A Na density lidar method and measurements of turbulence to 105km at the Andes Lidar Observatory

Abstract This study presents a method of measuring atmospheric turbulence from resonance-fluorescence lidar observations of atmospheric Na layer density fluctuations, which is used to characterize turbulent eddy diffusion ( k z z T u r b ) from 85km to 105km altitude over the Andes Lidar Observatory (ALO) in Cerro Pachon, Chile, (30.3°S, 70.7°W) at timescales between 12seconds and 78seconds. Na lidar measurements of heat, momentum, and constituent fluxes at turbulence scales have traditionally been limited to the 85-100km region due to signal limitations; however, the ”Na density” method described herein offers a 3x signal-to-noise ratio improvement over Doppler heat flux measurements, enabling determination of eddy diffusion to higher altitudes and at shorter timescales. The presented mean turbulent eddy diffusion profile is determined from 115h of zenith data acquired over 25 nights via the Na density method. Results are supplemented by off-zenith data for all 25 nights, which is used to examine atmospheric stability. Mean  k z z T u r b over the timescales analyzed ranged between  1 . 2 × 1 0 5 cm 2 s−1 and  5 . 7 × 1 0 5 cm 2 s−1 in the 85-100km region and increased from a minimum at 98km to a maximum of  4 . 5 × 1 0 6 cm 2 s−1 near 105km altitude. The mean profile is compared with the Doppler method for (i) the same ALO dataset discussed herein and (ii) vertical-only ALO data between 85km and 100km altitude. The critical region (95–110km) of diffusive coupling in the mesosphere and lower thermosphere (MLT) differs in forcing uniquely with a dominance of wind shear and associated instabilities. Source investigations pertaining to the induced transport difference from the critical to the lower MLT region (80–95km), where the effects of damped atmospheric gravity waves and convective instabilities are well-established, require an ability to gather data on the variability of turbulence with altitude, latitude, and season, as well as atmospheric wave activity.