Tropical Anvil Cloud Lifecycles and their Net Cloud Radiative Effect

Simultaneous measurements of pixel level broadband radiation and cloud vertical structure from active remove sensing from space confirm that counteracting net radiative effects from thick anvils and thin cirrus clouds are necessary to sustain the net zero cloud radiative effect of tropical convective clouds (Hartmann and Berry 2017). This suggests that the lifecycle of convective clouds, evolving from rainy cores into thin cirrus, is a primary mechanism required for the net top-of-atmosphere neutrality of tropical convective clouds. This idea is explored in a simple two dimensional eddy resolving model framework, where it is demonstrated that a thick anvil cloud evolving to a thin cirrus cloud will give compensating negative and positive cloud radiative effects over its lifetime. It is further demonstrated that the radiative heating of the upper level extended ice cloud, especially by longwave radiation, is essential to determining the lifetime and net radiative effect of the anvil cloud. Longwave radiative heating drives turbulence in the anvil cloud that results in recycling of water within the anvil and extending its duration. Without radiative heating the cloud simply sediments and slowly sublimates, giving a strong net negative cloud radiative effect. The lifecycle net radiative effect is very sensitive to the cloud microphysics, which is demonstrated by comparing simulations with two different schemes. The lifecycle is weakly sensitive to the humidity of the clear atmosphere, but primarily because this affects the radiative heating of the anvil cloud and its environment. Global climate models do not currently simulate the physics involved in maintaining the anvil cloud described here, so it is unlikely that they can accurately predict how the properties of tropical convective clouds and their associated anvil clouds will change in a warmed Earth.