Monthly Characteristics of Precipitation Distribution and Heavy Precipitation Frequency in Japan during the Passage of Tropical Cyclones

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  • 台風通過時における日本列島の降水分布と大雨発生頻度に関する月別の特徴
  • タイフウ ツウカジ ニ オケル ニホン レットウ ノ コウスイ ブンプ ト オオアメ ハッセイ ヒンド ニ カンスル ツキベツ ノ トクチョウ

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Abstract

The monthly differences in the average amount and frequency of precipitation at 661 stations in Japan during the passage of 100 tropical cyclones (tropical cyclone in this study means tropical cyclone that at least once exceed a maximum wind speed of 17.2m/s or greater, including the period in which the disturbance transforms into an extratropical cyclone) near Japan during 1981-1990 were investigated. The differences are discussed using monthly tropical cyclone characteristics (track, central pressure, etc.) and synoptic situations.<br>Tropical cyclone data from the “Tropical Cyclone Tracks in the Western North Pacific 1951-1990” and the daily precipitation data from the “AMeDAS annual report, ” edited by the Japan Meteorological Agency, were used. The precipitation amount recorded at each station during the passage of a tropical cyclone near Japan is defined by the period (days) for which every tropical cyclone exists between 25°N and 45°N plus one day.<br>Monthly average precipitation distributions indicate that the area with precipitation exceeding 31mm is large in June and September. The records for both these months show high occurrence rates of stationary fronts. Thus, the large areas with precipitations of 31mm or more could be considered to be due to the precipitation associated with stationary fronts.<br>Moreover, in September, the area with precipitation of 31mm or more is the largest and the average central pressure of tropical cyclones is low. A tropical cyclone that records a lower central pressure causes a large pressure gradient, thereby resulting in the transportation of a large amount of water vapor to the stationary front. This mechanism could explain the reason why the area with precipitation of 31mm or more is the largest in September.<br>On the other hand, the area with precipitation of 31mm or more is small in August. However, the number of stations that recorded heavy precipitation for this month is greater compared with that in other months. In addition, many of the stations recorded the maximum precipitation amount in August. The primary reasons considered responsible for this were: i) tropical cyclones in August exists for long durations between 25°N and 45°N; and ii) many tropical cyclones occurred in August and there is a possibility that more than two tropical cyclones passed near Japan.

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