A band-shaped precipitation system, associated with a cold front, caused heavy rainfall over northern Kyushu, Japan, on 29 June 1999. This precipitation system had a hierarchical structure with different horizontal scales; the precipitation system consisted of several mesoscale convective systems (MCSs), and each MCS consisted of a few convective cells that formed successively on its upstream side. Each of them had different spatial and time scales, as well as different traveling speeds and directions.
Low-level humid air from the southwest and middle-level dry air from the west continuously flowed into the precipitation system. This dry air was not colder than the surrounding atmosphere. The inflow of low-level humid air initiated the MCSs, and that of middle-level dry air enhanced, and maintained, the convective instability over the cold front. The heavy rainfall was brought under the maintenance condition of convective instability.
The cloud top heights of convective cells were found differently between the western and central parts of the precipitation system. The factors to determine them were examined using the successfu1 simulation results of a nonhydrostatic cloud-resolving model, with the horizontal resolution of 2 km. The intrusion of middle-level dry air into convective cells was considerably larger in the western part than in the central part. This caused the difference in the cloud top heights of the convective cells; the convective cells were less than 7 km high in the western part, whereas, in the central part, most were between 5 and 7 km high and at the height of the tropopause. In the western part, most of the convective cells completely lost their buoyancy because of the significant evaporative cooling of hydrometeors. These results show the two different effects of middle-level dry air; a huge amount inflow suppresses the development of convective cells, while it strongly enhances the convective instability.