A grid refinement study of trade wind cumuli simulated by a Lagrangian cloud microphysical model: the super‐droplet method

The impact of spatial resolution on the simulation of trade wind cumuli was investigated. The super‐droplet method, an efficient stochastic Lagrangian cloud microphysical model, was used to reduce uncertainties due to the empirical parameterisation of cloud microphysics and numerical diffusion for advection, which is inevitable in an Eulerian cloud microphysical model. We showed for the first time that cloud cover numerically converged with a grid resolution of 12.5 m. Our grid refinement analysis elucidated a significant contribution of small cumulus clouds to total cloud cover, as such cumuli are generated by small‐scale updrafts that can be resolved only at a fine resolution. Temporal evolution of (a) cloud cover and (b) the liquid water path (LWP) averaged over the whole calculation domain and (c) the LWP averaged over the whole cloudy grid. The red, green, pink, sky blue, and black lines indicate the results with Δx = 6.25, 12.5, 25, 50 and 100 m, respectively. The grey shade and blue line indicate the range between the maximum and minimum, and the median of the results of the previous intercomparison study conducted by the global energy and water cycle exchanges (GEWEX) cloud system studies (GCSS).