The Role of Large-Scale Vertical Motion in the Sugar-To-Flowers Trade Cumulus Transition

Trade-wind shallow cumuli occur over the subtropical oceans in regions with weak subsidence. Recent studies have categorized the mesoscale organization of these clouds into four states based on their spatial structure: Sugar, Gravel, Fish, and Flowers. A joint field campaign ATOMIC and EUREC4A conducted rigorous observations of these clouds and their relationships with large-scale meteorology. During the field campaign, it was observed that the Sugar clouds, which are very small and shallow, organize and transition into wider and deeper clusters of Flower clouds. This study further explores how the Sugar-to-Flowers transition responds to changes in large-scale vertical velocity. We use a Lagrangian large eddy simulation to represent the Sugar-to-Flowers transition. The environmental conditions are obtained from the ERA5 reanalysis. For the select case, large-scale vertical motions become positive during the transition period. We run additional simulations with altered large-scale vertical velocity, and analyze the effect on the transition. We perform a spectral analysis of the buoyant turbulence kinetic energy and the total water mixing ratio, and compare the spectral changes between simulations with stronger and weaker upward motions. The enhanced upward motions not only deepen the cloud layer, but also strengthen the buoyant turbulence kinetic energy at the mesoscale wavelengths. This reinforces mesoscale circulations, which redistribute the total water field, and helps accelerate the transition from Sugar to Flowers.