The appearance frequency of Cb-clusters and its diurnal variation were examined in relation to large-scale conditions based on the observational data from June to August 1979. The analysis was applied to the three stages in the seasonal march in that year, i.e., ''Pre-Meiyu'' (1-17 June), ''Meiyu'' (20 June-22 July) and ''Mid-summer'' (23 July-17 August). The main results are as follows. (1) The Cb-clusters appeared frequently not only in the Baiu frontal zone but also in North to Northeast China (Area N1) in ''Meiyu'', and in Central China (Area C2) in ''Mid-summer''. A remarkable diurnal variation of appearance frequency of the Cb-clusters with an evening maximum was observed in Area N1 in ''Meiyu'' and Area C2 in ''Mid-summer''. The daily maximum of ground surface temperature in these two cases was very high. (2) In Area N1 in ''Meiyu'', the Cb-clusters with diurnal variation were mainly embedded in large-scale cloud systems corresponding to slow-moving upper-level lows or troughs. The specific humidity in the lower layer increased from ''Pre-Meiyu'' to ''Meiyu''. These two factors, as well as the strong surface heating, would provide a favorable condition for the frequent appearance of Cb-clusters due to the increase of convective instability. The large-scale horizontal convergence in the lower layer was stronger at 12 UTC (20 BST) than at 00 UTC (08 BST), which would lead to the development of Cb-clusters in the evening. (3) Area C2 was covered by the subtropical high, and the low-level divergence was generally stronger at 12 UTC than at 00 UTC. In spite of these conditions, the isolated Cb-clusters developed very frequently in the evening. The very strong surface heating in the daytime could contribute to release of the strong convective instability by generating meso-scale upward motion as a trigger.
