Studies Of Planetary Wave Activities During Stratospheric Sudden Warming

In this paper,we use meteor radar,MST radar,and radiosonde observations combined with reanalysis data,to study the structures and propagation characteristics of planetary waves from the troposphere to the mesosphere and lower thermosphere during the Stratospheric Final Warming（SFW）in spring 2015 and Stratospheric Sudden Warming（SSW）and SFW in early 2016.By multi-site observations and global reanalysis data,the possible excitating sources of planetary waves are discussed.The main research results of this paper are as follows,The SFW of spring 2015 causes the zonal mean temperature of 10 h Pa in the polar region of the northern hemisphere to increase by nearly 20 K,and the zonal mean zonal wind decreases from more than 30 ms^-1to about-10 ms^-1.With the help of SFW,the polar circulation completes the seasonal transition from winter to spring in a short time.The SFW of spring 2015 occurres during mid-March to late March,which is relatively early.The earlier the SFW onsets,the greater the influence of planetary wave on it（Hu et al.,2014）.Our study shows that there are strong quasi 10-day wave（Q10DW）and quasi 16-day wave（Q16DW）activities with comparable amplitudes from the troposphere to the mesosphere and lower thermosphere.The peak amplitudes from the troposphere to the stratosphere are near the onset date of SFW.In addition,the wave amplitudes in the zonal wind are relatively larger than those in the meridional wind.However,in the mesosphere and lower thermosphere,the strong Q10DW and Q16DW waves are delayed more than 10 days relative to those in the lower atmosphere,and the maximum amplitudes appear after the temperature peak of SFW,which is roughly consistent with the results of Hough mode.This suggests that the Q10DW and Q16DW in the mesosphere and lower thermosphere are different from those in the stratosphere.The wave activities in the lower atmosphere may play an important role in the excitation of SFW through wave-flow interaction.However,the Q10DW and Q16DW in the mesosphere and lower thermosphere are the responses to the SFW event.The waves in the mesosphere and lower thermosphere may be the Rossby normal modes in situ excited and amplified by the instability of background flow from stratopause to lower thermosphere.There are minor SSW and SFW events occurring in early 2016 over the northern hemisphere.The SSW and SFW events occurr around February and March,respectively,during which the zonal mean temperature at 10 h Pa and 60°N increases by about 45 K and 36 K,and the mean zonal wind decreases by about 38 ms^-1 and 49 ms^-1.SSWs cause significant wind reversals in both the zonal and meridional winds at the three stations.The zonal wind reverses about 2-4 days earlier in the mesosphere and lower thermosphere than in the stratosphere.The meridional wind also reverses,especially during the SSW.Two separate quasi 6-day wave（Q6DW）activities are observed from the troposphere to the mesosphere and lower thermosphere during the SSW and the SFW,respctively.The height and latitude structures of the Q6DWs are roughly consistent with those of Q6DW with eastward propagation studied by Pancheva et al.（2018）.In addition,the amplitude of the Q6DWs in the stratosphere and lower mesosphere is greater in the zonal wind than in the meridional wind,and the Q6DWs during the SSW are stronger than those during the SFW,especially at the WH station.The vertical phase propagation indicates that the Q6DWs propagate downward in the stratosphere and upward in the mesosphere and lower thermosphere,suggesting that the Q6DWs are generated in the mesosphere.Q6DWs are found to travel eastward with zonal wave numbers 1-2 in the stratosphere and lower mesosphere.By studying the EP flux divergence and the meridional gradient of the quasi-geotrophic potential vorticity,instabilities are found in the mesosphere.Therefore,combined with the vertical propagation of the phase,Q6DWs may be in-situ excited by the instabilities in the mesosphere.Instability also occurs in the stratosphere over the mid-latitude,especially during the SSW.This is consistent with the larger wave amplitudes during the SSW than during the SFW.Thus,Q6DWs in the stratosphere at the three mid-latitude stations may be amplified by the stratospheric instabilities,especially during the SSW.