Study Of Gravity Waves In The Mid-Latitude Mesosphere And Lower Thermosphere Region

The mesosphere and lower thermosphere(MLT)region is the key area for the transition of the Earth's atmosphere into space,and plays an important role in the coupling between each layer in the geospace environment.The MLT region is strongly modulated by gravity wave(GW)perturbations.GWs transport heat,composition,momentum and energy from lower atmosphere to upper atmosphere,thus affect the dynamics structure,thermal structure and mean circulation of atmosphere,and become important in the vertical coupling between different layers of the atmosphere.In recent years,with the support of projects such as the Chinese Meridian Project,a large number of ground-based observations operated along 120° E longitude,aiming to monitor China's geospace environment.In this paper,we will study the propagation and characteristics of GWs in the mid-latitude MLT region with multi ground-based observations.Firstly,we developed a GW ray-tracing model based on the GW dispersion relation and the ray-tracing equation.By inputting the observed or specified initial GW parameters,selecting appropriate background atmospheric model,setting reasonable running steps and termination conditions,the forward or backward ray path and the potential source region of the GW can be estimated.Thus help the understanding of source and propagation of the GWs.Secondly,we used observational data from an all-sky airglow imager at Xinglong(40.2°N,117.4°E),a sodium lidar at Yanqing(40.4°N,116.00°E)and a meteor radar at Shisanling(40.3° N,116.2° E)to study the propagation of a mesoscale GW.During the night of March 1,2011,the imager identified a mesoscale GW structure in the OH airglow that had a wave period of 2 hours,propagated along an azimuthal direction(clockwise)with an angle of 163°,a phase speed of 73 m/s,and a horizontal wavelength of 566 km.Simultaneous measurements provided by the sodium lidar also showed a perturbation in the sodium layer with a 2-hour period.Based on the SABER/TIMED and radar data,we estimated that the momentum flux and the energy flux of the GW were approximately 0.59 m2/s2 and 0.22 mW/m2,respectively.Ray-tracing analysis showed that the GW was likely generated in the center of Lake Baikal owing to the existence of a jet-front system in the upper troposphere at that time.Thirdly,multi-year high frequency GW momentum fluxes and variances in the MLT region are revealed using four meteor radars along 120° E longitude at Northern Hemisphere mid-latitudes for the first time,which are located at Mohe(53.5° N,122.3°E),Beijing(40.3° N,116.2° E),Mengcheng(33.3° N,116.5° E)and Wuhan(30.5° N,114.2° E),respectively.The seasonal and latitudinal variation of GW momentum fluxes in the mid-latitude are discussed.The monthly mean zonal horizontal momentum fluxes and background mean zonal winds show significant coupling throughout the year,which agree well with the selective filtering mechanism.The latitudinal variations in the seasonal variation of GW momentum fluxes are mainly due to the latitudinal variation of mean wind in the lower atmosphere and GW sources.The background mean wind and temperature structure play decisive roles in the seasonal variation and height variation of GW variances,during which the latitudinal variations are also mainly due to the latitudinal difference of GW sources.Besides,we also analyzed the unexpected anomalies in seasonal variation and interannual variation,and possible physical mechanisms were proposed.These findings are significant for extending our understanding of MLT regions.