Convective clouds are a basic element of precipitation phenomena in a humid environment. The structure and mechanism of clouds are important problems because they determine the morphology and the time variation of the convective systems. Previous studies focused on the effect of evaporative cooling which accelerates the downdraft, because the low-level outflow from the downdraft plays an important role to maintain convective clouds. In a highly humid environment, however, the evapolative cooling are ineffective. There have been few studies about convective cloud in such a highly humid environment. The present paper studies the structure and mechanisum of convective clouds and their clusters using Doppler radar observation in Miyakojima region during the Baiu season and three dimentional cloud-resolving model. Three important results were obtained in this study.

First, the environment around Miyakojima islands during the Baiu season were humid and a highly humid layer was present. Convective clouds in this region had charcteristics of oceanic convective clouds when the Baiu front was located to the north of Miyakojima islands.

Second, the convective system which formed in the highly humid environment on 6 June 2003 was studied in detail. This convective system had a multiscale structure which was consisted of three elements: convective cells, cell groups and cell-group lines. Some cell groups formed in lines when cell groups developed at the same time. Cell groups consisted of two types of cells: long-lasting type and shrot-lived type. Cell groups consisted of the former type included only two long-lasting convective cells. On the other hand, the latter type were often observed during event. The long-lasting convective cells didn't form low-level outflow but the convective cells maintained long time by re-development. It is considered that the cells re-developed by intensification of upward acceleration which resulted from falling a large amount of precipitation loading and condensation heating by entrainment of almost saturated air.

Third, this paper examined the mechanisum of the re-development using the cloud-resolving model. Moreover, some sensitivity tests to examine the humidity of the environment. If the humidity was more low, the convective cell did not re-develop because the condensation heating was suppressed. Thus, it was revealed that the convective cells had lasted for long time by re-development due to unloading in the highly humid environment.

This paper clarified the existence of long-lasting convective cells in the highly humid environment and their mechanism during the Baiu season.