High-resolution observations and modeling of turbulence sources, structures, and intensities in the upper mesosphere

New capabilities for imaging small-scale instabilities and turbulence and for modeling gravity wave (GW), instability, and turbulence dynamics at high Reynolds numbers are employed to identify the major instabilities and quantify turbulence intensities near the summer mesopause. High-resolution imaging of polar mesospheric clouds (PMCs) reveal a range of instability dynamics and turbulence sources that have their roots in multi-scale GW dynamics at larger spatial scales. Direct numerical simulations (DNS) of these dynamics exhibit a range of instability types that closely resemble instabilities and turbulence seen in PMC imaging and by ground-based and in-situ instruments at all times and altitudes. The DNS also exhibit the development of “sheet-and-layer” (S&L) structures in the horizontal wind and thermal stability fields that resemble observed flows near the mesopause and at lower altitudes. Both observations and modeling suggest major roles for GW breaking, Kelvin-Helmholtz instabilities (KHI), and intrusions in turbulence generation and energy dissipation. Of these, larger-scale GW breaking and KHI play the major roles in energetic flows leading to strong turbulence. GW propagation and breaking can span several S&L features and induce KHI ranging from GW to turbulence scales. Intrusions make comparable contributions to turbulence generation as instabilities become weaker and more intermittent. Turbulence intensities are highly variable in the vertical and typically span 3 or more decades. DNS results that closely resemble observed flows suggest a range of mechanical energy dissipation rates of ε ~10⁻³–10 W kg⁻¹ that is consistent with the range of in-situ measurements at ~80–90 km in summer.