Interaction between tropical cyclones and idealized baroclinic waves

Michael Riemer
Forschungszentrum Karlsruhe, Germany

The potential influence of tropical cyclones on the midlatitudes is widely underestimated. 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 the diabatically forced upper-level outflow of 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 our 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.

Currently the interaction of the tropical cyclone with an ongoing baroclinic development of one of two paradigms of baroclinic life-cycles is being investigated. These simulations can be seen as a more realistic testbed for the tropical cyclone - midlatitude flow interaction. Piecewise inversion of potential vorticity and the partitioning of the flow in its rotational and divergent parts should allow a quantitative assessment of the influence of the tropical cyclone on the midlatitude flow pattern and the development of low pressure systems located downstream of an ET event.