Subjects: Geosciences >> Space Physics submitted time 2016-05-04
Abstract： Sodium lidar is recognized as a useful tool to investigate gravity wave (GW) which influences the circulation, structure and composition of middle and upper atmosphere notably. For further understanding GW and their characteristics, more sufficient observational data within global models, especially those at low latitude are extremely expected. Meanwhile, long-term observations are also quite helpful for the GW analysis. As one part of Meridian Project, the observational Na lidar system at Hainan (19. 50 degrees N, 109. 1 degrees E) employed the double wavelengths laser beam, which is consists of three channels for Mie scattering, Rayleigh scattering and broadband sodium fluorescence. A pulsed Nd:YAG laser was rebuilt to trigger off two green laser beams simultaneously, one of which was 532 nm and used to excite the Mie and Rayleigh scattering, and the other laser beam pumped by a pulsed dye laser was about 589 nm and with 30 mJ per pulse. Resonant fluorescence from the sodium layer was received by a Cassegrain telescope with a primary mirror of 1000 mm diameter. The lidar profiles are stored automatically in a computer. The temporal and spatial resolution was about 167s and 96 m, respectively. Seasonal variations of the GW activity and their spectra at Hainan were studied with comparison to the previous reports and the predictions of theories of GW saturation and dissipation. The seasonal variations of GW activity and characteristics spectra at Hainan are mainly presented as follows: (1) The mean RMS atmospheric density perturbation over Hainan are 5. 63 %, which at solstice are obviously larger than that near the equinox. And the GW activity in winter is still active, which are quite different from those reports at the middle latitude area. (2) Due to the linear theory, the m spectra were calculated which show power law shapes and their range of variation were between -2. 14 and -3. 56 with an annual mean value of -2. 93. The calculated omega spectra were between -1. 22 and -2. 36 with an annual mean value of -1. 80, respectively. (3) The amplitudes of atmospheric density perturbation spectra at m = 2 pi/8km, 2 pi/4km, 2 pi/2km, 2 pi/1km and omega = 2 pi/60min, 2 pi/40min, 2 pi/25min all exhibit large nightly variability as well as large seasonal variations, with the maxima occurring near the solstice. While in winter, their amplitudes indicated that the GW activity is still active, which are consistent with the RMS atmospheric density perturbation annual variations. (4) The seasonal variation regularity of the GW activity at Hainan and the possible GW source there were discussed. It is concluded that the reaction of the Qinghai-Tibet plateau with the convection and the equatorial undercurrent may be the main reason of the GW behaviors at Hainan. The seasonal variation of GW activity at Hainan were measured based on 3 year's observation. The RMS atmosphere perturbation, the vertical wave number spectral amplitudes at 2 pi/8km 2 pi/4km 2 pi/2km and 2 pi/1km and omega spectra with those frequencies at 2 pi/60min, 2 pi/40min, 2 pi/25min all show that the semiannual maxima occur near the solstices, while the GW activity in winter is still active, which is quite different from the middle latitude area. The vertical wave number spectra and frequency spectra were also calculated which were fitted well with the theoretical value. It is concluded that the fact of the Qinghai-Tibet plateau's topography, intense convection of South China Sea and the equatorial undercurrent could be the main reason of the GW's behaviors at Hainan.
Peer Review Status:
Awaiting Review
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