Quantifying Kelvin-Helmholtz instability dynamics observed in noctilucent clouds: 2. Modeling and interpretation of observations

A companion paper describes high-resolution, ground-based imaging of apparent Kelvin-Helmholtz instabilities (KHI) observed in noctilucent clouds (NLCs) near the polar summer mesopause. Here we employ direct numerical simulations of KHI at Richardson numbers from Ri=0.05 to 0.20 and relatively high Reynolds numbers to illustrate the dependence of KHI and secondary instabilities on these quantities and interpret and quantify the KHI events described by Baumgarten and Fritts (2014). We conclude that one event triggered by small-scale gravity waves provides clear evidence of strong KHI initiated at Ri similar to 0.05-0.10. Events arising in a more uniform shear environment exhibit KHI and small-scale dynamics that compare reasonably well with modeled KHI initiated at Ri similar to 0.20. Our application of numerical modeling in quantifying KHI dynamics observed in NLCs suggests that characteristics of KHI, and perhaps other small-scale dynamics, that are defined well in NLC displays can be used to quantify the dynamics and spatial scales of such events with high confidence. Specifically, our comparisons of KHI observations and modeling appear to indicate a "turbulent" viscosity similar to 5-40 times the true kinematic viscosity at the NLC altitude. This offers an alternative, or an augmentation, to more traditional radar, lidar, and/or airglow measurements employed for such studies of small-scale dynamics at coarser spatial scales during polar summer.