DISTRIBUTION OF SUMMER WIND IN THE KUSHIRO DISTRICT HOKKAIDO

Abstract

The summer climate of Kushiro, locating on the coast of eastern Hokkaido, is charac-terized by sea fog, which results in relatively low temperature, high humidity, and insufi-cient sunshine. The sea fog in this district is explained as follows. The prevailing south-erly air flows, which have travelled over the cold sea surface of the NW-Pacific, are cooled from its lower level to form sea fog. The sea fog invades as an advection fog in the Kushlro District. At eight stations in the Kushiro District (Fig. 1), wind observations with recording windvanes were carried out for three years from 1970 to 1972, for the purpose of air pollution resarches. In the present study, the author analysed the data from May to August for the three years. Some field observations and analyses of winds were made in June and August, 1972. In addition, the observations on wind-shaped trees were made in order to make clear the detailed distribution of the prevailing wind in summer. The results obtained are summerized as follows. (1) The wind roses for every three hours in a given period at each observation station (Fig. 2-Fig. 6), show that there is a marked diurnal change. Calm and weak northerly winds prevail from mid-night to early morning. In the forenoon, the wind direction changes clockwise from the north to the south (i.e., the beginning of sea breeze). At noon, winds reach its maximum speed. After about 15h, winds become gradually weak or calm, and then are replaced by weak northerly winds. This wind system seems to be a character of land sea breezes. (2) The sea breeze sets in on the coast first, and then gradually invades inland (Fig. 7 and Fig. 8). From the daily records of wind direction and speed at each station from May to August in 1972, frequencies of the onset time of sea breeze are shown in Fig. 9. Fig ure 10 illustrates the relationship between the mean onset time of sea breeze (Ts in hour) and the distance from the coast to the station (D in km). The relation is represented by the following equation Ts=0.11 D+8.00. (3) The diurnal change of wind speed at each station is shown in Fig. 11. This figure was made by averages of 26 days in June, 1972. Each station shows a remarkable regularity with maximum in the daytime and minimum early in the morning. The wind speed increases for 4-5 hours after the sea breeze invasion and reaches its maximum nearly at noon. (4) An observation of wind shaped trees enables the author to draw detailed stream lines in the district (Fig. 12). The stronger wind regions appear in the eastern part of the district, on the north-western slope of the Lake Harutori, west of the Nishi Port and the inland marshy ground (Kushiro Shitsugen). Topographical features, land uses, vegetations and geographical distribution of urbanized areas influence slightly on the wind directions. (5) Fig. 13 clarifies the relation between the mean wind speed of the prevailing winds in summer at the eight stations and the degree of deformation of wind-shaped trees of larch (Larix leptolepis) near each station. The result is represented by the following equation Ws=0.51 G1+2.57 where, Ws means mean wind speed (m/sec) of prevailing wind direction at each observation station, GL the degree of deformation of wind-shaped trees of larix.