Researches And Observations Of Gravity Wave Activities With Meteor Radar In 40°N China

High-frequency(periods less than 2 hours)gravity waves(GWs)play an important role in the dynamics of the mesosphere and lower thermosphere(MLT).The study of high-frequency GW activity,GW momentum flux and its effect on the mean zonal flow is important for understanding the structure and dynamics of MLT.Meteor radar data are used to study the high-frequency GW activity of the MLT in 40°N China in this paper.Firstly,a composite day extension of the Hocking(2005)analysis technique was used to study diurnal and seasonal variations in atmospheric short-period GWs in the MLT in this study.Observations were made using meteor radar wind data collected in Langfang,China(39.4° N,116.7° E).GW activity was found to be strong over a 24-hour period,above the 95% confidence level,during almost every month of the year,as determined by an analysis of the corresponding power spectra.A 12-hour period of particularly high activity was also evident in April and October for zonal wind variance,as well as in January,April,May,and December for meridional wind variance.Additional periods were observed for the first time,including 4-,6-,and 8-hour cycles with confidence intervals greater than 95%.This suggests the possibility that GW activity could be modulated by solar-heating tidal wave harmonics.The zonal and meridional wind variances also exhibited annual,semi-annual,and quasi-4-month oscillation periods.Lastly,weak quasi-3-month and strong quasi-2.4-month cycles were observed at specific altitudes.Secondly,meteor radar data collected over Langfang,China(39.4° N,116.7° E)were used to estimate the momentum flux of short-period GWs in the MLT,using the Hocking(2005)analysis technique.Seasonal variations in GW momentum flux exhibited annual oscillation(AO),semiannual oscillation(SAO),and quasi-4-month oscillation.Quantitative estimations of GW forcing toward the mean zonal flow were provided using the determined GW momentum flux.The mean flow acceleration estimated from the divergence of this flux was compared with the observed acceleration of zonal winds displaying SAO and quasi-4-month oscillations.These comparisons were used to analyze the contribution of zonal momentum fluxes of SAO and quasi-4-month oscillations to zonal winds.The estimated acceleration from high-frequency GWs was in the same direction as the observed acceleration of zonal winds for quasi-4-month oscillation winds,with GWs contributing more than 69%.The estimated acceleration due to Coriolis forces to the zonal wind was studied;the findings were opposite to the estimated acceleration of high-frequency GWs for quasi-4-month oscillation winds.The significance of this study lies in estimating and quantifying the contribution of the GW momentum fluxes to zonal winds with quasi-4-month periods over mid-latitude regions for the first time.Thirdly,the wind fields and high-frequency GW activity in the MLT are compared using the data of two meteor radar stations with the same latitude and longitude difference of 30° between Kuerle(41.5°N,86.9°E)and Shisanling(40.3°N,116.2°E).The background mean wind field between two stations has a phase difference of 2 hours,which has obvious longitude dependence;the GW activity has no obvious correlation,which may be related to the different gravity wave sources near the two stations.