庄内平野の強風に関する中気候学的研究 清川ダシと北西季節風

Abstract

In the region along the Mogami River in the Shonai Plain, Northeastern Honshu, the strong easterlies which blow mainly from SE and ESE and the strong westerlies which blow mainly from NW and WNW prevail (Table 1). The strong easterlies are the local wind, called “Kiyokawa-dashi”, in the Shonai Plain. The most of this westerlies correspond to the winter monsoon winds over East Asia. Some properties of these winds can be demonstrated by the daily mean wind speed at the Karikawa AMeDAS station (38°48'N, 139°59'E) and the Sakata Meteorological Station (38°54'N, 139°51'E) (Fig. 3); Both stations are on the Shonai Plain, Karikawa is located at the eastern side, where the Mogami River comes out on the Plain, and Sakata at the westernside, on the coast of the Sea of Japan. In this report, topographic effects were investigated on the basis of the statistical comparison of the meso-climatological properties of these winds.The vertical structure of the wind aloft on the occasion of the easterlies and the wester lies at Karikawa are examined by the aerological data at Sendai (38°16'N, 140°54'E). The easterlies develop under the condition of the wind direction of S in the layer below the summit level (about 800 mb) in the Tohoku District, Northern Honshu (Fig. 4 (a)). Moreover, the wind speed at Karikawa relate to the wind speed below the summit level with positive correlation and that on the heigher levels with negative correlation (Table 2). On the other hand, the velocity of the westerlies depend on the upper wind speed with the direction of W or NW (Fig. 4 (b), Table 2).The sea level pressure distribution shows also respective characteristic features in each case of the easterlies and the westerlies at Karikawa. On the occasion of the easterlies, the low on the Japan Sea side of the Japanese Islands and the high on the Pacific side appear clearly and the contours run almost in north-south direction on the pressure distribution pattern. This pattern of pressure distribution corresponds with the southerlies which develop in the lower layer below the summit level in the Tohoku District on the occasion of the easterlies as described above. In particular, the low pressure on the Japan Sea side strengthen in the case of the strong easterlies ; the east to west pressure gradient in the region of the Japan Sea side become large (Fig. 5, Fig. 6 (a), (b)). This pressure pattern suggests the upwind meso-high and the Lee meso-low explained by ATKINSON (1981) and BARRY (1981); this also indicates that the southerlies in the lower layer are induced by the topographic effects of the Tohoku District. On the occasion of the westerlies, the topographic effects are also readily seen on the pressure distribution maps ; the lee low and the upwind high develop and the pressure gradients are particularly large in the region of the Pacific side (Fig. 7, Fig. 8 (a), (b)). The trend of the isobars are northwest-southeast direction, which almost equivalents to the general wind direction.In order to evaluate the topographic effects in various dimensions, the relationships between the wind speed and the pressure gradient are examined. In regard to the pressure gradients, the space means of the Tohoku District and of the Japan Sea side region and the difference between Shinjo and Sakata are calculated. The wind speed at Karikawa and these pressure gradients are related to each other with strong linear correlation (Fig. 9, Table 3). In particular, the easterlies at Karikawa are related closely to the space mean of the pressure gradient in the Japan Sea side region and the westerlies are related relatively well to that in the Tohoku District. In addition, it is atracted special attention that the pressure gradient between Shinjo and Sakata strongly correlate with the easterlies than with the westerlies.