–500 km in response to strong MW sources. Additionally, the ISS orbit enables repeat observations over all sites at an ∼4‐day cadence and 2–3 successive swaths at an ∼1.5 hr cadence over all sites near the ISS orbit inclination of 51.6°. AWE swaths enable assessments of along‐track, cross‐track, and 2‐dimensional wavenumber spectra of GW temperatures, T′ ² , and velocity covariances, < u _h ' w ′>, for u _h ′ = ( u ′, v ′) spanning GW horizontal wavelengths, λ _h ∼ 30–1,000 km or longer. These spectral evaluations reveal the major contributions to GW pseudomomentum fluxes, ρ < u _h ' w ′>, to occur at GW λ _h ∼ 30–300 km.

Sporadic E (Es) layer plays essential role in ionosphere‐atmosphere coupling. Theoretical simulations show that the atmospheric gravity waves (GWs) in the lower thermosphere are the main modulatory source on Es dynamics at small spatial scales and short periods. Based on Sanya Incoherent Scatter Radar, direct observational evidence for the mesoscale GW modulations on low‐latitude Es layer is firstly obtained by four‐dimensional electron density with extreme fine resolutions of 37.5 m in range and 50 s in time. The wave‐like horizontal structures, with ∼10 min period and ∼55 km wavelength, indicate the typical features of mesoscale GWs. Our results confirm that, interacting with the background tidal winds, the upward propagating GWs modulate the horizontal structures and drift velocity of Es layers at different altitudes with increasing amplitude to cause the rapid oscillations in range.

High frequency Gravity waves (GWs) promote mixing in the mesosphere and lower thermosphere (MLT) via energy transport and momentum deposition. Kelvin-Helmholtz instabilities (KHI) accompany strong shear throughout the atmosphere and influence GW breaking and mixing processes. Recent observations of GW-KHI interactions in the MLT revealed the formation of KHI tube and knot dynamics (T&K) that were reproduced in simulations of unperturbed shear layers. However, simulation studies to-date have not explored the dominant parameters of GW-KHI interactions and their mixing implications. We present observations and modeling results of KHI advecting through a stationary GW in the MLT. On 18 July 2018, the advanced mesospheric temperature mapper (AMTM) at Tierra del Fuego captured stationary 30-50 km GWs and orthogonal, 8-10 km KHI advecting through the wave field. We conduct direct numerical simulations (DNS) to assess the influence of shear layer depth and Richardson number (Ri) on GW-KHI interactions observed in the OH layer. DNS reveal the intensification and rapid evolution of KHI triggered by shear layer advection through successive GW phases. Shear layers initialized with Ri = 0.15 yield KHI localized to one GW wavelength, whereas KHI from layers with Ri = 0.05 form faster and extend across multiple GW wavelengths. For Ri = 0.05, T&K control the transition to turbulence for deeper shear layers, whereas shallower layers result in localized KH billow core breakdowns at earlier times. We assess the impact of these different KHI scales and shear intensities to determine how KHI-GW interactions promote mixing in different atmospheric environments.

Breaking gravity waves (GWs) are a major source of turbulence and momentum deposition in the middle atmosphere. In spite of the importance of this phenomenon, the small‐scale vortex structures associated with wave breaking remain poorly documented under realistic conditions. We perform a three‐dimensional compressible simulation of orographic GWs observed above Syowa Station, Antarctica, using 125 m horizontal grid spacing from the ground to the mesosphere. In the 75–80 km region upward‐propagating waves break and exhibit numerous horseshoe‐shaped vortices whose bends extend obliquely downward. By examining the time evolution of a representative horseshoe‐shaped vortex, we identify the mechanism responsible for the downward elongation of the vortex tube: (a) horizontal vorticity aligned with the GW phase lines is amplified as the wave approaches the overturning condition; (b) a convectively driven downward flow displaces the vortex tube downward, creating the initial horseshoe shape; (c) vertical shear associated with the GW tilts and stretches the tube, reinforcing the downward flow. The direction of the initial vortex tube is determined largely by the baroclinic effect of the GW. The present study also addresses the relationship between the vortex tube structure and the mean flow acceleration (0.3–0.4 m s ⁻¹ min ⁻¹ ). The vortex tube deformation associated with wave breaking can be interpreted as a manifestation of the irreversible cascade that transfers the GW momentum to the mean flow.

New high‐resolution measurements by Sanya incoherent scatter radar (ISR) employing a wide‐bandwidth chirp signal now enable spatiotemporal resolutions of 37.5 m and 50 s. We have applied these new capabilities for observations of Kelvin‐Helmholtz instability (KHI) dynamics in the low‐latitude ionosphere E region. The KHI evolution exhibited ∼±150 m oscillations in altitude, billows “periods” of ∼3–8 min due to horizontal advection, and 1–3 km vertical extents in the sporadic E layers, consistent with the classic understanding of KHI at these altitudes. Our results confirm the quasi‐period character of KHI and first reveal the KHI modulates the plasma layers by horizontal advection in direct observation. They also demonstrate the benefits of improved spatiotemporal resolutions for more general studies of the low‐latitude E region ionosphere.

Kelvin-Helmholtz Instabilities (KHI) are known to be significant drivers of atmospheric turbulence. Recent observations show KHI forming with misaligned or angled billow segments that develop connecting vortex tubes and knots (T&K); these features promote distinctive, event-defining instability and mixing characteristics that were not accounted for in prior idealized studies. Though T&K have been shown to increase mixing in KHI events with low Richardson numbers (Ri), their influence in weakly KH-unstable, less-idealized environments is unknown. Here we present modeling results of KHI in the stratosphere to assess the impact of T&K dynamics in weakly KH-unstable environments. Radiosonde wind and temperature profiles from 22 February, 2006 near Lamont, Oklahoma measured vertically offset shear and stability peaks near 16.15 km with a minimum Ri = 0.11. Direct numerical simulations (DNS) of this event reveal decreasing shear and increasing stratification, where Ri increases to 0.2 as the shear and stratification peaks move to a common altitude. The resulting KHI exhibit T&K features forming adjacent to, and in superposition with, secondary convective instabilities (CI) rather than superseding them as in prior T&K studies with Ri = 0.05. Newly-discovered “crankshaft” instabilities distort the billows and give rise to secondary KHI with delayed, elevated dissipation. KHI that exhibit T&K dynamics are found to accumulate ~60% greater mixing than axially-uniform KHI with equal or lower mixing efficiency. The substantial increase in mixing suggests significant contributions of T&K dynamics to KHI events throughout the atmosphere that remain unaddressed in general circulation models’ turbulence parameterizations.

A companion paper by Fritts et al. reviews extensive evidence for Kelvin–Helmholtz instability (KHI) “tube” and “knot” (T&K) dynamics at multiple altitudes in the atmosphere and in the oceans that reveal these dynamics to be widespread. A second companion paper by Fritts and Wang reveals KHI T&K events at larger and smaller scales to arise on multiple highly stratified sheets in a direct numerical simulation (DNS) of idealized, multiscale gravity wave–fine structure interactions. These studies reveal the diverse environments in which KHI T&K dynamics arise and suggest their potentially ubiquitous occurrence throughout the atmosphere and oceans. This paper describes a DNS of multiple KHI evolutions in wide and narrow domains enabling and excluding T&K dynamics. These DNSs employ common initial conditions but are performed for decreasing Reynolds numbers (Re) to explore whether T&K dynamics enable enhanced KHI-induced turbulence where it would be weaker or not otherwise occur. The major results are that KHI T&K dynamics extend elevated turbulence intensities and energy dissipation rates ε to smaller Re. We expect these results to have important implications for improving parameterizations of KHI-induced turbulence in the atmosphere and oceans. Significance Statement: Turbulence due to small-scale shear flows plays important roles in the structure and variability of the atmosphere and oceans extending to large spatial and temporal scales. New modeling reveals that enhanced turbulence accompanies Kelvin–Helmholtz instabilities (KHIs) that arise on unstable shear layers and exhibit what were initially described as “tubes” and “knots” (T&K) when they were first observed in early laboratory experiments. We perform new modeling to explore two further aspects of these dynamics: 1) can KHI T&K dynamics increase turbulence intensities compared to KHI without T&K dynamics for the same initial fields and 2) can KHI T&K dynamics enable elevated turbulence and energy dissipation extending to more viscous flows? We show here that the answer to both questions is yes.