Propagation Characteristics of Gravity Waves Around Critical Layers Revealed by the Andes Lidar

Abstract By analyzing horizontal wind and temperature data from the Andes Lidar Observatory Na lidar at Cerro Pachón, Chile (30.3°S, 70.7°W) from May 2014 to July 2019, we investigated the evolution process, propagation characteristics, and influencing factors of gravity waves (GWs) encountering the critical layer. Adopting the criterion that the critical layer is observed when the intrinsic frequency reaches 1.3 times the inertial frequency, accompanied by a sharp decrease in vertical wavelength, we identified 40 critical layer events, with an occurrence rate of 58.9%, indicating that critical layers are common phenomena in the MLT of the Andes mountain area. All observed GWs encountering the critical layer propagate along the wind, with rapid decreases in vertical wavelength and energy and momentum absorption by the background. When the critical layer appears, 44.2% of the wave energy is below 20 J/kg, and 30.4% of momentum flux is below 50 m 2 /s 2 . Contrary to the predictions of linear theory, not all waves are fully absorbed at the critical layer. Some carry residual energy to higher altitudes until reaching a second (19.1%) or third (4.4%) critical layer. These effects must be considered in the future GWs parameterization models. The occurrence rate of critical layers shows significant seasonal and altitude variations, being higher in autumn and lower in summer, mainly at 90–91 km, influenced by background wind and thermal structure. The critical layer usually forms below the mesopause, and its occurrence is negatively correlated with mesopause altitude, as a smaller buoyancy frequency favors its formation.