Climatology of extra tropical transitioning duration in the North Western Pacific

Given its meridional extent and location within the Pacific basin, Japan is regularly impacted by strong winds from cyclones at different stages of their lifecycle. To quantify the associated risk of damage to properties, catastrophe models such as the ones developed by RMS aim to simulate wind fields from thousands of stochastic storms that extrapolate historical events. In a recent study using 25 years of reanalysis data, Kitabatake (2011) estimated that 40 % of all Pacific tropical cyclones completed their transition as an extra tropical system. From a cat modeling point of view the extent of the wind field observed during these transitioning episodes is critical.

The duration of extra tropical transition is a key parameter to model storms accurately throughout their lifecycle for the purpose of damage assessment. We derive a climatology of extra tropical transition duration from ERA-interim (0.7°) and GFS-FNL (1°) data in the North Western Pacific. We put this work in the perspective of the climatology of Kitabatake (2011), which uses JRA-25 (1.25°) data. The statistical characteristics of extra tropical transitioning duration depend on the exact definition of duration. We investigate the duration using both of Hart (2003) cyclone phase spaces, and the alternative cyclone phase space proposed by Wood and Ritchie [submitted to J. Climate] in their climatology of the North Eastern Pacific extra tropical transition. The results highlight the complex issues involved in prescribing distributions of transitioning durations for risk assessment applications.