広域からみた梅雨前線帯の特徴

Abstract

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.