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The large-scale

Detailed features of heavy rainfalls in three medium-scale disturbances developed in Baiu front over the southwestern Japan Islands have been studied basing on time series of 10-min rainfall amounts and 3-radar PPI composite photographs.For rainfall analysis, the rainfalls were classified into two categories, continuous and convective rains, according to intensity fluctuation and intensity threshold (2 mm/ 10 min).Results revealed that the wave-shaped echo system associated with one medium-scale disturbance consists of thin weak widespread echo in the front and in the north of the disturbance and of convective echo cells distributed around the crest or the trailing portion of the wave. The convective echo cells are mostly organized into mesoscale echo clusters arranged with 100*150 km spacing.Results also revealed that the wave-shaped medium-scale rainfall system consists of two specified areas. The one characterized by continuous rain spreads to north and east of the wave crest, corresponding to thin weak widespread echo area. The other characterized by convective rain is the heavy rainfall zone in the crest or the trailing portion of the wave, corresponding to the areas of convective echoes organized into mesoscale cluster.Applying spectral analysis method to time series of 10-min rainfall amounts, it is confirmed objectively that the rainfalls in the heavy rainfall zone are due to mesoscale pulsations of convective rain.The stratification over the convective rain area is evidently different from that over the continuous rain area, i.e., the lower layer is unstable in the convective rain area (southern convective echo area), while stable in the continuous rain area (northern thin weak widespread echo area).

Detailed analysis on the field of horizontal moisture flux and moisture budget is made in the vicinity of the Japan Islands for the 16 days period between 29 June and 14 July 1972, which includes the period of extremely heavy rainfalls.The results of the analysis indicates that the strong precipitation is primarily related to the increase of the horizontal moisture convergence in the lower layer below 700 mb, while the role of the moisture convergence in the upper layer above 700mb is insignificant.The areal mean amounts of the observed precipitations, which are accurately estimated from the data of about 2000 rain-gauge stations, are well agreed with the amounts of the calculated precipitations (apparent moisture sink).By comparing the situations of moisture budgets for subdivided periods (i.e., light, heavy and extremely heavy rainfall periods), we are able to show that the situations in the extremely heavy rainfall period is remarkably different from those in remainders. During the extremely heavy rainfall period, the low-level moisture convergence mentioned above is mainly caused by the southerly transversal moisture influx across the lateral boundary of the cloud zone rather than the longitudinal flux within the cloud zone.It is also concluded that the moisture flux from the subtropical Pacific, i.e., from the outside of the cloud zone, into the cloud zone is primarily important to the occurrence of heavy and extremely heavy rainfalls.

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