A New Paradigm for Tropical Anvil Clouds

In the deep tropics, extensive anvil clouds produce a peak in cloud cover below the tropopause. The dominant paradigm for cloud cover argues that this anvil peak results from a layer of enhanced convective detrainment. However, cloud cover also depends on the lifetime of cloudy air after it detrains, which raises the possibility that the anvil peak may be the signature of slow cloud decay rather than enhanced detrainment. Here we measure the cloud decay timescale in cloud-resolving simulations, and find that cloudy updrafts that detrain in the upper troposphere take much longer to dissipate than their shallower counterparts. We show that cloud lifetimes are long in the upper troposphere because the saturation specific humidity becomes orders of magnitude smaller than the typical condensed water loading of cloudy updrafts. This causes evaporative cloud decay to act extremely slowly, thereby prolonging cloud lifetimes in the upper troposphere. As a consequence, extensive anvil clouds still occur in a convecting atmosphere that is forced to have no preferential detrainment layer in the upper troposphere. Our results support a new paradigm for tropical anvil clouds, which states that their existence depends on the microphysics of slow cloud decay rather than on a preferential detrainment layer.