Characteristics of Tropical Cyclone Outer Size and Structure Associated With Extratropical Transition

There is a lack of consensus on how tropical cyclone outer winds may change, if at all, due to extratropical transition. Hence, this study examines changes in North Atlantic tropical cyclone outer size and structure using a large, multidecadal sample of cases from reanalysis data. These results suggest that tropical cyclone outer size and structure typically remain unchanged until after extratropical transition end. In those minority of cases with strong expansion during extratropical transition, increases in tropical cyclone outer winds begin first in the lower troposphere during extratropical transition and build upwards over time. This broadening of the azimuthal-mean outer winds is also associated with an increasingly asymmetric outer wind field with the strongest winds concentrated downstream of the tropical cyclone. These storms that expand most strongly during transition are typically smaller at transition start and eventually become embedded in more strongly baroclinic environments by extratropical transition end.