Large-scale organization development in precipitating shallow cumulus convection

Clouds forming in the atmospheric boundary layer, typically the lowermost 4 km of the atmosphere, have a large impact on the Earth's energy balance and are one of the largest sources of uncertainty in climate projections. The development of precipitation in shallow cumulus clouds changes the spatial structure of convection and creates large-scale organization. The development of convective organization is studied using large-eddy simulations (LES) of the trade-wind boundary layer observed during the Rain In Cumulus over the Ocean campaign. The LES employ extensive horizontal domains, up to 160 x 160 km in the horizontal directions, and fine resolution (40 m). The development of large-scale organization develops rapidly and has a large impact on the turbulent-flow statistics. The structure of the flow organization and turbulence strongly depend on the LES domain size. In contrast, mean profiles do not depend on computational domain size. It is shown that large-scale organization primarily affects the horizontal fluctuations in the flow through the creation of local cloud-system circulations rather that changes to the individual cumulus-topped convective elements.