SEASONAL CHANGES OF WATER VAPOR TRANSFER PATTERN AND WATER VAPOR DISTRIBUTION DURING BAI-U SEASON IN EAST ASIA

Seasonal changes of water vapor distribution have been studied in this investigation in terms of changes of water vapor transfer pattern at the level of 850 mb and pressure system during Bai-u season (June and July) in East Asia (Fig. 1). A data set from 51 observational stations during the years 1977 to 1981 were used in this study. This data set was converted into 3°×3° mesh point data.<br> In this paper, water vapor amount and water vapor transfer are defined by mixing ratio [g. kg^-1] and water vapor amount passing through 1 [m2] area per unit second [g. m^-2• s^-1], respectively.<br> A water vapor transfer field is distinguishable by the location of its main axis. Distri-bution of water vapor transfer amount averaged for the whole period is shown in Fig. 2-a. From the southern part of continental China to the southern part of Japan, the value in-dicates more than 70_??_80 [g•m^-2.s^-1]. As a result, the boundary amount was set at 80 [g•m^-2•s^-1] for recognition of the main axis in each water vapor transfer field. The occurrence frequency of eastward, northward and westward water vapor transfer exceeding 80 [g•m^-2-s^-1] in amount are presented in Fig. 2-b_??_d. Regarding eastward transfer, the high frequency zone lies along 30°N latitude. Separate high frequency regions of north-ward transfer are located in the western part of the South China Plain (A), around the Ryukyu Islands (B) and to the south of Japan (C). To the east of Ryukyu Islands (D), a high frequency zone of westward transfer appears in the last part of July. From the facts mentioned above, classification of water vapor transfer fields was for-mulated by location of northward axis and presence of the westward axis in the range of 25°_??_30°N (Fig. 3-a). The main axis was defined as a ridge line of transfer amount exceeding 80 [g•m^-2•s^-1] along the direction of transfer. Water vapor transfer fields could be classified into five types (Fig. 3-type A_??_D).<br> Each pattern can be distinguished seasonally from the. others except the type Bc (Fig. 4-a). Consequently, the seasonal migration of the main axis is illustrated schematically as Fig. 4-b.<br> Changes of the water vapor transfer pattern are followed by seasonal northward migration and the vanishing of the frontal zone (Fig. 5) which is related to the seasonal change of the general circulation. Seasonal changes of the water vapor transfer pattern, however, are unexplainable from the location of frontal zone.<br> The main axis is located at the longitude where the zonal gradient of height is greatest (Fig. 6). Westward migration of the main axis (type C→type Bj→type A) can be explained in terms of the movement of the maximum pressure gradient zone on the western periphery of the Subtropical High as pressure rises around the South China Sea or Ryukyu Islands propagation of the humid area in the North China Plain and the westward retreat in the South China Sea. It also corresponds to the change of water vapor distribution from type Bj to type A. In spite of the decreasing water vapor amount in the west part of Japan, the contour line of 11 [g•kg^-1] shifts slightly northward.<br> type D: Water vapor amount decreases in the eastern part of the Chinese continent and the south of Japan. These decreases correspond to the retreat of the contour line (13 [g•kg^-1]) on the Chinese continent and the appearance of a comparatively dry area in the south of Japan.<br> In general, seasonal changes of water vapor distribution are synchronous with the appear-ance of water vapor transfer patterns.