Cloud size distributions of shallow cumulus cloud populations are calculated using the Large-Eddy Simulation (LES) approach. A range of different cases is simulated, and the results are compared to observations of real cloud populations. Accordingly the same algorithm is applied as in observational studies using high-altitude photography or remote sensing.

The cloud size density of the simulated cloud populations is described well by a power-law at the smaller sizes. This scaling covers roughly one decade of sizes, with a power-law exponent of -1.70 which is comparable to observations. A scale-break is present at the top end of this region. When the cloud size is non-dimensionalized by the scale-break size, the cloud size densities of all cases collapse over all sizes. This corroborates the idea of a universal description of the whole cloud size density, with the scale-break size as the only variable. The intermediate dominating size in the cloud fraction and mass flux decompositions is directly related to the presence of the scale-break in the number density. Despite their large number, the smallest clouds contribute very little to the total vertical mass transport. The intermediate size of the dominating clouds is insensitive to the resolution of LES.

The exact definition and complete control over all conditions in LES has several important advantages: the possibility of reproducing obtained results for similar settings, and therefore the possibility to carry out systematic impact studies of key parameters in the system. On top of this LES offers almost unparalleled statistical possibilities.