Yaku-shima is a mountainous island, with area of approximately 500 km2 and peak of 1, 935 m, located in the western North Pacific around 30° N, 131° E (Fig. 1). This island is known as one of the highest precipitation areas in Japan where annual mean precipitation in the mountainous region exceeds 7, 000 mm. In the present study, distribution of monthly and annual mean precipitation is presented, and that of heavy rainfall days when daily precipitation exceeding 100 mm is observed at least one station in the island is investigated by using station data in so far as possible. For the mean distribution, data of 16 stations are available, while for heavy rainfall distribution data of 23 stations are utilized (Table 1).
First, distribution of monthly and annual mean precipitation is presented. Monthly mean precipitation at some stations is over 1, 000 mm in June or September (Fig. 2). Annual mean precipitation exceeds 5, 000 mm in the inland area, with the highest value of 7, 373 mm in the southeastern part (Fig. 3).
Next, precipitation distribution of heavy rainfall days from 1996 to 1998 is analyzed as follows : for each heavy rainfall day when rainfall amount over 100 mm/day is observed at anystations in the island, “relative precipitation (RP)” is calculated as a ratio of daily precipitation at each station to the maximum daily precipitation observed in the island on the same day. The cause of rainfall on each heavy rainfall day is identified as typhoon (Ty), extratropical cyclone (Ec), stationary front (Sf), or cold front (Cf), by inspecting daily surface weather charts analyzed by the Japan Meteorological Agency. Then, “averaged relative precipitation of heavy rainfall days (ARP)” is computed by averaging RP at each station classified by each synoptic system (Fig. 4).
For ARP distribution due to Ty, ARP is very high in the inland area but in contrast it is very low in the coastal area. For ARP distributions due to Ec, Sf, and Cf, the highest ARP (over 0.6) area is located in the southeastern part of the inland area. However, the moderate ARP (0.3-0.5) area is expanding more westward in case of Cf, but it is located in only eastern part in case of Ec.
Applying a cluster analysis to monthly mean precipitation and ARP, Yaku-shima Island is divided into 5 regions (Fig. 7). In region A, located in the coastal area except for the eastern coast, precipitation is relatively low in each month and ARP is also low in all cases. In region B of the southeastern coastal area, precipitation is high in March and April, and ARP is high in case of Ec, Sf, and Cf. In region C of the southeastern inland area, precipitation is high in each month and ARP is high in all cases. In region D of the northern inland area and the northeastern coastal area, precipitation is high in August and September, and ARP is high in case of Ty. In region E of the central inland area, ARP is high in case of Ty, Sf, and Cf.
