The Dissipation of Shallow Cumulus Clouds: Dynamics, Microphysics, and Environmental Signatures

Abstract. The dissipation stage of shallow cumulus clouds remains less understood than their growth and mature stages, despite occupying a large fraction of the cloud lifetime. Here, we investigate the dissipation of isolated shallow cumulus clouds using high-resolution simulations across a range of initial cloud condensation nuclei (CCN) concentrations. We show that, while the growth stage is characterized by relatively smooth and monotonic evolution of bulk cloud properties, dissipation is marked by an overall decline accompanied by pronounced oscillations.

Dynamically, dissipation is characterized by weakening of the coherent large-scale circulations and an increasing relative contribution of small-scale turbulence. In non-precipitating polluted clouds, continued upward transport of moist air from cloud base allows lower-cloud updrafts to persist even as the upper cloud decays. In contrast, in precipitating clean clouds, rain formation strengthens downdrafts. The microphysical evolution also depends strongly on aerosol loading: clean precipitating clouds exhibit rapid depletion of small droplets and growth of large drops, whereas polluted non-precipitating clouds undergo weaker temporal changes. Dissipation also leaves a clear imprint on the cloud's surrounding environment. Enstrophy increases outside the cloud, indicating enhanced vorticity near the cloud–environment interface that extends outward through detrainment. A humid halo develops and expands around the cloud, exhibiting oscillatory behavior in both precipitating and non-precipitating cases.

Overall, our results show that shallow-cumulus dissipation is a distinct and prolonged stage involving coupled changes in cloud dynamics, microphysics, turbulence, and the near-cloud environment. Therefore, dissipation should be explicitly considered when interpreting instantaneous observations and representing shallow convection in models.