Lidar application to middle atmospheric dynamics

Abstract

LIDAR (LIght Detection And Ranging) is an optical remote sensing technique which can be used to probe middle atmosphere (stratosphere & mesosphere) from where RADAR (RAdio Detection And Ranging) system fails to get scattering. The Mie and Rayleigh lidar system installed at National Atmospheric Research Laboratory (NARL), Gadanki (13.5°N, 79.2°E) has been operating at 532 nm green laser with increased energy of ~600 mJ/pulse and pulse repletion frequency of 50 Hz since 2007. From the Rayleigh lidar observations, vertical profiles of atmospheric density and temperature can be obtained above ~25-30 km (where the aerosols are almost negligible) at high spatial and temporal resolutions. The temperature profiles often show mesospheric inversion layers (MILs), the causative mechanisms of which are yet to be understood. In the present study, the improved performance of the lidar system is demonstrated by showing the height profile of temperature and its error obtained with the high power laser (~12 W per pulse) on 20 January 2007 when compared to the same obtained using the low power laser (~5 W per pulse) on 05 February 2007 over Gadanki. The temperature errors observed at ~80 km are ~3.5 K, ~18 K with high and low power lasers respectively. A large MIL has been observed on 20 January 2007 above ~78 km with amplitude of ~31 K from the lidar temperature operated with high power laser. The dominant gravity wave (GW) period and vertical wavelengths are found to be T~66 min and λ z ~6.4 km in the inversion region. The wave saturation ratio and eddy diffusion coefficient due to the GW breaking are calculated and it is found that the wave gets saturated at ~84-85 km and the eddy diffusion coefficient increases from ~25 m 2 /sec above the inversion region (~83 km). This result suggests that the occurrence of this large MIL event is probably due to gravity wave breaking.