Influence of an Upper-level Trough on the Formation of a Baiu Frontal Depression that Caused a Torrential Rainfall Event in Southern Kyushu, Japan on July 4, 2020

The influence of an upper-level trough on a Baiu frontal depression (BFD) that caused a heavy rainfall event in southern Kyushu, Japan, on July 4, 2020, was examined using numerical simulations with and without the upper-level trough. The numer-ical simulation with the upper-level trough (CNTL) reproduced a reasonable well-developed BFD and heavy rainfall in southern Kyushu. Conversely, the numerical simulation without the upper-level trough (NOUT) produced a weaker BFD and notable south-ward rainfall shift compared with the situation in the CNTL. The weaker BFD for the NOUT was due to weaker convection than that of the CNTL over mainland China. Thus, strong convection over mainland China was essential for the formation and develop-ment of the BFD that caused heavy rainfall in southern Kyushu. Additional sensitivity experiments, in which the strength of the upper-level potential vorticity anomalies was reduced to 75, 50, and 25% of the CNTL, showed that the spatial rainfall distribu-tion shifted southward and resulted in a change in precipitation amounts in southern Kyushu because of the weakening of the BFD.