Investigation of Tropical Cyclone Extratropical Transition and Downstream Flow Effects

As a tropical cyclone (TC) moves poleward and recurves to the east, it begins to interact with the baroclinic environment associated with the midlatitude regime. This interaction of the TC with the midlatitude environment can lead to the development of a hybrid extratropical cyclone that contains characteristics of both the original TC and a midlatitude cyclone in a process known as extratropical transition (ET). Recent studies have shown that there is potential for recurving TCs to perturb the midlatitude waveguide, which can result in a downstream high-amplitude response in the flow, and occasionally development of high impact weather events.

Here we present WRF-based case studies of two North Atlantic hurricanes from September 2011 that underwent ET and subsequently impacted Western Europe. Hurricane Katia (2011) moved into the midlatitude regime, interacted with an upstream trough, and developed into a significant extratropical cyclone that propagated east across the Atlantic and impacted the British Isles causing 1 death and $157 million USD in damages. A week later, Hurricane Maria was absorbed by the baroclinic regime over the Canadian maritimes during ET. However, the upper-level flow downstream of Maria became quite perturbed generating a midlatitude cyclone that impacted Europe approximately 3-4 days later. In this presentation we will discuss the physical characteristics of each case and diagnose the physical processes that resulted in such different midlatitude evolutions and outcomes.