Study On Characteristics Of Inertia-gravity Waves In The Mesosphere/lower Stratosphere(MLT) By Lidar

Using the 11.6 h wind,temperature and Na atom number density data observed by the Na lidar system at Andes Lidar Observatory(30.3°S,70.7°W)from 2212 UT 8 June to 1024 UT 9 June 2016,we study a typical upward propagating inertia-gravity wave(IGW)in the mesosphere and lower thermosphere(MLT).The spectral analysis indicates that the period of the IGW is about 6.6 h,and its horizontal and vertical wavelengths are about 826 km and 7.5 km,respectively.Hodograph technique shows that the wave propagates in the horizontal direction of about 38.6° west of north.The horizontal and vertical group velocities are calculated to be 20.4 ms-1 and 0.2 ms-1,respectively.We roughly estimate the intrinsic frequencies and horizontal wavelengths of the 6.6h IGW by combining the GW dispersion relation with the Doppler shift equation.The upward propagation IGW has a horizontal wavelength of 826 km.It propagates along the background wind of about 7.4 ms-1 in its horizontal propagation direction,its intrinsic period of 8.7 h is larger than its ground-based period of 6.6h.thus,its intrinsic horizontal and vertical group velocities are 20.4 ms-1 and 0.2 ms-1.Since the horizontally velocities of these waves are much larger than their vertical velocities,they propagate almost horizontally with very small elevation or depression angle of about 2-3°.These observed wave parameters are in the typical ranges presented in presented in previous studies.Since many significant wave sources exist in the different atmospheric layers,the observational events of several dominating waves are a relatively common phenomenon.With the help of data from HWM07 and MSISE00,the ray-tracing analysis indicates that the upward propagating IGW probably originates from the stratospheric unbalances flow over the Antarctic.The maximum horizontal wind amplitudes of 9.8 ms-1 is smaller than its intrinsic horizontal phase speeds of 24.5 ms-1,Thus,The IGW do not reach the threshold of instability.However,the Richardson number and N2 show the occurrence of dynamical and convective instabilities in some height and time due to the superposition of different perturbation components.Hence,the instability arising from the superposition effect may have a significant influence on wave saturation and amplitude constraint in the MLT.GW parameterizations do not include such effects,therefore may miss an important wave dissipation mechanism.According the IGW polarization relations,the perturbation components of the meridional wind,temperature and atmospheric density are derived from the zonal wind perturbation.The observed perturbations in the meridional wind and temperature are consistent with the results derived from the polarization relations,which indicates that the observed quasi monochromatic IGW satisfies the polarization relations of GWs.The calculated perturbation phase of Na atom number density is in good agreement with that of atmospheric density,confirming that the Na atom in the MLT can be regarded as a tracer of IGW activity.