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 (kzzTurb) from 85 km to 105 km altitude over the Andes Lidar Observatory (ALO) in Cerro Pachón, Chile, (30.3°S, 70.7°W) at timescales between 12 seconds and 78 seconds. Na lidar measurements of heat, momentum, and constituent fluxes at turbulence scales have traditionally been limited to the 85–100 km 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 115 h 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 kzzTurb over the timescales analyzed ranged between 1.2×105 cm2s−1 and 5.7×105 cm2s−1 in the 85–100 km region and increased from a minimum at 98 km to a maximum of 4.5×106 cm2s−1 near 105 km 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 85 km and 100 km altitude. The critical region (95–110 km) 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 (80–95 km), 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.
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