A heavy rainfall band was observed over southern Kyushu on 23 June 2001. This heavy rainfall was not predicted by a nonhydrostatic model (NHM) that was operated twice a day to support the field experiment of X-BAIU-01. The reason for this failure was examined in comparison with X-BAIU-01 special observation and satellite data. The maritime wind convergence zone was retrieved over the sea southwest of Kyushu Island from NASA QuickSCAT data, before the heavy rainfall system formed. This wind convergence was also found in the initial field of NHM. The lower atmosphere observed by aerosondes was very humid, while that simulated by NHM was considerably drier. The existence of this low-level humid air was also supported by total precipitable water (TPW) data derived from Tropical Rainfall Measuring Mission Microwave Imager (TMI) and Defense Meteorological Satellite Program Special Sensor Microwave/Imager (SSM/I). These comparisons indicate that the fact that the lower atmosphere was drier than it is in reality, was the reason that the NHM failed to simulate the heavy rainfall. This indication was ascertained by humidifying the lower atmosphere of NHM to correspond to the observation (MOD), and applying the four-dimensional variational data-assimilation technique to TMI-derived TPW data (4DV). For MOD, the band-shaped rainfall was reproduced successfully. For 4DV, the simulated precipitation intensity was not strong enough to forecast a heavy rainfall, because 4DV could not retrieve the vertical profile of water vapor by using only the TMI-derived TPW data. Humid air concentrated in the lower atmosphere is necessary to induce heavy rainfall over the Baiu frontal zone.
