42% of all the hurricanes that developed over the Atlantic Ocean between 1899 and 1996 underwent extratropical transition. Many of these ex-hurricanes crossed over Europe as extratropical systems. These systems may cause gale force winds and heavy rainfall over Europe and pose a significant threat to life and property. Therefore accurate prediction of extratropical transition is of utmost importance. At present, numerical forecast models do not always handle extratropical transition well. Contributory factors to poor numerical forecasts might be errors in the relative locations of the tropical cyclone and baroclinic wave or in the structure of either system. We investigate the sensitivity of extratropical transition to these factors using a series of idealized numerical calculations relating to the interaction of a baroclinic wave with a hurricane-like vortex. In particular, we investigate the flow evolution as a function of the position of the vortex relative to the baroclinic wave and of the evolution of the baroclinic wave.

The calculations use a three-dimensional, nonlinear, hydrostatic limited area model based on the primitive equations. At this stage moist processes are not included. The model is initialized using a zonal jet with a maximum just below the tropopause. In some cases an additional barotropic, meridional shear of the zonal wind is added to the initial wind field. The initial conditions are in thermal wind balance giving a strong meridional temperature gradient in the troposphere. Time integrations in which these basic states are perturbed near the tropopause give baroclinic waves with different stuctures. A hurricane-like vortex is inserted at different integration times corresponding to different stages of maturity of the baroclinic wave and at different positions relative to the baroclinic wave. Following this insertion, the integration is recommenced. Results of model runs with and without a vortex are compared and the influence of the structure of the baroclinic wave is investigated.