What Drives the Life Cycle of Tropical Anvil Clouds?

The net radiative effects of tropical clouds are determined by the evolution of thick, freshly detrained anvil clouds into thin anvil clouds. Thick anvil clouds reduce Earth's energy balance and cool the climate, while thin anvil clouds warm the climate. To determine role of these clouds in climate change we need to understand how interactions of their microphysical and macrophysical properties control their radiative properties. We explore anvil cloud evolution using a cloud-resolving model in three-simulation setups of increasing complexity to disentangle the impacts of the various components of diabatic heating and their interaction with cloud-scale motions. The first phase of evolution and rapid cloud spreading is dominated by latent heating within convective updrafts. After the convective detrainment stops, most of the spreading and thinning of the anvil cloud is driven by cloud radiative processes and latent heating. The combination of radiative cooling at cloud top, latent cooling due to sublimation at cloud base, latent heating due to deposition and radiative heating in between leads to a sandwich-like, cooling-heating-cooling structure. The heating sandwich promotes the development of two within-anvil convective layers and a double cell circulation, dominated by strong outflow at 12-km altitude with inflow above and below. Our study reveals how small-scale processes including convective, microphysical processes, latent and radiative heating interact within the anvil cloud system. The absence or a different representation of only one component results in a significantly different cloud evolution with large impacts on cloud radiative effects. Plain Language Summary Clouds have a large influence on climate. Thick clouds reflect part of the solar (or shortwave) radiation back to space and therefore cool the climate. On the other hand, wispy and thin high clouds do not reflect much of solar radiation. They form high in the atmosphere at cold temperatures and therefore keep part of the terrestrial (or longwave) radiation within the atmosphere. They warm the climate, similarly to greenhouse gasses. The evolution of thunderstorm clouds is of particular interest as it involves a transition from the thick clouds that cool the climate to the thin high clouds that warm the climate. We study small-scale processes that drive this transition and their delicate balance and interactions. Tiny differences in how ice crystals form, grow, shrink, or interact with solar or terrestrial radiation can lead to large differences in the climatic role of thunderstorm clouds. Such processes are currently not represented in models we use for climate projections. Our findings may ultimately lead to improvements in the representation of thunderstorm cloud life cycles in climate models and therefore increase the trust in projections of future climate.