Study On The Characteristics Of Quasi-16-day Planetary Wave Activity During The 2009 Stratospheric Sudden Warming

Stratospheric sudden warming（SSW）occurring in the polar stratosphere region of winter hemisphere is a special atmospheric phenomenon,during which time the global atmosphere can be affected to different degrees.The interaction between planetary waves（PWs）coming from the troposphere and background atmosphere is considered as an important excitation mechanism of SSW.Therefore,exploring the characteristics of PWs in the troposphere and the stratosphere is helpful for us to understand the SSW involved dynamic process throughly.In this paper,we studied the activities and propagations of quasi-16-day PWs in the troposphere and the stratosphere by using radiosonde data at Fairbanks station（64.8°N,147.9°W）in Alaska,United States and MERRA reanalysis data during2008-2009 winter,and further analyzed the relationship among the quasi-16-day PWs,the background atmosphere and the SSW.The main conclusions are as follows:1.We first analyzed the background atmosphere changes in 2008-2009 winter by using MERRA data.During the 2009 major SSW,the zonal mean temperature increased by 55 K poleward of 60°N at 10 hpa,accompanied by a sharp deceleration of the zonal mean zonal wind.The tropical stratosphere and high-latitude mesosphere cooled down slightly in the meantime because of atmosphere coupling.The zero wind line moved longititudally from the equator to the pole and vertically from the mesosphere to the lower stratosphere.The stratopause continued to shift down during the SSW,then broke down and was reformed at higher altitudes.The axis of the polar night jet moved poleward and downward before SSW,then was uplifted to the location where the stratopause was.2.We discussed the activities of the different modes of the quasi-16-day PWs in the stratosphere during this SSW based on MERRA data.All the four wave modes,i.e.W1,W2,E1 and E2 modes,showed different levels of enhancement in high-latitude stratosphere before and after SSW.The vertical structure showed that the four modes reached their maximum amplitudes in the middle and upper stratosphere within the latitudal range of 50°N-80°N,and then continued to weaken when extending downward,which was similar to the extension trend of the zero wind line from high to low altitudes.By comparison,we found that the W2 mode showed the largest increment and the strongest convergence,causing the maximum westerly deceleration of the background wind larger than 4ms^-1day^-1,indicating that the W2 mode played a dominant role in this SSW.The W2 mode of quasi-16-day PW in the stratosphere propagated upward vertically and kept longitudinally quasi-standing structure in the mid-high latitudes,and then propagated to the northern pole and the equator respectively.Besides,the E-P flux of the W2 mode showed that:the eddy heat flux and momentum flux were transmitted upward and poleward in the mid-high latitudes on January 19 and 24;while the upward eddy heat flux was weakened,and the eddy momentum flux transmitted to the equator increased significantly on January 29.3.We also studied the quasi-16-day PW’s activities and.propagation at Fairbanks station in the troposphere and lower stratosphere（TLS）during this SSW by using radiosonde and MERRA data.The stratospheric atmosphere at Fairbanks station has experienced severe warming and west wind reversal,while the tropopause appeared around 10 km.There were many spectral peaks of PWs below the tropopause,while the stratosphere was dominated by the low-frequency PWs.During this SSW,the temperature,zonal wind and meridional wind disturbances in the TLS all showed strong quasi-16-day oscillation between 0-25 km,while there were three peaks,two peaks and one peak in the vertical direction for the three parametes respectively.The meridional wind component had larger amplitudes than the zonal wind component.Strong quasi-16-day PWs appeared in the troposphere continuously,while only appeared in the stratosphere before and after the SSW.The local Quasi-16-day PWs were mainly composed of the W2 mode,which showed a vertically standing wave structure in the troposphere,and propagated upward in the stratosphere.The refraction index of the quasi-16-day W2 mode showed that there existed reflection layers in 0-3km and 10-15 km on January 2009,leading to a standing wave structure between these two reflection layers.After the onset day of the SSW,another reflection layer appeared in the lower stratosphere（17-25 km）,when the westward background wind reversed to eastward and hindered the upward propagation of the quasi-16-day PWs.During the SSW,the temperature and height of the tropopause were modulated by the quasi-16-day PWs,which enhanced the coupling between the troposphere and the stratosphere.The results from the radiosonde observations and the MERRA data have a strong consistency,verifying the authenticity and reliability of the above mentioned conclusions.