Predictability of a Mesoscale Convective System That Caused Heavy Precipitation in Northern Kyushu, Japan, on 9 July 2023, as Revealed by the Assimilation of Himawari-9 Observations

Mesoscale convective systems (MCSs) frequently produce heavy rainfall and trigger disasters, yet accurately forecasting them remains a challenge. This study aims to provide valuable insights into predictability of an MCS that produced record-breaking rainfall over northern Kyushu, Japan, on July 9, 2023. The assimilation of all-sky infrared radiances from Himawari-9 clearly improves the initial conditions, resulting in an accurate ensemble forecast. Ensemble-based correlations reveal close relationships between the precipitation amount in northern Kyushu and low-level water vapor flux, horizontal convergence, and mid-tropospheric humidity. A comparison of better and worse ensemble members in terms of the precipitation amount indicates that the better members exhibit a moist absolutely unstable layer, which may promote the development of the MCS. Moreover, immediately before the onset of the heavy precipitation, the better ensemble members exhibit stronger low-level water vapor flux than the worse members. Notably, such differences are unclear several hours prior to the onset, indicating limited predictability of the target MCS due to rapid forecast error growth.