In order to reduce the damage caused by the heavy rainfall, it is important to predict the development of cumulonimbus clouds from the stages of their formation and development. It is generally believed that cumulonimbus clouds form when the updrafts generated in the lower layers exceed the condensation altitude. However, previous studies have observed cases in which several small cumulonimbus clouds gather and merge to form a cumulonimbus cloud.

This case has not been reproduced in a model, and the factors of the merge of the cumulus clouds and the development of convective clouds have not been clarified. Since this case was observed in summer, we conducted an idealized experiment focusing on thermals, which are air masses that are supplied with heat and water vapor from the ground surface and have higher temperatures and higher water vapor mixing ratios than their surroundings gain buoyancy and rise, to clarify this process.

In this study, an idealization experiment was conducted by setting up the cloud-resolving model CReSS with an assumed ground surface in the urban area based on LANAL (Local Objective Analysis) data near the location where the previous study was conducted as the initial condition. The integration time was 12 hours, from 5 a.m. on August 6, 2018 to 5 p.m. on August 6, 2018, with output at 30-second intervals.

The analysis of the experiment yielded several insights into the merge of cumulus clouds. In the case of the horizontal wind analysis, there were two upwelling thermals with higher vapor mixing ratios and higher virtual potential temperatures than the surrounding area. They had horizontal wind components that approached each other, so the thermals came close to each other but did not merge. The analysis of the vertical flow also showed that the cumulus clouds merged with each other. The thermal rose and generated a cumulus cloud, and the two cumulus clouds merged into one as if they were sewn together by the compensating downdraft that was generated in conjunction with the thermal.

Based on the results of this study, two mechanisms can be considered for the phenomenon of the merge of the cumulus clouds and the development of convective clouds. First, the occurrence of a thermal may change the surrounding horizontal wind field, causing the two thermals to approach each other by chance. Another possibility is that there is a weak area of compensating downdraft between two thermals or cumulus clouds, and cumulus clouds develop and merge through that weak area.