A Case Study of a Heavy Precipitation Event along the Baiu Front in Northern Kyushu, 23 July 1982: Nagasaki Heavy Rainfall

Abstract

A case study is made of an exceptionally heavy rainstorm that hit the northwestern coastal area of Kyushu, Japan, on 23 July 1982. The 5hr rainfall accumulation was as high as 412mm at the city of Nagasaki.The precipitation occurred along the warm front associated with a medium scale disturbance that developed along the Baiu front. The atmosphere in the pre-storm period was very moist through virtually the entire troposphere and its static stability was conditionally unstable with a lifted index of -1.5*, thus exhibiting characteristics more similar to the tropical atmosphere than the typical pre-storm environment over the midwestern United States in spring seasons.Prior to the development of heavy precipitation over the Nagasaki area, an intensive rainband was propagating with a speed of 40km hr-1 eastsoutheastward over northwestern Kyushu.Its structure is found to be different from the structure of typical tropical squall clusters in that the heaviest rainfalll occurred in the middle of the rainband without a well defined leading edge.A dramatic change occurred in the rainband when its southern tip reached the Nagasaki area; it stopped propagating and stayed there for the next 5hr. Its line structure changed to a blob structure at the meso-* scale (*100km).The timing of the precipitation development at Nagasaki coincided with the peak period of the incursion of very moist air associated with the southwesterly low-level jet. During the period of heavy precipitation, a new cloud cluster formed over the sea 300km west of Nagasaki. It kept developing and propagating eastward with a speed of 60km hr-1, and eventually merged with the rainstorm over Nagasaki.These observations strongly suggest that the Nagasaki rainstorm was trapped orographically.Several other heavy precipitation events are cited which also occurred in association with medium scale disturbances along the Baiu front and in which the precipitation developed over the coast areas upwind of the low-level jets.

Behavior of Cumulonimbus-Cloud Groups in a Slow-Moving Meso-Scale Cloud Cluster over the Ocean

“…One of characteristic phenomena associated with cloud clusters around Japan is heavy rainfall which continues for several hours in the meso-* scale region. In the analysis of a stationjtry cloud cluster, Ogura et al (1985) pointed out that convective echoes formed several tens of kilometers away from the western edge of an intensive convectiveecho system which caused heavy rainfall and that they moved eastward to merge into the system. Ninomiya et al (1981, 1988 a and b) and Akiyama (1984 a and b) studied another cloud cluster.…”

Structure and Behavior of Mesoscale Cloud Clusters Traveling over the Baiu-Frontal Zone

“…The discrepancies in wind speed and direction between Kamigoshima and Fukue was large at 2300 JST and 2400 JST 14 July, suggesting that a horizontal shear line with a convergence zone existed to the south of the echo groups. Mesoscale cloud clusters to the west of their accompanying shear line were pointed out by Ogura et al (1985) and Ninomiya et al (1988b). Cumulonimbus clouds in Cb group B2 and Cb group B3 were formed to the north of the shear line.…”

“…Movement of mesoscale cloud clusters seemed to be influenced by the behavior of convective clouds of three different scales. However, the dimension of convective cells in Ogura et al (1985) is close to that of the Cb groups in Iwasaki and Takeda (1989). It is difficult to distinguish between the behavior of convective cells and Cb groups on the basis of the results of different case studies.…”

“…Ogura et al (1985) showed that new convective cells (dimension -10-30km) formed successively to the west of the slow-moving mesoscale cloud cluster. Iwasaki and Takeda (1989) analyzed a slow-moving cloud cluster over the ocean, they found that new Cb groups formed to the west of pre-existing groups, while the old Cb groups disappeared to the east, each Cb group having a slow-moving stage.…”

Meso-β to -γ-Scale Wind Circulations Associated with Precipitating Clouds near Baiu Front Observed by the MU and Meteorological Radars

“…It is frequently accompanied by a meso-a-scale cyclone, of which distribution of active cloud clusters is inhomogeneous and this makes heavy rainfalls much localized. Movements and evolutions of cloud systems near the meso-a-scale disturbance were reported by many studies based on satellite data (e.g., Akiyama 1984;Ogura et al 1985;Ninomiya et al 1981Ninomiya et al ,1988aIwasaki and Takeda 1989), but continuous highresolution tracking of each system was impossible. By using C-band meteorological radar of the Japan Meteorological Agency (JMA), Ogura et al (1985) investigated a cloud cluster that changed from line to blob structure in front of a meso-a-scale disturbance.…”