Title: Extratropical Transition of Tropical Cyclones: From Climatology to Prediction

Presenter: Dr. Melanie Bieli

Abstract: Tropical cyclones (TCs) undergoing extratropical transition (ET) pose a serious 
threat to coastal regions in the midlatitudes. 
In this talk, I will present the first climatology of ET that encompasses all major global TC 
basins and is based on a consistent set of data, time period, and method. Using best-track 
data from 1979-2017 to define the tracks of the storm centers, we identify storms that 
undergo ET by means of their paths in the cyclone phase space (CPS), calculated from 
geopotential height fields in reanalysis datasets. Two reanalyses are employed and compared 
for this purpose, the Japanese 55-year Reanalysis (JRA-55) and the ECMWF Interim 
Reanalysis (ERA-Interim). The results are used to study the seasonal and geographical 
distributions of storms undergoing ET, inter-basin differences in the statistics of ET 
occurrence, and the differences between the ETs defined by CPS and those defined by the 
'extratropical' labels (determined subjectively by human forecasters using a wider range of 
data) in the best-track archives.
In the second part, I will introduce a simple statistical model that predicts whether a TC will be 
extratropical at a future time using seven features describing the TC and its environment. The 
model is based on North Atlantic and Western North Pacific TCs from 1979-2016. According 
to the Matthews Correlation Coefficient, it has skill in predicting ET at lead times up to three 
days in both ocean basins.