The climatological significance of extratropical transitioning on typhoon precipitation over japan

Extratropical transitioning (XTT) is a frequent and significant occurrence for typhoons in the Northwest Pacific Basin. The XTT process poses many challenges from a forecasting perspective. The XTT process also poses many challenges from a catastrophe modeling perspective. While the wind field can become highly asymmetric, the precipitation field can broaden significantly, if not intensify, thereby placing a larger area under the threat of flood. Unlike in the United States, many Asian Countries are insured against flood damage from typhoons. Japan is particularly prone to floods from typhoon because of its combined high typhoon activity, its mid-latitude location, and steep orography.

This paper presents a climatological analysis of the impacts of XTT on typhoon precipitation in Japan. Best Track data from the Japan Meteorological Agency is used to demonstrate that XTT storms are responsible for more than 50% of the total typhoon activity for Japan. Ten years of TRMM data as well as longer record AMEDAS (station) data are used to demonstrate that XTT storms account for disproportionately more typhoon precipitation than non-XTT storms. Available satellite imagery is used to demonstrate how the XTT process leads to significant enhancement of the cold conveyer belt and warm frontal precipitation. Because XTT typhoons typically re-curve over or south of Japan, the enhanced cloudiness and precipitation tend to extend over Japan. A conceptual model is presented to provide a physical explanation of why XTT storms can contribute more significantly to flooding and flood loss in Japan than non-XTT storms.