Comment on acp-2022-112

Abstract

Based on the full-diurnal-cycle sodium (Na) lidar observations at Beijing (40.41 °N, 116.01 °E), we firstly report pronounced downward extensions of the Na layer bottomside to below 75 km near mid-December, 2014. Considerable Na atoms were observed even as low as ~72 km, where Na atoms is short-lived. To our knowledge, this represents the lowest altitude where considerable Na atoms have ever been detected by Na lidar. More interestingly, an unprecedented Na density of ~2500 atoms/cm3 around 75 km was observed on December 17, 2014. Such high Na atoms concentration was two orders of magnitude larger than that normally observed at the similar altitude region. The variations of Na density on the layer bottom were found to be accompanied by warming temperature anomalies and considerable perturbations of minor chemical species (H, O, O3) in the upper mesosphere. Different from the previous reported metal layer bottom enhancements mainly contributed by photolysis after sunrise, these observational results suggest more critical contributions were made by the Na neutral chemical reactions to the Na layer bottom extensions reported here. The time-longitudinal variations of background atmospheric parameters in the upper mesosphere and stratosphere from global satellite observations and ERA reanalysis data indicated that the anomalous structures observed near the lidar site in mid-December, 2014 were associated with planetary wave (PW) activities. The anomalies of temperature and O3 perturbation showed opposite phase in the altitude range of 70~75 km and 35~45 km. This implied that the vertical coupling between the mesosphere and stratosphere, possibly driven by the interactions of PW activities with background atmosphere, contributed to the perturbations of background atmosphere. Furthermore, the bottom enhancement on December 17, 2014 was also accompanied by clear wavy signatures in the main layer. The wavy structures had downward phase propagations and agreed well with the variation of zero zonal wind measured by a nearby meteor radar, suggesting the downward transportation of Na species from above 80 km to below further promotes the formation of the unprecedented Na layer bottom enhancement on December 17, 2014. These results provide a clear observational evidence for the Na layer bottom response to the planetary-scale atmospheric perturbations which modulated Na chemical reactions. The results of this paper also have implications for the response of the metal layer to vertical coupling between the lower atmosphere and the mesosphere.