Migration of a Cold Air Mass Related to Rain Belt Formation on the Chinese Continent and Atmospheric Circulation Systems during the Baiu Season

Relationships between the behavior of a cold air mass having a triggering effect on the formation of the synoptic scale rain belt on the Chinese continent (Takahashi, 1989) and atmospheric circu-lation systems during the Baiu season(June and July) havd been investigated. In the first step, sev-enteen cases in which rain belt formation occurred in the presence of a cold air mass (Takahashi, 1989) were examined and classified in terms of the migration path of the cold air mass. Moreover, atmospheric circulation systems related to the behavior of the cold air mass were examined to get a synthetic comprehension of the Baiu phenomena.<br> Data sets used in this study are atmospheric data at 12 GMT from 131 stations (Fig. 1) and precipitation records in East Asia during the years 1977 to 1981. Furthermore, daily synoptic charts published in Japan and China, and 5-day mean 500 mb level height data (10° × 10° grid) were used in analysis of temporal changes in the atmospheric circulation.<br> Figures 2, 3, and 4 give examples of inter-diurnal changes in the 850 mb level temperature distri-bution, which are shown as anomaly distributions from a 10-day mean field. Migration of the remarkable minus anomaly area which corresponds to a cold air mass can be traced back in thir-teen cases, and classification is conducted according to the path (type E and type W of Fig. 5). In the case of type E (Fig. 6-a and Fig. 7-type E), a cold air mass migrates southward between the quasi-stationary ridge (Baiu ridge; _??_) in 90° E-100° E and trough (Baiu trough; _??_) in 120° E-130° E at the 500 mb level. For type W, two circulation patterns related to the southeastward migration of a cold air mass are recognized. One corresponds to the traveling trough (type WT; Fig. 6-c); the other is related to the Baiu ridge in 50° E-60° E and the Baiu trough in 100° E-110° E (type WR; Fig. 6-b and Fig. 7-type WR). Type W (type WR and WT) and type E coincide with the precipitation distribution pattern type B-I (in which a rain belt is formed to the north of 30° N) and type B-II (rain belt is formed to the south of 30° N) classified by Takahashi (1989).<br> Examples of temporal changes in the synoptic field at the 850 mb level on type WR and type E, which are associated with the Baiu ridge and Baiu trough, are shown in. Fig. 8 and Fig. 9, respec-tively. From these illustrations, the mechanisms described below are found to be essential for the migration of a cold air mass in the lower troposphere. A paired anticyclone and depression in the lower troposphere corresponding to the Baiu ridge and Baiu trough builds up a wind system which drives cold air from a higher latitude to a middle latitude, and the southward or southeastward extension of the anticyclone causes further southward migration of the cold air mass. The differ-ence in the migration path to the north of 40° N between type WR and type E is due to the locality of the Baiu ridge and Baiu trough.<br> and July of 1979 (Fig. 17). With the passage of the 500 mb level trough, the 850 mb level height ascends and temperature descends (_??_) in the northern area of the Tibetan Plateau. The high pres-sure area of type WT consists of cold air overall, but in the case of type WR, air mass in high pressure area originates from the warm anticyclone corresponding to the quasi-stationary Baiu ridge.<br> Three circulation types are recognized concerning the migration of the cold air mass that has a triggering effect on rain belt formation. One is attributed to the traveling upper trough north of the Tibetan Plateau, whose cold air mass is accumulated in the lower troposphere by the wall effect of the plateau on the northerly cold wind. Other types are related to the quasi-stationary ridge and trough. In these cases, a cold air mass migrates southward or southeastward between a lower anticyclone and a depression corresponding to the upper ridge and trough.