Statistics of gravity waves in the lower stratosphere over Beijing based on high vertical resolution radiosonde

The seasonal variation of inertia gravity-wave activity in the lower stratosphere (17-24 km) over Beijing is studied based on the high vertical resolution radionsonde observations (from December 2001 to February 2003) of Beijing Observatory (116 degrees 28'E, 39 degrees 48'N). Some of the important gravity-wave parameters, such as intrinsic frequency, and propagation direction, are estimated according to the polarized relation among gravity-wave wind components and temperature. Time series of wave energy show that the largest wave amplitudes occur during the winter and the least during the summer, and the average of the KE:PE ratios is about 2.6. Zonal and meridional wind perturbations have almost the same roots of mean variances, which shows that the wave energy in the lower stratosphere is isotropic horizontally. The motion and temperature fields are dominated by waves with vertical wavelengths of 1.5-3 km, which occupy above 80% samples, with a mean value of about 2.3 km. The horizontal wavelengths are mainly distributed between 100-800 km, averaging 445 km. The ratio of the mean horizontal to vertical wavelength is about 200:1, which indicates that the wave propagates nearly horizontally, with a very small vertical angle. The intrinsic frequency is estimated by fitting a polarized ellipse to the wind perturbations after band-filtering waves with a wavelength of 1.5-3 km, and the results show that the intrinsic frequency is dominated by 1f-3.5f, with an averaging value of 2f, corresponding to an intrinsic time period of 9 hours. Wave energy is found to propagate mainly upward, and in the horizontal direction, there is clear azimuthal anisotropy with predominate northwest propagation against the prevailing wind.