Cold pools and mesoscale organization of 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 affecting the boundary layer energy balance.

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 (RICO) campaign. The LES employ extensive horizontal domains, up to 330 x 330 km in the horizontal directions, and fine resolution (40 m).

The cloud structure transitions from uniformly scattered cumulus to organized cloud clusters when sufficient precipitation develops. 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 the 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 than changes to the individual cumulus-topped convective elements.