Southerly Transversal Moisture Flux into the Extremely Heavy Rainfall Zone in the Baiu Season

Abstract

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.