FRONTAL ZONES AND PRECIPITATION DISTRIBUTION IN THE RAINY SEASON OVER EAST ASIA

Abstract

Some climatological aspects of the frontal zones at the ground and 850 mb-surface (Figs. 1_??_3) and the precipitation distributions (Figs. 4_??_6) from decade to decade in May, June and July over East Asia were dealt with. The following results were obtained: (1) In the second decade of May, the rainy season appears in South China and the third decade in Japan. In the first decade of June, a marked frontal zone is formed from South China through East China Sea to the Pacific coast of Japanese Islands. Sometimes the frontal zone tends to disappear in South China, causing a pause of the rainy season. In the second decade of June, the frontal zone develops from the area around the lower course of the Yangtze River to the. Pacific coast of Japan; that is the beginning of the Mai-yu season over China and the Bai-u season over Japan. In the last decade of June it shifts slightly to the north. In the first decade of July, it runs from Middle China crossing over southern part of Korea to southwestern Japan. From the last decade of June to the first decade of July the rainy season reaches to its climax. In the second decade of July the frontal zone goes up further north. The rainy season ceases normally in this decade. (2) The position of the frontal zone at the 850 mb-surface is deviated 2_??_6° latitudinally to the north from that at the ground-surface in the beginning period of the rainy season over China. However, there is no positional correlation over Japan and her eastern sea. In the climax period of the season the latitudinal deviation is 2_??_4° over China and Japan. In the last period of the season, there is no positional correlation over China and, on the contrary, 2_??_4° latitudinal deviation is found in the area around Japan and the Pacific. This change in the tendency of positional deviation of frontal zones between the 850 mb-and ground-surface over China and Japan can be explained in relation to the shifting of the jet stream in the rainy season. (3) Characteristics of the precipitation distribution are analyzed only in the area over Japan and her surroundings. On the distribution maps of decadal precipitation from May to July one can observe always a zonal pattern along the Pacific coast of Japan (Figs. 4_??_6). On the other hand, the beginning of the season defined by the change in precipitation and the appearance of the maximum in the season shift from south to north (Figs. 7_??_8). The amount of maximum precipitation becomes smaller in accordance with the distance from the tonal area along the Pacific coast of Japan (Fig. 9). Therefore, it must be pointed out that the shifting of the frontal zone does correspond to the appearance of the maximum at any points and does not indicate the climatological distribution pattern of precipitation in the rainy season over Japan.