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

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.