Several regions in western Japan experienced a record-breaking early onset of the rainy season called Baiu in mid-May 2021, which is attributed to the northward movement and enhancement of the Baiu frontal zone. This study investigates large-scale atmospheric circulation that contributes to the early onset of Baiu. Diagnostic and statistical analyses based on reanalysis datasets reveal that both enhanced convection over the western Indian Ocean associated with the Madden–Julian oscillation and a blocking high near western Russia promote the excitation of Rossby waves to propagate downstream along the upper-tropospheric jet, and thus contributing to the northward movement of the Baiu frontal zone. The anomalous convection over the western Indian Ocean and the subtropical western North Pacific also may affect anticyclonic circulation anomalies to the northeast of the Philippines in the lower troposphere, which promotes moisture inflow toward western Japan and consequently intensifies the Baiu frontal zone. Numerical and quantitative analyses of the circulation anomalies near Japan based on a linear baroclinic model confirm the aforementioned results. The results indicate that the anomalous convection over the Asian monsoon region and the blocking high near western Russia are the primary factors contributing to the early onset of Baiu.

We statistically investigate characteristics of “senjo-kousuitai”, quasi-stationary linear precipitation systems, in East Asia using high-resolution satellite precipitation and reanalysis data to understand whether these events are common there. We define an elongated precipitation system in the satellite precipitation data as a senjo-kousuitai event.

Our results show that the contribution of senjo-kousuitai to heavy rainfall is high in western Japan, especially in Kyushu, the Nansei Islands, and the East China Sea. Among the environmental factors favorable for the occurrence of senjo-kousuitai, low-level water vapor flux and vertical wind shear are essential to the development of senjo-kousuitai. As a typical case, we examine large-scale circulations associated with senjo-kousuitai events in Kyushu in the Baiu season (June to July), and found that they are generally characterized by the intensified Pacific High over the south of Kyushu and pressure trough to the north of Kyushu. This circulation pattern results in a stronger pressure gradient and higher low-level wind speeds over Kyushu. With respect to the previously noted importance of water vapor and wind speed for better prediction of senjo-kousuitai, we show that not water vapor but higher wind speeds are the main factor for the enhancement of low-level water vapor flux.

This study investigated the predictability and causes of the heavy rainfall event that brought severe disasters in Kyushu in July 2020 with a global numerical weather prediction system composed of the NICAM (non-hydrostatic icosahedral atmospheric model) and the LETKF (local ensemble transform Kalman filter). We performed ensemble data assimilation and forecast experiments using the NICAM-LETKF system with 1,024 members and 56-km horizontal resolution on the supercomputer Fugaku. The results showed that 1,024-member ensemble forecasts captured the probability of heavy rainfall in Kyushu about five days before it happens, although a 10-day-lead forecast is difficult. Ensemble-based lag-correlation analyses with the 1024-member ensemble showed very small sampling errors in the correlation patterns and showed that the moist air inflow in the lower troposphere associated with a low-pressure anomaly over the Baiu front was related to this heavy rainfall in Kyushu.

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 numerical 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 southward 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 development 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 distribution shifted southward and resulted in a change in precipitation amounts in southern Kyushu because of the weakening of the BFD.