The impact of extratropical transition on the downstream flow: idealised modelling study

A tropical cyclone undergoing extratropical transition (ET) has a direct influence on the synoptic-scale midlatitude circulation. However, arguably the more significant influence is the excitation of a Rossby wave train on the midlatitude potential vorticity gradient by a poleward moving tropical cyclone. This wave train then propagates downstream and alters the midlatitude flow pattern. Thus an ET event taking place in the western North Atlantic can initiate explosive cyclogenesis in the eastern Atlantic and western Europe and might trigger severe precipitation events in the Mediterranean. Numerical weather forecasts frequently fail to capture this downstream influence.

To investigate the interaction of a tropical cyclone with the midlatitudes we are performing full physics numerical experiments using the PSU/NCAR MM5 modelling system with idealised initial conditions. Periodic boundaries in the zonal direction allow a channel configuration. A vortex-following nesting renders a higher resolution around the modelled tropical cyclone possible. In an initial experiment the midlatitude flow is represented by a balanced straight jet stream. A tropical cyclone approaches the jet stream from the south. The interaction between the tropical cyclone and the midlatitude circulation leads to the formation of a distinct jet streak. In the left exit region of the jet streak an extratropical cyclone develops and deepens rapidly. A Rossby wave train can be seen to propagate downstream from the ET event leading to the development of further baroclinic systems. A variation in strength and size of the tropical cyclone allow to asses the sensitivity of these results on the structure of the tropical cyclone.

Currently the interaction of the tropical cyclone with more realistic midlatitude flow patterns, consisting of developing baroclinic waves exited by localised as well as periodic pertubations at different vertical levels, is being investigated. The respective life-cycles of these waves constitute a variety of synoptic patterns for the tropical cyclone to interact with. In particular the life cycles developing from localised and periodic pertubations exhibit distinct atmospheric states downstream of the ongoing ET. Piecewise inversion of potential vorticity,complemented by the partitioning of the flow into its rotational and divergent parts, is performed to assess quantitativly the influence of the tropical cyclone on the midlatitude flow. This approach should help to determine which mechanisms during ET are important for the excitation of a Rossby wave train and for the promotion of rapid extratropical development located downstream of a transitioning tropical cyclone.