Objective Gravity wave activities play a key role in atmospheric circulation, and the energy and momentum coupling between the lower atmosphere and the upper atmosphere can be realized via wave propagation from the troposphere to the mesosphere. Based on the Rayleigh scattering of atmospheric molecules, temperature structures in the middle atmosphere can be effectively measured with lidar, and the temperature perturbation induced by wave propagation can be employed to study gravity wave activities. In the past few decades, lidar observations of gravity wave activities in the middle atmosphere have been carried out at many locations globally, and different regional characteristics of gravity wave activities are found at different latitudes. According to the longtime observation data accumulated by the Rayleigh lidar at Haikou (19. 9 degrees N, 110. 3 degrees E), a data analysis process is designed in this study for the retrieval of temperature structures in the middle atmosphere and the identification of atmospheric gravity wave events. It is successfully applied to the regional characteristic investigation of gravity wave activities in the middle atmosphere. Hainan Province locates in the South China Sea, and atmospheric activities in this low-latitude region are significant to the terrestrial climate changes in the global atmospheric circulation system. The developed data analysis process may be helpful to the broader application of the dataset for the Chinese Meridian project, and research on the regional characteristics of gravity wave activities in Hainan Province could be meaningful to building global climate parameterization models. Methods Based on the Rayleigh lidar observations, the temperature structure in the middle atmosphere (30-65 km) is retrieved according to the method introduced by Chanin and Hauchecorne. However, the accuracy of calculation results of atmospheric temperature is related to the signal- to-noise ratio (SNR) of lidar observation data, and a proper reference altitude is also very important to precisely retrieve the temperature structure in the middle atmosphere. Therefore, the relative measurement error in temperature is calculated according to the SNR distribution in the observation data, and it is preset to less than 5% at different altitudes. Additionally, simultaneous measurement results from SABER/TIMED and COSMIC are utilized for comparison in this designed data analysis process to find the proper reference for the accurate retrieval of temperature profiles. The power spectral density of atmospheric temperature perturbations is calculated with the Fourier transform of the autocorrelation function, and atmospheric gravity wave events are identified from the sequences of lidar observation data. After the high-frequency components are removed with the wavelet analysis method, the vertical wavelength and the observed wave period of dominant gravity waves are extracted according to the calculation results of the vertical wavenumber spectrum and the temporal frequency spectrum. Results and Discussions A data analysis process (Fig. 5) is developed for accurately retrieving the temperature structure and identifying gravity wave events in the middle atmosphere, and it is successfully applied to the analysis of the Hainan lidar observation dataset. Atmospheric density and temperature structures (Fig. 2) are retrieved for the middle atmosphere over Haikou (19. 9 degrees N, 110. 3 degrees E), and 202 gravity wave events in the middle atmosphere are successfully identified from Hainan lidar observations from January 2011 to July 2013. Vertical wavelengths and the observed wave periods are extracted for every gravity wave event, and statistical analysis (Fig. 7) shows that those lidar- observed gravity waves in the middle atmosphere over Haikou are typically 5- 9 km in vertical wavelength and 5- 13 h in observed wave periods. Comparison with the Rayleigh lidar observation results at Arecibo (18 degrees N, 66 degrees W), Gadanki (13. 5 degrees N, 79. 2 degrees E), and the middle and higher latitudes demonstrates that these lidar-observed characteristics of gravity wave activities over Haikou are different from those reported at other locations in the world. This indicates that atmospheric gravity wave activities may have obvious regional characteristics, and lidar observation results in the middle atmosphere are typically influenced by the properties of wave sources and the background atmosphere. Conclusions Based on the Rayleigh lidar observation dataset of the Chinese Meridional project, a data analysis method on the retrieval of temperature structures in the middle atmosphere and the identification of atmospheric gravity wave events have been investigated in our study. The temperature structure in the middle atmosphere (30-65 km) is retrieved accurately by the Chanin- Hauchecorne method, and the calculation results are compared with satellite measurements from SABER/TIMED and COSMIC. Gravity wave events are identified by calculating the power spectral density of atmospheric temperature perturbations after the dominant wave components are extracted through the wavelet analysis method. A specific data analysis process is well designed, and 202 gravity wave events in the middle atmosphere are successfully identified from Hainan lidar observations from January 2011 to July 2013. Vertical wavelengths and the observed wave period are extracted for every gravity wave event, and statistics show that lidar-observed gravity waves in the middle atmosphere over Haikou (19. 9 degrees N, 110. 3 degrees E) are typically 5- 9 km in vertical wavelength and 5-13 h in wave period. This data analysis method designed by us can be applied to the research on temperature variations and gravity wave activities in the middle atmosphere over China and may be helpful to further data assimilation of the Chinese Meridional project.
