Baiu Frontal Zone Research Articles

広域からみた梅雨前線帯の特徴

The large-scale aspects of the Baiu Front, heavy rainfall and associated phenomena are analyzed over the Far East during the 40day period from June 16 to July 25,1968, mainly by using 5-day mean fields of various meteorological elements.The Baiu Front over the Far East forms a belt-shaped zone of maximum cloudiness, which lies east and west along the northern boundary of the monsoon or tropical airmass. This cloud zone coincides well with the zone of maximum rainfall amount. Heavy rainfalls there indicate successive convective activities. Owing to the convective transport of moisture, sensible heat and release of latent heat, a midtropospheric warm belt and a deep moist layer are formed within the Baiu Front.The most notable feature of the mean Baiu Front is the appearance of the core of the super-geostrophic strong wind in the lower troposphere during the heavy rainfall periods. It is suggested that the low-level jet is intensified by the vertical mixing of horizontal momentum due to active convections there.Medium-scale disturbances develo p successively in the Baiu frontal zone which provides favorable conditions for their generation as inferred from the small Richardson Number.The results of water-vapor budget in the vicinity of the Baiu Front reveal that a large amount of water-vapor convergence within the frontal zone is mainly by northeastward flow along the northwestern rim of the Northern Pacific Subtropical anticyclone. It is also concluded that the evaporation from the western Pacific is one of the important vapor sources which maintain intense rainfall within the Baiu Front.

“Moist Tongue” over Japan in the Baiu Season (Preliminary Report)

At the time of heavy rainfall, very humid air named “moist tongue” is found in the upper air levels over Japan. This air, extending as a tongue from a moist air to the west or to the southwest, has been repeatedly observed during the rainy periods. In this paper, the moist air area around Japan was analyzed statistically from the vie wpoint of dew-point distribution during the baiu season (June and July). The observed frequency of dew-points above 6°C., 8°C., 10°C., and 12°C. indicates that between the 1st and the 10th of June in the average of three years, 1960-1962 (Figs. 1-4) the axis of the moist area lies in South China and extends to the east; and that between the 1st and the 10th of July (Figs. 5-8) the axis lies in central China and extends to the east. Each group of figures for June and July shows that the core of the moist area exists approximately in SW China with its axis moves toward the north in the early summer when the baiu frontal zone moves toward the north.The observed frequency of high dew-points is shown in Table. Analyzing each observation, the authors conclude that the “moist tongue” appearing over Japan emanates, in many cases, from the above mentioned moist air area.