Mesoscale Radiatively-Induced Anvil Spreading

In idealized 2D simulations, we represented the generation of optically thick anvil clouds by adding (“injecting”) cloud ice in a layer over a time period of a few hours. We found that (1) there is no spreading without radiative heating, (2) mesoscale motions are required for spreading but cloud-scale motions and/or turbulence are not, and (3) solar radiation does not reduce the spreading. More realistic full life-cycle simulations support the conclusions reached with the injected anvil simulations.

In all the simulations, the spatial gradients of radiative heating at the anvil cloud edges produce a mesoscale circulation that spreads the anvil cloud outward at a significant rate (about 1 m/s). We call this mesoscale radiatively-induced anvil spreading (MRAS). As the cloud spreads due to the mesoscale circulation, the radiative heating gradients also spread. The result is a positive feedback that lasts as long as there is sufficient cloud ice. As a result of the radiatively-induced anvil spreading, the simulated anvils have greater infrared (IR) warming (greenhouse) effects than they would have without the radiatively-induced anvil spreading. This is due to both the greater spatial extent and the longer lifetimes of the radiatively interactive anvils.