Gravity wave characteristics in the middle- to high-latitude Southern Hemisphere are analyzed using simulation data over 3 yr from a high-resolution middle-atmosphere general circulation model without using any gravity wave parameterizations. Gravity waves have large amplitudes in winter and are mainly distributed in the region surrounding the polar vortex in the middle and upper stratosphere, while the gravity wave energy is generally weak in summer. The wave energy distribution in winter is not zonally uniform, but it is large leeward of the southern Andes and Antarctic Peninsula. Linear theory in the three-dimensional framework indicates that orographic gravity waves are advected leeward significantly by the mean wind component perpendicular to the wavenumber vector. Results of ray-tracing and cross-correlation analyses are consistent with this theoretical expectation. The leeward energy propagation extends to several thousand kilometers, which explains part of the gravity wave distribution around the polar vortex in winter. This result indicates that orographic gravity waves can affect the mean winds at horizontal locations that are far distant from the source mountains. Another interesting feature is a significant downward energy flux in winter, which is observed in the lower stratosphere to the south of the southern Andes. The frequency of the downward energy flux is positively correlated with the gravity wave energy over the southern Andes. Partial reflection from a rapid increase in static stability around 10 hPa and/or gravity wave generation through nonlinear processes are possible mechanisms to explain the downward energy flux.
An intensive observation of the stratosphere has been made using 10 radiosondes every 3 h for the time period of 11−12 May 2006 at Shigaraki, Japan (34. 85°N, 136.11°E). Horizontal wind and temperature data were successfully obtained with high accuracy in the height region up to about 36 km. The sampling time intervals are 2 s corresponding to a nominal vertical resolution of about 10 m.Two packets of wavelike fluctuations whose phases propagate downward are detected around a height of 34 km (hereafter referred to as Wave-A) and of 24 km (Wave-B) in the obtained vertical profiles of horizontal winds. Wave parameters are estimated using a hodograph analysis under an assumption that these fluctuations are due to inertia-gravity waves (IGWs). The ground-based wave periods are 11 and 21 h, the horizontal wavelengths are 850 and 900 km, and the vertical wavelengths are 6.0 and 2.6 km, for Wave-A and Wave-B, respectively. It is also shown that both IGWs propagate energy upward and northnorthwestward relative to the background wind. The validity of the assumption is confirmed by the accordance of two independent estimates of the ground-based frequency. The horizontal structure seen in the horizontal divergence field calculated from European Centre for Medium-rangeWeather Forecasts (ECMWF) operational analysis data is consistent with the estimated wave parameters.Sources of the two IGWs are examined by a ray tracing method. Both IGW rays are traced back to the level and latitude of the mid-latitude westerly jet. Detailed examination for temporal variation of the wave structure indicates that the IGWs meandered eastward slightly south of the mid-latitude jet, turned north-northwestward, ascended rapidly where the background wind direction was changed to southward, and reached the middle stratosphere over the observation site. An interesting point is that both locallydefined Rossby number and cross-stream Lagrangian Rossby number are large in the regions where the IGW packets were situated during propagation around the jet from several days. Therefore, it is likely that the IGWs were generated in the vicinity of the unbalanced westerly jet through the spontaneous adjustment processes.
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