Heavy Rainfall in Hokkaido Island, Japan

Abstract

The climatological characteristics of heavy rainfalls in Hokkaido Island are investigated on the basis of areal frequency distribution of heavy rainfalls (the amount of daily precipitation above 100mm) and geographical distribution of the maximum daily precipitation. The causes of heavy rainfalls and the maximum daily precipitation are investigated on the daily synoptic weather charts, and their geographical distributions are also investigated. Relation between the regional characteristics of heavy rainfalls and the location of the Polar front is discussed. The statistical period is 25 years, from 1973 to 1977 and the data of 256 stations are analysed.
As a result, Hokkaido Island is divided into two climatic regions, the one is ‘high frequency heavy rainfall region (HR)’ and the other is ‘low frequency heavy rainfall region (LR)’. The bordering line of these two regions runs approximately from ENE to WSW, from Shiretoko peninsula, via Shiranuka Hills, Ishikari Mountains, Hidaka Mountains, Iburi Mountains to the southern part of Oshima Peninsula.
To the south of this line is the HR in which local extremly high frequency heavy rainfall regions are located on the eastern to southern slopes of the mountain ranges. In this region the frequency of heavy rainfalls is more than once per year, and the maximum daily precipatation exceeds 300mm. The main causes of heavy rainfalls here are extratropical cyclones passing to the south of Hokkaido Island from Japan Sea or from Pacific Ocean, most often in August and September. This types of heavy rainfalls are mainly caused by the moist air flow from these cyclones and the effect of orographic rifting strengthens the rainfalls. In these cases the Polar front is usually located to the south of Hokkaido Island.
On the other hand to the north of the line, the frequency of heavy rainfalls decreases drastically to less than once per five years. Here the main causes of heavy rianfalls are fronts and tropical cyclones, mainly in July and August. This time the Polar front lies in the northern part of Hokkaido Island and this location corresponds to its mean northernmost position. The region in which the main cause of heavy rainfalls is such northernmost Polar front is classified into ‘frontal heavy rainfall region’. In addition, extremly low frequency heavy rainfall rigion in which no heavy rainfalls were recorded through whole investigated period are discerned.