A Study of Inertia-Gravity Waves in the Middle Stratosphere Based on Intensive Radiosonde Observations

Abstract

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 locally-defined 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.