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Based on the full-diurnal-cycle sodium (Na) lidar observations at Beijing (40.41 ∘ N, 116.01 ∘ E), we 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 are short-lived. More interestingly, an unprecedented Na density of ∼ 2500 atoms cm -3 around 75 km was observed on 17 December 2014. Such high Na atoms concentration was 2 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, O 3) 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 O 3 perturbation showed opposite phase in the altitude range of 70–75 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 and modulation of gravity wave (GW) filtering by stratospheric wind, contributed to the perturbations of background atmosphere. Furthermore, the bottom enhancement on 17 December 2014 was also accompanied by clear wavy signatures in the main layer. The strong downwelling regions are likely due to the superposition of tide and GW, suggesting the wave-induced adiabatic vertical motion of the air parcel contributed greatly to the formation of the much stronger Na layer bottom enhancement on 17 December 2014. These results provide a clear observational evidence for the Na layer bottom response to the planetary-scale atmospheric perturbations in addition to tide and GW through affecting the chemical balance. 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. [ABSTRACT FROM AUTHOR]

Based on the established LIDAR chain along the 120°E meridian in China, the gravity wave (GW) activity between 80 and 105 km and the seasonal behavior of temporal and spatial spectra at Beijing, Hefei and Hainan were studied with three years of continued observations. The averaged GW-induced atmospheric density perturbations are near 6%, which in summer are obviously larger than in winter. The semiannual maxima occur near the solstice and the minimum emerges around the equinox at different latitudes. Besides, as a disparity, the density perturbation of GW is still active considerably in winter at low latitude at Hainan. The spectra of vertical wave number Fa(m) and observed frequency Fa(ω) show power law shapes, of which the average is near −3 and −1.7, respectively, and both spectra with special values all exhibit similar seasonal behavior as the atmospheric density perturbations shows. This behavior is explained by multiple effects possibly originating from Tibet plateau (TP) and the main GW source in China was roughly calculated by the LIDAR observation method for the first time located at the TP boundary. [ABSTRACT FROM AUTHOR]

• Na density inversion with narrowband filters for daytime measurement is discussed based on spectral analysis. • Annual mean diurnal variations of Na layer is obtained, showing considerable daytime bottom extension and nighttime top extension. • Large height-scale nighttime extension on Na layer topside shows obvious seasonal dependence, appearing frequently from early summer to autumn, while disappearing in winter months. • A case of nighttime Na layer top extension shows both gravity wave and tides signatures. Diurnal observations of metal Na layer can be implemented by making use of narrowband Faraday filters. On one hand, the narrowband spectral filters help to effectively suppress the background noise; on the other hand, the narrow bandwidth leads to different transmissions of Na layer resonance fluorescence signal and stratosphere Rayleigh scattering signal. The influence of the narrow spectral transmission on Na density retrieval cannot be canceled out by Rayleigh normalization and would result in a systematic error. In this paper, the Na density retrieval method when narrowband Faraday filters are used in the lidar receiver is discussed based on the transmission spectrum of Faraday filters and echo spectra of Rayleigh scattering and Na fluorescence. The results of Na layer diurnal variations that have been obtained from Na lidar observations at Beijing, China are presented. Considerable diurnal variation in Na density on the layer top and bottom are clearly revealed on a logarithmic scale. Intriguingly, larger height-scale nighttime extensions on the Na layer topside have been frequently observed from early summer to autumn. These observational results can provide valuable evidence for studying solar effect on the metal layer variation, atmospheric dynamical and chemical processes in the mesosphere and lower thermosphere region. [ABSTRACT FROM AUTHOR]

Based on application of the atomic filter technology in a signal detection system of lidar, the diurnal observation of sodium lidar were obtained using the system at the National Space Science Center of the Chinese Academy of Sciences at Beijing Yanqing station (40.5° N, 116° E) in April 2014. During the lidar observation period, among the 103 cases of continuous daytime observations, the longest time was 181 h. In the case of a continuous observation period of 5 days (13–18 October 2014), the signal-to-noise ratio reached to 19:1 at 12:00–13:00 Local Time of the daytime, when the spatial and time resolutions were respectively set to 96 m of 167 s. The improvements resulted in the highest detection level of any existing diurnal lidars in China. Some interesting phenomena such as the sporadic sodium layer have also been observed during the daytime. The daytime capability extended the observing time range of the earlier systems that were limited to only nighttime observations. This innovation provides a useful method for the studies of diurnal tides, photochemistry, gravity waves, and correlative modeling studies. [ABSTRACT FROM AUTHOR]

• High Power-Aperture lidar system with two wavelengths and three channels for Rayleigh scattering, Mie scattering and Na fluorescence simultaneous detecting was completed at Beijing (Yanqing District, 40.3°N, 116.2°E). The excellent detecting performance of the lidar as photon counts shows a considerable high value of which the sodium layer echo peak value can reach to more than 37000 counts. • A relevant advanced lidar detecting standard has been realized. High resolution mode (15 m, 6.7 s) has been implemented while photo count per shot reaches to a considerable high value as 861, and the Power-Aperture Product can reach to 0.7 Wm2 which is equivalent to the largest LZT lidar (6-meter diameter, 5 w power, 1500 photocount per shot, 0.6 Wm2 at UBC, Canada), ALO (1000 photocount per shot 0.6 Wm2 at Cerro Pachón, Chile) and STAR (Boulder, Colorado US) lidar as their high level standard at the moment. • Based on this High Power-Aperture lidar, the first report of observations of turbulence spectrum in North hemisphere over China was provided, which had been extremely difficult to be observed due to the resolution and the signal/noise rate as in before for most researches. The observed turbulence spectrum was found in consistent with the only report by ALO lidar at Chile in South hemisphere observed in 2017 [Y. Guo et. al., Geophys.Res.Lett., 2017 ] and well fitted with the theory prediction, which can effectively help us to further understand the entire global gravity wave behavior. With the support of the Meridian Project of China, an integrated two-wavelength and three-channel lidar system was completed for atmospheric detection in Yanqing, Beijing. This lidar system emitted a 589 nm laser beam for detecting the 80–110 km sodium atomic layer. A 532 nm laser system was used to detect the 30–80 km atmospheric temperature and density and also to provide the atmosphere aerosol information from the ground to a height of 30 km. Because all of the technique details were carefully designed, this lidar obtained a very good signal and achieved the full-scale detection of the atmosphere from near-ground to a height of 110 km. With the conventional mode (3 minutes, 100 m), the atmospheric 532 nm Rayleigh detecting channel could reach a height of 80 km, and the 589 nm channel photo counting peak could reach 37000. This specification actually reached the high power-aperture sodium lidar standard in advance, which caused the high-resolution detection mode (15 m, 6.7 s) to be realized. For the high-resolution detecting mode of our Yanqing Lidar, observations of the spectrum of a gravity wave in the turbulence region were given according to the advantages of the high signal/noise ratio. Additionally, the properties of the turbulence spectrum and their variations were first obtained in the northern hemisphere over China. The observed gravity wave turbulence spectra well fitted with the theoretical prediction. These results achieved new field detections, which will lead us to further discover the nature and essence of a gravity wave and deeply study the behavior of upper atmosphere circulation on a large scale. [ABSTRACT FROM AUTHOR]