Growth rates of turbulence length scales in precipitating shallow cumulus convection

Large-scale organization development in precipitating trade-wind shallow cumulus boundary layers is an important process that affects the boundary layer energy balance, has the potential to strongly modulate the convecting environment, and is 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. This development of convective organization is studied using large-eddy simulations of the boundary later observed during the Rain In Cumulus over the Ocean campaign. The LES employ extensive horizontal domains, up to 320 × 320 km in the horizontal directions, and fine resolution (40m). The cloud structure transitions from uniformly scattered cumulus to organized cloud clusters when sufficient precipitation develops. The structure of the flow organization and turbulence strongly depend on the LES domain size. In contrast, the mean profiles do not depend on the computation 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 than changes to the individual cumulus-topped convection elements.