The purpose of this study is to clarify maintainance mechanisms of rainbands over the East China Sea during the Early Baiu Season. Structures and maintainance processes of a rainband, whose structure changed before and after passing Okinawa island on 25 May during a field experiment of CREST (Core Research for Evolutional Science and Technology)/LAPS (Lower Atmospheric and Precipitation Study) in 2003, were investigated with dual Doppler radar analysis and Japan Meteorological Agency Meso Regional Objective Analysis (MANAL) data.

The rainband extended from the southwest to the northeast with the length of 300 km. The stratiform region was ahead of the system movement with the width of 60 km and the convective region was in the northwest side of the system with the width of 30 km. The system was generated over the East China Sea 300 km to the northwest Okinawa island around 1930 JST 24 May 2003. The system moved southeastward with the speed of 4.7 m/s and passed Okinawa island from 0530 JST to 0630 JST on 25 May. After the movement of 150 km southeastward from Okinawa island, the rainband disappeared around 2000 JST 25 May. The atmosphere ahead of the system was all layers almost saturated below 400 hPa before the system passed Okinawa island. After the system passed Okinawa island, the line structure was not able to maintain only in the part of the convective region which passed Okinawa island, and scattered convective cells existed in the east side of Okinawa island. However, the convective region elongated from the southwest to the northeast again after five hours of the passeage.

Before passing Okinawa island, the south-southwesterly wind ahead of the system movement and the westerly wind behind the system movement converged below 2 km in altitude. The convergence line extended from the southwest to the northeast. The height of the reflectivity in the convective region core was always in the lower layer below 2 km in altitude. The echo top height was 9 km in the southwest side and was 6〜 7 km in the northeast side. The convective region was composed of convective cells. The deep convective cells whose maximum echo height of 18 dBZe was over 7 km and the shallow convective cells whose maximum echo height was under 5km extended from the southwest to the northeast with the length of 200〜300 km. They were generated along the convergence line and the maximum longevity was 100 minutes. The deep convective cells whose maximum echo height of 18 dBZe was over 7 km formed short convective line with length of 30 km extended from the northwest to the southeast. They were generated on the southeast side of deep convective cells which generated along the convergence line, and the maximum longevity was 30 minutes. In both of the deep and shallow convective cell which were one of the convective cells pararell to the system, the moist air from the southeast was lifted along the convergence line and the inflow from the southeast continued for a long time below 1 km in altitude. In the deep convective cell, the downdraft was formed on the southeast side of the core of the deep convective cell under the echo top height (7 km) and it did not reach the lowest layer. In the shallow convective cell, the downdraft was formed on the northeast side of the core of the shallow convective cell below 3 km in altitude. In the deep convective cell which were parpendicular to the system, the moist air from the southeast converged with the outflow from the deep convective cell generated along the convergence line at the altitude of 2〜4 km and the moist air from the southeast were lifted. Soon after the convective cell which was parpendicular to the system was generated, the downdraft nearby the core of the reflectivity reached the bottom of the layer and the convective cell declined faster than the convective cells which were paralell to the system.

After passing Okinawa island, the convergence line of the lower layer disappeared and the south-southwesterly wind was superior from 1 km to 3 km in altitude. In the part of the convective region which passed Okinawa island, scattered convective cells remained and the shallow convective cells were not generated. The height of the reflectivity core and the echo top height rose 1〜2 km in the convective region. The life time of the convective cells was less than 60 miniutes. In one of the convective cells, the southeastely wind converged with westerly wind at the altitude of 3〜4 km. The moist air from the southeast was lifted at 3 km in altitude. After the convective cell was generated, the downdraft from the convective cell was formed below 4 km in altitude and reached the bottom of the layer and was diverged at the lowest layer.

By the process of the structual change in the convective region before and after the passage of Okinawa island, it is found that the inflow from the southeast of the convective cells paralell to the system was maintained for a long time before it passed Okinawa island. The reason is that the downdrafts which flew out from the convective cells were formed at the position which it did not cut the inflow of the moist air from the southeast. The echo top in the convective region rose after it passed Okinawa island. It is understood that the convective region was in the developing stage. However, the part of the convective region which passed Okinawa island was not able to maintain a liner structure by accompanying the disappearance of the convergence line of the lower layer, where the convective cells were not generated along the convergence line. It can be considered that the disappearance of the convergence line was caused by the cut off of the westerly wind behind the system motion below 1 km in altitude. It is considered that the maintenance mechanisim of the convective reigion is that the convective cells generated along the convergence line of the lower layer extended from the southwest to the northeast and the inflow of the moist air from the southeast continued for a long time. Especially, shallow convective cells existed a lot on the northeast side of the convective region before it passed Okinawa island and they were not genarated after passing Okinawa island. It can be said that shallow convective cells are necessary to compose the convective region of the rainband. During the Early Baiu Season, the rainband which the stratiform region was ahead of the system movement and the convective region was behind of the system movement is a feature of rainbands generated over the East China Sea. It can be considered that the inflow of the moist air ahead of the system movement to the convective region always continued because the downdraft that flew out from the convective region and the stratiform region doesn't reach the bottom of the layer, then the system was maintained. In this research, it is clarified that the rainband generated over the East China Sea during the Early Baiu Season was self-maintaining system keeping the continuous supply of the moist air to the system.