Diurnal variation of atmospheric metal Na layer and nighttime top extension detected by a Na lidar with narrowband spectral filters at Beijing, China.

• 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]