Niels Bohr Institutet > Kalender - det sker på NBI > 2009 > Master Thesis by Annik...
The Extratropical Transition of Tropical Cyclones over the North Atlantic Ocean:
The post-transformation Evolution
Master Thesis by Annika Hald Collins
Advisor: Aksel Walløe Hansen
Tropical cyclones developed over the Atlantic some times recurve to the north or north-east without making landfall and enter the baroclinic midlatitudes. A tropical cyclone which enters a baroclinic zone undergoes a transition to an extratropical cyclone and continues its lifespan as extratropical. This process is known as extratropical transition.
The extratropical transition is defined in two stages, the transformation stage and the post-transformation evolution. The post-transformation evolution begins when the cyclone has obtained extratropical features, as fronts within the cyclone. The cyclone can either reintensify or decay during the post-transformation evolution. In some cases the reintensification can be explosively and become a threat to coastal areas and shipping.
The purpose of this thesis is to study the post-transformation evolution of the tropical cyclones going through an extratropical transition over the North Atlantic Ocean, in the efforts to find why some cyclones rapid reintensify and why others do not reintensify at all.
The post-transformation evolution is classified in 3 scenerios, no reintensification, < 4 hPa, reintensification, > 4 hPa, but < 1 hPa per hour, and rapid reintensification > 1 hPa per hour.
It is found through a climatology study from 1991-2001, that 15% of transitioning cyclones rapid reintensify during post-transformation evolution. The rapid reintensification cases only occurred during the months of September and October, in the western part of the North Atlantic Ocean. The preferable track for these cases was moving north-northeastward close to the east coast of North America and then transitioning along that path. The majority complete the transformation stage in the vicinity of Newfoundland.
It was found through the case studies of cyclones moving toward Newfoundland that the location and growth rate of the baroclinic instability is very important in determining the post-transformation evolution. Also, very distinctive processes have been found through for the rapid reintensification cases.
The results of this thesis, including the current understanding of extratropical transition, will be presented during the talk.