Study Of The Variability Of Atmosphere In The MLT Region Based On The Observations From Meteor Radars

The mesosphere and lower thermosphere(MLT)are the transition region between the lower and upper atmosphere,where complex dynamic processes and climatology are dominant.These dynamic processes and climatology are mainly modulated by various atmospheric waves,including planetary waves,tides and gravity waves,which can propagate upward to the MLT region with large energy.This energy deposits in the MLT,thus affecting the atmospheric horizontal wind,neutral atmospheric density,and atmospheric temperature in the region.In particular,the study of horizontal winds in the MLT region is necessary for understanding the interaction between the lower and upper atmosphere and the variation in atmospheric circulation.Therefore,we report the variability of the global atmosphere in the MLT region based on observations from meteor radar and atmosphere models.In addition,the neutral density and horizontal wind observed by the meteor radars are used to examine the response of neutral density,wind,and temperature in the MLT region to stratospheric sudden warmings(SSWs).The detailed results are as follows:We report a seasonal variation and its latitudinal feature in the horizontal mean wind in the MLT region observed by 10 meteor radars located at the Svalbard(78.3°N,16°E),Troms(?)(69.6°N,19.2°E),Mohe(53.5°N,122.3°E),Beijing(40.3°N,116.2°E),Mengcheng(33.4°N,116.5°E),Wuhan(30.6°N,114.4°E),Kunming(25.6°N,108.3°E),Fuke(19.5°N,109.1°E),Darwin(12.3°S,130.5° E)and Davis(68.6°S,77.9° E)stations.The semiannual oscillation(SAO)predominates at low latitudes,while the annual oscillation(AO)is obvious at low latitudes.In both zonal and meridional winds,the AO amplitude increases slowly from Svalbard to Fuke as latitude decreases,while the SAO amplitude rapidly increases as latitude decreases.The reversal of the zonal wind from eastward to westward occurs at an altitude of approximately 90 km because of gravity wave forcing.As latitude decreases,the altitude of the zonal wind reversal decreases.The time of the meteor radar eastward zonal wind maximum occurs at different latitudes and slowly shifts from July to May.WACCM and HWM07 appear to well capture the seasonal and latitudinal variations in the zonal wind component,especially the temporal evolution of the eastward zonal wind maximum shifts from July to May as the latitude decreases.However,the WACCM and HWM07 meridional winds show the difference between the meteor radar observations.We report a seasonal variation and its latitudinal feature in the diurnal tide and semidiurnal tide harmonic fitted by the horizontal mean wind in the MLT region observed by 10 meteor radars located at different latitudes.The semidiurnal tide predominates with two peaks throughout the year,while the diurnal tide has one peak at low latitudes.However,the diurnal tide predominates with two peaks,while the semidiurnal tide has three peaks at low latitudes.In both zonal and meridional winds,the diurnal tide amplitude increases obviously from Svalbard to Fuke as latitude decreases,while the semidiurnal tide rapidly decreases as latitude decreases.WACCM and GSWM can simulate the seasonal and latitudinal variations in the atmospheric tide well.Meanwhile,the GSWM has similar variations to the meteor radar observations,and WACCM has similar values to the meteor radar observations.The neutral density and horizontal wind observed by four meteor radars,including the Svalbard and Troms(?) meteor radars at high latitudes and the Mohe and Beijing meteor radars at middle latitudes,are used to examine the response of neutral density,wind,and temperature in the MLT region to stratospheric sudden warmings.The neutral density over Svalbard and Troms(?) at high latitudes increased at the beginning of SSWs and decreased after the zonal mean stratospheric temperature reached the maximum.However,the neutral density over Mohe at midlatitudes decreased in neutral density at the beginning of SSW and increased after the zonal mean stratospheric temperature reached the maximum.The zonal wind at high latitudes shows a westward enhancement at the beginning of SSWs and then shows an eastward enhancement after the stratospheric temperature reaches a maximum.However,the zonal wind at midlatitudes shows an opposite variation to that at high latitudes,with an eastward enhancement at the onset and changing to westward enhancements after the stratospheric temperature maximum.By analyzing the relationship between the variation in density,temperature and wind in the MLT region during SSW events,we found that SSWs may cause a simultaneous change in temperature and atmospheric circulation,which leads to an obvious change in neutral density.