The Sugar-to-Flower Trade Cumulus Transition in a Future Climate

Subtropical trade-wind cumulus clouds have been found to occur in four patterns of mesoscale organisation, which have been labeled Sugar, Gravel, Fish and Flowers. Sugar consists of small, low clouds with low albedo, Gravel clouds form along apparent gust fronts, Fish exhibits skeletal, often fishbone-like features, while Flowers are large circular aggregates surrounded by nearly cloud-free areas. The Sugar cloud state can evolve into the Flower cloud state. We present a study of the Sugar-to-Flower trade-wind cumulus transition at the end of the 21st century and compare it to the transition as it occurs in present-day conditions. Present-day conditions are represented by the ERA5 meteorology in the area studied by the ATOMIC and EUREC4A field campaigns, the Caribbean north-east of Barbados, in early February 2020, when a Sugar-to-Flower transition was observed. Future conditions are constructed from the present-day ERA5 meteorology by applying to it the large scale environmental change over the course of the 21st century in the region, as simulated by select models in the Coupled Model Intercomparison Project Phase 5 (CMIP5). The present-day and future conditions are used to drive Lagrangian large eddy simulations of the Sugar-to-Flower transition. We characterize and quantify the response of the simulated Sugar and Flower cloud states, their transition, and their cloud radiative effect to climate change in select Representative Concentration Pathway (RCP) scenarios, and discuss the results in the context of current understanding of the trade-wind cumulus response to large scale environmental change.