Is it radiation or precipitation that drives mammatus cloud development in cirrus anvils?

Mammatus clouds are the cloudy pouch-like protuberances seen hanging from mid to high level clouds. They are interesting, not simply for their dramatic appearance, but also because they provide clues about the inner workings of anvil cirrus. Past literature has most commonly suggested that mammatus subsidence is associated with hydrometeor evaporative cooling, accelerated by the presence of dry lower troposphere air. Here we use a combination of thermodynamic reasoning, a cloud model, and three dimensional radiative transfer simulations to show that dry layers can contribute to mammatus development also because they are radiatively transparent to thermal radiation, and this magnifies the brightness temperature contrast between the cloud and its below cloud environment. It is this contrast that allows for mechanical work to be done within radiatively absorbing cloud near the anvil cirrus base, sustaining development of the cloud features that are observed.