Flower-Type Organized Trade-Wind Cumulus: A Multi-Day Lagrangian Large Eddy Simulation Intercomparison Study

Abstract

Shallow cumulus cloud fields in subtropical marine trade wind environments, particularly over the tropical Atlantic Ocean, show distinct organizational patterns. Among these, Flower-type clouds are characterized by expansive stratiform cloud patches surrounded by regions of scattered convection. The objectives of this study were (a) to construct a case study of a time period during the ${\text{EUREC}}^4$A/ATOMIC field campaign when Flower-type organization was observed, (b) to evaluate the fidelity of a multi-model ensemble of large eddy simulations of that case, and (c) to analyze the interaction between cloud and precipitation processes and mesoscale organization in the simulations. The simulations follow a quasi-Lagrangian trajectory, allowing mesoscale features to develop over time in a domain that follows the boundary-layer airmass. The results show a broad agreement in simulated thermodynamic properties across different LES codes, with Flower-type cloud patches appearing within hours of each other. The consensus among models is consistent with observations made during the ${\text{EUREC}}^4$A/ATOMIC field campaign on the specific day of interest. The cloud structure reveals three distinct peaks in the joint probability densities of cloud base and cloud top height, with the dominant peak at any given time influenced by the stage of cloud organization. The simulated cloud system evolution reveals consistent occurrence of maxima in liquid water path and rain rate before Flower reaches its maximum length scale. Targeted sensitivity tests reveal a weak relationship between Cloud Droplet Number concentration and the extent/degree/type of organization.