Turbulence, Richardson number (Ri) distributions, and parametric instabilities in the turbopause region (96–105 km) from Na LIDAR measurements at the Andes Lidar Observatory (ALO)

Turbulence in the Mesosphere and Lower Thermosphere (MLT) region is responsible for vertical mixing and transport of constituents and heat and the formation of the turbo-pause. A study of turbulence at the Andes Lidar Observatory (ALO) by Philbrick et al. (2021) found, for 25 nights of lidar observations, the probability of Ri < 1/4 decreased with altitude above 100 km, whereas the power in turbulence increased. The objective of this study is to understand the atmospheric process responsible for the observed increase in turbulence power with altitude. Conventionally turbulence is caused by instabilities due to convection (Ri < 0), and Kelvin-Helmholtz Instabilities (KHI), 0 < Ri < 1/4. These criteria are based on laminar flow, a waveless basic state. However, wave-modulated states requiring Floquet theory may dominate the MLT region and can generate instabilities and turbulence under more stable conditions (Ri > 1/4, Klostermeyer, 1990). It was determined in this study the probability of Ri > 1/4 to be >70% at 105 km, consistent with parametric instability (PI) where large tidal induced wind shears and gravity wave presence are contributing factors.