Structural changes of low level wind field of tropical cyclones in idealised extratropical transition scenarios

Tropical cyclones undergoing extratropical transition (ET) pose a serious threat for land-based and marine activities in the midlatitudes. Significant structural changes in the wind field occur during ET. Although the maximum wind speed typically decreases, the areal coverage of hazardous surface winds increases considerably and strong asymmetries develop. In order to provide accurate and timely warnings for an ET system these changes must be taken into account. Furthermore, reinsurance companies need information about the general behaviour of the surface wind field during ET for their risk assessment of the affected areas.

The structural changes of the surface wind field during ET are not yet well understood. Previous investigations of the asymmetric wind field in a tropical cyclone have concentrated on the tropical phase of the storm. In this study we use data from a variety of ET scenarios in numerical experiments with idealised initial conditions to investigate the structural changes in the low level wind field of the transitioning tropical cyclones. It is found in a number of cases that the asymmetric distribution of surface winds is much more complex than a simple superposition of the translation and the axisymmetric wind speed would suggest. Thus, the influence of further parameters including SST, midlatitude forcing and frontogenetical processes is considered.