Simultaneous upward and downward propagating inertia-gravity waves in the MLT observed at Andes Lidar Observatory

Based on the temperature and zonal and meridional winds observed with an Na lidar at Andes Lidar Observatory (30.3 degrees S, 70.7 degrees W) on the night of 20-21 July 2015, we report simultaneous upward and downward propagating inertia-gravity waves (IGWs) in the mesosphere/lower stratosphere (MLT). The ground-based periods of the upward and downward IGWs are about 5.4 h and 4.8 h, respectively. The horizontal and vertical wavelengths are about 935 km and 10.9 km for the 5.4 h IGW and about 1248 km and 22 km for the 4.8 h IGW, respectively. Hodograph analyses indicate that the 5.4 h IGW propagates in the direction of about 23 degrees west of north, while the 4.8 h IGW travels northward with an azimuth of 20 degrees clockwise from north. These wave parameters are in the typical IGW wavelength and period ranges; nevertheless, the downward propagating IGWs in the MLT are rarely reported in previous observations. The ray-tracing analysis suggests that the 5.4 h IGW is likely to originate from the stratospheric jet adjustment over the Antarctic, while the 4.8 h IGW source may be above the MLT because it is unlikely to propagate downward through a reflection in the realistic atmospheric wind field. Although both IGWs do not reach their amplitude thresholds of instability, the Richard number reveals that the dynamical and convective instabilities occur intermittently, which indicates that the instability arising from the multiple-perturbation superposition may have a significant influence on wave saturation and amplitude constraint in the MLT.