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2. Observation and data analysis

     We used sounding data observed by Vaisala Digicora2 MW 15 rawinsonde in Pohang(  E,  N ), Korea during a year of 1998. The ascending rate of the balloon is approximately 50 - 60 , and all the observation variables are recorded every 10 second. As a result, the vertical resolution of data is roughly 50 - 60 m. However, because the rawinsonde automatically smooth horizontal wind data using low-pass filter in order to reduce observational errors, the vertical resolution of horizontal wind data becomes approximately 500 m. For the convenience, all observed variables used in this study are interpolated into 50 m grids using cubic-splint method.
     Figure 1 shows time-height cross sections of monthly mean temperature and zonal wind observed at Pohang. In this figure we used sounding data that reached the higher altitude than 30 km. Temperature in the troposphere decreases rapidly with height, while temperature in the stratosphere increases slowly with height. The vertical lapse rate remains nearly constant in the stratosphere regardless of the change of season. This suggests that the static stability in the stratosphere is nearly constant during a year of 1998.  The jet stream in winter(January and December) are much stronger than that in summer(July, August). Figure 1b also shows the clear seasonal variation of zonal wind that the zonal wind in the winter stratosphere is generally westerly, while that in the summer stratosphere is easterly. However, it should be taken note that easterly wind regions are also observed near z = 33 km in January and November.

     For the analysis of gravity wave characteristics in the stratosphere and the troposphere, the temperature and wind profiles within 17 - 30 km and 2 - 9 km altitude ranges are used, respectively. The rapid changes of temperature and wind at the tropopause can be excluded in the data analysis using the two separated analysis regions. The sounding data observed at 00UTC and 12UTC are used except for July in which there were only 7 soundings that reached the higher altitude than 30 km at 00UTC and 12UTC. Accordingly, in July, the sounding data observed at 06UTC and 18UTC are also used in addition to 00UTC and 12UTC.
     To obtain gravity wave perturbation from temperature and wind profiles in the stratosphere and the troposphere, basic state profiles are estimated by fitting second order polynomial into sounding profile for individual variables, and perturbation profiles are calculated by removing the basic state profile from the each sounding profile.