Formation of Tropical Anvil Clouds by Slow Evaporation

Tropical anvil clouds play a large role in the Earth's radiation balance, but their effect on global warming is uncertain. The conventional paradigm for these clouds attributes their existence to the rapidly declining convective mass flux below the tropopause, which implies a large source of detraining cloudy air there. Here we test this paradigm by manipulating the sources and sinks of cloudy air in cloud-resolving simulations. We find that anvils form in our simulations because of the long lifetime of upper-tropospheric cloud condensates, not because of an enhanced source of cloudy air below the tropopause. We further show that cloud lifetimes are long in the cold upper troposphere because the saturation specific humidity is much smaller there than the condensed water loading of cloudy updrafts, which causes evaporative cloud decay to act very slowly. Our results highlight the need for novel cloud-fraction schemes that align with this decay-centric framework for anvil clouds. Plain Language Summary Clouds influence the Earth's energy budget by reflecting sunlight and intercepting terrestrial radiation. The extent to which clouds modify these flows of energy is highly sensitive to the vertical distribution of cloud fraction, and changes in cloud fraction are a dominant source of uncertainty in future climate projections. Here we show that the prominent high cloud-fraction peak in simulations of the tropical troposphere is primarily produced by long cloud lifetimes, which result from the fact that very little condensed water can evaporate into cold air. Our results provide a revised interpretation for the extensive anvil clouds found in the deeply convecting tropics and highlight the importance of developing new cloud-fraction schemes for use in climate models that explicitly depend on cloud condensate lifetime.