Tropopause remote control of anvil cirrus evolution

What are the primary processes that drive low-latitude anvil cirrus evolution? Anvil evolution has often described by assuming it can be treated as a vertically well-mixed layer; the mixing is maintained by cloud- scale circulations driven by radiative heating contrasts between cloud base and cloud top. In-situ measurements from CRYSTAL-FACE show that this is not always the case: in reality, at least some anvils are stably stratified, and merely float in an environment with equivalent stratification. From microphysical and thermodynamic observations, numerical and analytical arguments suggest that radiative heating gradients do exist and maintain mixed layers. However, the region of strong radiative interaction is highly concentrated within slabs just 100 m deep located at cloud base and top. Rather than driving deep vertical motion, these thin layers can create a disequilibrium with the clear environment that drives anvil spreading in the form of gravity currents; internal anvil stability is thereby maintained. We further show that if a second layer of optically thin cirrus near the tropopause is also present, as is often the case, the upper part of the anvil equilibrates to radiative temperatures warmer than would normally be associated with a clear upper atmosphere. The associated reduction to heating gradients at anvil cloud top corresponds to reduced anvil spreading and turbulent kinetic energy, by as much as 19% and 40%, respectively. These results suggest that tropopause cirrus can affect climate indirectly, by capping anvil cirrus dynamic energy.