Observation and backward trajectory of an inertio-gravity wave in the lower stratosphere

A Doppler lidar observation of an inertio-gravity wave in the mid-latitude lower stratosphere is presented. The wave packet characteristics (vertical and horizontal wavenumbers, intrinsic and apparent frequencies) are inferred from the analysis of the hodograph of the horizontal wind fluctuations. Those parameters are used as initial conditions for the calculation of the wave packet trajectory backwards in time in the atmosphere. These calculations are realized by ray-tracing techniques, with background fields (wind and stability) provided by the European Center for Medium-Range Weather Forecasting analyses. Sensitivity tests are performed in order to estimate the robustness of the computed trajectory. It is argued that the generation of the wave has taken place in the upper troposphere, where evidence of large synoptic scale Rossby wave disturbances are found. Our results support the fact that geostrophic adjustment (and possibly shear instabilities) associated with such disturbances could be an effective mechanism for the generation of inertia-gravity waves in the mid-latitude.