Modeling the implications of Kelvin‐Helmholtz instability dynamics for airglow observations

Abstract

A companion paper describes high-resolution, ground-based imaging of apparent Kelvin-Helmholtz instabilities (KHI) observed in OH airglow at~87 km over the Andes Lidar Observatory at 30°S. Here we employ direct numerical simulations (DNSs) and large eddy simulations (LESs) of KHI at Richardson numbers from Ri = 0.05 to 0.20 and relatively high Reynolds numbers of Re~2500 to 10,000 to illustrate the dependence of primary and secondary KHI on these quantities for the purpose of quantifying KHI dynamics observed by ground-based airglow imagers. Our DNS and LES reveal significant variations of both primary and secondary KHI scales and amplitudes with varying Ri and Re. Lower Ri and higher Re yield stronger and deeper initial 2-D KH billows. Low Re for a given Ri either yield larger-scale 3-D secondary instabilities or suppress them altogether. Secondary instability scales decrease as Re increases for a given Ri. Corresponding variations in implied KHI airglow signatures include (1) stronger airglow intensity variations for larger KHI wavelengths and depths (higher Ri), (2) stronger 2-D and 3-D responses for the initial KHI shear layer displaced somewhat from the airglow layer, and (3) stronger 3-D responses for the lower Ri and intermediate Re yielding the larger secondary instability scales.

A fast SWIR imager for observations of transient features in OH airglow

“…8 show further small-scale features, which we assume to be KHI billows. Fritts et al (2014) show structures based on model calculations of the OH airglow response to KHIs looking very similar to these. Yamada et al (2001) and Hecht et al (2014) present similar phenomena in their measurements.…”

“…Where the superposition of both waves takes place, one can see even smaller structures in the order of about 2 km, which we assume to be Kelvin-Helmholtz instability billows (compare Fritts et al (2014); Hecht et al, 2014).…”

PMC Turbo: Studying Gravity Wave and Instability Dynamics in the Summer Mesosphere Using Polar Mesospheric Cloud Imaging and Profiling From a Stratospheric Balloon

“…Applications of DNS to KHI for various Reynolds and Richardson numbers, Re and Ri , respectively, for idealized shear flows and MSD arising from superposed higher‐ and lower‐frequency motions have yielded other comparisons that provide further evidence of the validity of DNS descriptions of such flows. Specifically, comparisons of PMC and OH airglow imaging and modeling have revealed tendencies for enhanced KHI accompanying significant GW amplitudes (Baumgarten & Fritts, ; Fritts, Baumgarten, et al, ; Fritts, Wan, et al, ; Hecht et al, , ). These features are consistent with regions of preferred KHI capping local GW breaking in MSD (Fritts et al, ) and apparent in radar and lidar profiling noted above.…”

“…PMC and airglow imaging have also revealed features aligned along the plane of Kelvin‐Helmholtz (KH) billow rotation that intensify, interact, and ultimately break down to smaller‐scale turbulence (Baumgarten & Fritts, ; Hecht et al, ). These secondary instability features comprise counterrotating vortices, with spanwise (normal to the evolution plane) wave numbers, that arise in the outer (inner) portions of the KH billows for smaller (larger) Ri that are relatively more (less) unstable (Fritts, Baumgarten, et al, ; Fritts, Wan, et al, ). Of these, the events exhibiting the most rapid evolutions are those having the smallest Ri , the deepest KH billows, and the largest Re .…”

Numerical Simulations of High‐Frequency Gravity Wave Propagation Through Fine Structures in the Mesosphere

“…The Reynolds number ( Re ) is calculated from the GW length scale as where ν = μ / ρ is the kinematic viscosity. We apply a turbulent kinematic viscosity of ν =3 ν 0 based on estimates of an elevated effective viscosity due to preexisting turbulence (Baumgarten & Fritts, ; Fritts, Baumgarten, et al, ; Fritts, Wan, et al, ; Hecht et al, , ) in the manner of Fritts, Laughman, et al (), where ν 0 is the true kinematic viscosity ∼1.5×10 −5 m 2 s −1 at ground level and ν ∼2.8 m 2 s −1 is the kinematic viscosity specified in the model at 80 km. For GW with λ z =10 km, this results in Re ≈10 5 where FSs arise accompanying flow instabilities.…”