Evaluating the impacts of extratropical transitioning on typhoon losses via synoptic case studies

The economic impacts of tropical cyclones that have undergone extratropical transitioning (XTT) can be dramatic. One reason is that buildings and their owners in the middle and high latitudes may be less well prepared for storm-force winds and heavy precipitation. Another reason is that these storms typically have larger damage footprints from both a wind and precipitation perspective.

This paper presents a case study comparison of two typhoons to demonstrate the impacts of XTT on property losses in Japan. Typhoon Tokage occurred in October 2004 and began XTT as it approached landfall on Shikoku Island. An important aspect of the XTT process was a trough approaching simultaneously from the northwest – over the Sea of Japan. Surface circulation became significantly elongated from southwest to northeast and winds remained strong on the left hand side and over the north- and west-facing coasts and prefectures of Japan. Precipitation also increased significantly as a result of XTT. Insured losses from Tokage were reported to be around 88.5 B JPY. Similarly, typhoon Etau in August 2003 made landfall in the almost identical spot as Tokage with nearly the same intensity. However, a significant difference from Tokage was that Etau did not transition as it recurved northeastward across the spine of Japan until it reached Hokkaido. As a result, insured losses were significantly less than those for Tokage – 5 B JPY. The greatest losses from Etau occurred in fact in Hokkaido, from flooding.

Synoptic and mesoscale analyses are presented for these two typhoons using 20 km resolution JRA-25 data. Reasons for the enhanced winds on the left hand side during the transitioning process are presented. Larger scale NCAR Reanalyses for other XTT and non-XTT storms are shown that suggest the NW trough/SE typhoon interaction version of XTT is a typical occurrence. Given that a majority of XTT occurs as the typhoon is south and/or east of mainland Japan, and that the wind footprint is large , it is demonstrated that such storms lead to significantly more damage than non-XTT storms.