Here we examine to what extent changes in the anvil cloud fraction can be understood from changes in the clear-sky radiative convergence, in a variety of contexts, configurations and timescales. In particular, the anvil cloud fraction also depends on the mixing of cloud ice condensates with the upper-tropospheric air 8, 9, 10. The behavior of anvil clouds does not only depend on the clear-sky radiative convergence, but can also be affected by local convective entrainment and microphysical processes.

Due to enhanced S, the clear-sky pressure gradient of subsidence is reduced, reducing D r, which by mass conservation leads to reduced anvil cloud fraction. When the surface warms, anvils and D r rise nearly isothermally, and find themselves in a more stable atmosphere because of the dependency of S on atmospheric pressure. This mechanism is based on thermodynamic arguments and supported by radiative-convective equilibrium simulations from General Circulation Models (GCMs) and cloud-resolving models 10, 11, 12 as well as observations 6, 13.

This argument has been invoked to predict a decrease of anvil cloud fraction with temperature, in the framework of the stability-Iris mechanism 11. Owing to mass continuity, a maximum D r of horizontal mass convergence in clear-sky regions is associated with a maximum of horizontal mass divergence in convective regions, and therefore with the fraction and altitude of anvil clouds. These findings should also be considered when assessing the impacts of geo-engineering techniques. Therefore, the control of tropical anvils by clear-sky radiative cooling and static stability in the upper troposphere can explain a large diversity of the responses of anvil cloud fraction to natural and anthropogenic perturbations. This includes the response to interannual and long-term surface temperature changes, to the direct effect of carbon dioxide, and the decrease of anvil cloud fraction after explosive volcanic eruptions. Based on satellite observations, meteorological reanalyses, and climate model simulations, we show that these arguments can be used to interpret the response of anvil cloud fraction to a range of perturbations in the current climate and under climate change. It has been suggested that variations in the height and spatial extent of these clouds can be understood from basic physical arguments related to the conservation of mass and energy in the clear-sky areas of the tropics. Anvil clouds produced by deep convection cover extensive areas of the tropics, and their response to external perturbations matters for the Earth’s climate sensitivity.