Decaying Active Clouds Produce the Scale Break in the Power‐Law Size Distribution of Cumulus Clouds

Abstract Cumulus clouds exhibit a wide range of horizontal scales, spanning from tens of meters to several kilometers. While their horizontal size distribution (HSD) generally follows a power‐law, it consistently displays a distinct scale break. Such an HSD pattern of cumulus clouds has been extensively reported, but the underlying mechanism is so far unclear. This study explores it by analyzing cumulus cloud samples generated by the large eddy simulations. By taking into account the evolution processes of individual clouds as well as their precursor bubbles, that is, a compact entity of updrafts originating from the surface layer, we identified a critical linkage between cloud lifecycle and the emergence of scale break. For the two categories of cumulus clouds (active and forced) that are divided into two life stages (growing and decaying), the scale break was found to appear only in decaying active ones. The HSD of decaying forced clouds and that of growing ones exactly satisfy a power‐law, consistent with their precursor bubbles. The initial HSD of young bubbles undergoes modification through the temporal size evolution of clouds near their maximum horizontal cross‐sectional area layer (maximum area layer), thus shaping the HSD of cumulus clouds. During the decaying stage, active clouds experiences non‐monotonic variation of the averaged vertical velocity, slowing their size down as they traverse the level of neutral buoyancy. The medium‐size snapshot cloud samples increasing in this way causes a scale break in the statistics, suggesting that the scale break is essentially an emergent property of cumulus cloud dynamics.