Christoph Zuelicke & Dieter Peters Leibniz-Institute of Atmospheric Physics, Kuehlungsborn, Germany
The relation between poleward breaking Rossby waves and stratospheric inertia-gravity waves is investigated for ten field campaigns in the winters 1999 =96 2002. They were performed at Kuehlungsborn in the North German Lowlands. While Rossby wave breaking takes place over Northern Europe in ~ 40 % of the winter days the ten field campaigns can be considered as characteristic for such situations. Inertia-gravity wave properties have been diagnosed from radiosonde and VHF radar observations and appeared to be flatter (vertical wavelength lambda_z ~ 1.74 km), slower (intrinsic period tau ~ 8.82 hrs) and larger (total energy e ~ 15.6 m^2 s^-2) than the climatological mean for Northern Germany= .
The ten field campaigns were hindcasted with the nonhydrostatic PSU/NCAR 5th-generation mesoscale model (MM5). It was set up with a resolution of 24 km horizontally and 250 m vertically allowing for the simulation of waves with a wavelength of ~ 200 km horizontally and ~ 1.4 km vertically. The stratospheric inertia-gravity wave action in 16 - 20 km height was parameterised with an empirical formula based on the synoptic-scale cross-stream Lagrangean Rossby number accounting for the flow deceleration. For a statistically satisfying result with more than 80 % explained variance both generation and (critical level) absorption processes needed to be taken into account. The wave propagation model for the 2 - 20 km height layer was based on WKB theory for a vertically changing wind field.
In the presented case studies we found a close statistical relationship between the synoptic-scale upper tropospheric / lower stratospheric flow deceleration and stratospheric inertia-gravity waves. Hence, these events of poleward breaking Rossby waves provided an optimal environment for their generation by the tropospheric jet streak and further propagation into the stratospheric jet.