Formation of a Rain Belt and the Structure of the Front during the Baiu Season on the Chinese Continent

The formation of the rain belt and the structure of the Barn front at the lower troposphere on the Chinese continent during the rainy season (June and July) have been studied. As the first step, the daily precipitation distributions were classified in terms of the morphology of the precipitation area and the formative location of the rain belt. Next, the concurrent relationships between the changes of synoptic fields and the formation of the rain belt were investigated. Finally, the characteristic features of the frontal system for each type were analyzed to show the manifold structures of the Baiu front on the Chinese continent.<br> Data sets used in this paper are daily precipitation records from 234 rain gauge stations (Fig. 1-a) and atmospheric data at 12GMT from 85 stations (Fig. 1-b) during the years 1977 to 1981.<br> The daily precipitation areas having synoptic scale can be classified morphologically into two groups: a circular precipitation area (here in after, CPA) whose diameter is 1, 000km or more, and a belt-shaped precipitation area (here in after, BSPA), oriented in an almost east-west direction. The BSPA, which is mainly seen south of 35°N on the Chinese continent (Fig. 3-a), can be divided into two types in terms of the initial formation site (Fig. 3-c). Furthermore, the BSPA sometimes appears to west-southwest of the CPA. Accordingly, the transition patterns of the precipitation area resulting in the formation of the BSPA could be classified as follows:<br> Type A (7 cases): The BSPA is formed after the passage of the CPA (Fig. 4).<br> Type B-I (11 cases). The precipitation area which appeared on the Tibetan Plateau moved or extended eastward without forming a CPA, and the BSPA built up to the north of 30°N (Fig. 5 type B-I).<br> Type B-II (6 cases): Similar to type B-I, but the precipitation area moved or extended southeastward, and the BSPA built up to the south of 30°N (Fig. 5 type B-II).<br> Figure 6 indicates the appearance period of the BSPA for each type. Type A and type B-II took place in early to mid-June, and type B-I occurred frequently after mid-June. On the Chinese continent, the occurrence of The BSPA is chiefly due to type B-I, which lasts longer than other types but often is punctuated with intermissions.<br> The BSPA of type A corresponds to the convergence line due to the southerly and the northerly or westerly trend in the western part of the synoptic scale cyclone which coincides with the CPA (Fig. 7 type A-a). In this case, the meridional gradient of temperature around the BSPA is not recognized (Fig. 7 type A-b and Fig. 12 type A). The synoptic scale cyclone appears to develop according to the incursion of cold air from a higher latitude to the east of the Tibetan Plateau (Fig. 9).<br> The formation of the BSPA in the absence of a CPA (type B-I and type B-II) is closely related to the appearance of a cold air mass in the lower troposphere. The BSPA is formed where the humid air, in a state of convective instability from south of the BSPA, is lifted up by the cold air mass and releases its instability (Fig. 12 type B-I, type B-II ), Therefore, the BSPA of type B-I and type B-II is considered to correspond to the front with a comparatively large meridional temperature gradient.<br> The cold air mass in type B-I can be traced back to the eastern foot of the Tibetan Plateau (Fig. 10) and is accompanied by an anticyclone in the lower troposphere (Fig. 7 type B-I and Fig. 11 type B-I ). On the other hard, the cold air mass in type B-II is derived from the Yellow Sea or the Japan Sea as a cold northeasterly or easterly air current (Fig. 7 type B-Il and Fig. 11). This cold air current seems to be established by an anticyclone to the east of North China Plain (Fig. 8 type B-II). Furthermore, the equivalent potential temperature to the north of the BSPA is lower in type B-II than in type B-I (Fig. 14 type B-II ).