THE DISTRIBUTION OF PREVAILING WINDS OVER THE PLAINS IN HOKKAIDO IN SUMMER

Abstract

In this paper, the distribution of prevaling wind direction and wind velocity in the following six regions in Hokkaido was studied: the Ishikari Plain, the Shari-Abashiri Region, Konsen-Genya, the Tokachi Plain, Sarobetsu-Genya and the Lake Saroma and the surrounding Region as shown in Fig. 1. The wind conditions were made clear by the field observations of wind-shaped trees and the wind observation at the agrometeorological stations. The main species of wind-shaped trees observed in this study were Larix leptolepis, Fraxinus mandshurica, Alnus japonica, Populus spp., Picea jezoesis, Betula platyphylla var, japonica. The grade and direction of deformation of wind-shaped larch trees (Larix leptolepis) was measured at about 140 points in the Tokachi Plain in August, 1973 and 1974. Five wind systems can be recognized in the Tokachi Plain: 1) The first branch of the westerly winds from the Karikachi Ridge blows northeastwards in the Plain. 2) The second branch of the westerly blows eastwards crossing the central part of the Plain. 3) The third branch of the westerly winds changes the direction from the region around Memuro to south or southeast. 4) The southeasterly winds from the Pacific Ocean blow up into the lowland along the Tokachi River. 5) The other southeasterly winds blow up northwards along the Hidaka Mountains. The observations in the Sarobetsu-Genya were made in November, 1973, and August, 1974. The main species of the observed wind-shaped trees were Fraxinus mandshurica, Alnus japonica and Larix leptolepis. Sarobetsu-Genya can be divided into three wind regions: 1) The first branch of the westerly winds from the Japan Sea changes their direction slightly northeastwards due to the local topographical conditions. 2) The second branch of the westerly winds from the Japan Sea blows eastwards. 3) The third branch of the westerly winds changes their direction slightly southeastwards into the lowland area along the Teshio River after crossing the Teshio Region. Further, an attempt was made to clarify the relation between the mean wind velocity (m/s) observed at the meteorological stations and the grade of wind-shaped trees. The relation is expressed by the following equation: _??_ where, Wsp is the mean wind velocity (m/s) observed at the agrometeorological stations in spring (March, April and May) and Gspl is the grade of wind-shaped trees of Larix leptolepis deformed by the westerly winds in the Tokachi Plain. For summer, the relation is expressed by the following equation: Ws(m/s)=0.86+1.07 Gsfawhere, Ws is the mean wind velocity (m/s) obsrved at the agrometeorological stations in summer from May to September and Gsfa is the grade of wind-shaped trees of Fraxinus mandshurica and Alnusjaponica in the Ishikari Plain and the Sarobetsu-Genya. This means that the mean wind velocity in summer is approximately equal to the value obtained by adding 0.9_??_1.1m/s to the grade value of wind-shaped trees of Fraxinus mandshurica and Alnus japonica. The distribution of mean wind velocity (m/s) estimated by wind-shaped trees over the Plains in Hokkaido, can be summarized as follows: The strongest wind regions with mean wind velocity of 4.0_??_5.0m/s are found (i) on the mouth of Ishikari River in the Ishikari Plain, (ii) the area along the coast of the Sea of Okhotsk and the southern mountain regions in the Shari-Abashiri Region, (iii) the area around the Lake Saroma, (iv) the Shiriu Cape, Hamanaka Bay and the Kombumori around regions in the Konsen-Genya, (v) the area around the City of Shintoku and the Urimaku regions in the Tokachi Plain, and (vi) in the area of about 5_??_10km wide along the Sea of Japan in the Sarobetsu-Genya.