Diurnal Differences in Tropical Maritime Anvil Cloud Evolution

Satellite observations of tropical maritime convection indicate an afternoon maximum in anvil cloud fraction that cannot be explained by the diurnal cycle of deep convection peaking at night. We use idealized cloud-resolving model simulations of single anvil cloud evolution pathways, initialized at different times of the day, to show that tropical anvil clouds formed during the day are more widespread and longer lasting than those formed at night. This diurnal difference is caused by shortwave radiative heating, which lofts and spreads anvil clouds via a mesoscale circulation that is largely absent at night, when a different, longwave-driven circulation dominates. The nighttime circulation entrains dry environmental air that erodes cloud top and shortens anvil lifetime. Increased ice nucleation in more turbulent nighttime conditions supported by the longwave cloud-top cooling and cloud-base heating dipole cannot compensate for the effect of diurnal shortwave radiative heating. Radiative-convective equilibrium simulations with a realistic diurnal cycle of insolation confirm the crucial role of shortwave heating in lofting and sustaining anvil clouds. The shortwave-driven mesoscale ascent leads to daytime anvils with larger ice crystal size, number concentration, and water content at cloud top than their nighttime counterparts.

Automated summary

Satellite observations show that tropical maritime convection exhibits a peak in anvil cloud fraction in the afternoon, which cannot be explained by the diurnal cycle of deep convection peaking at night. Idealized cloud-resolving model simulations reveal that anvil clouds formed during the day are more widespread and longer lasting due to the influence of shortwave radiative heating, which leads to lofting and spreading of the clouds through a mesoscale circulation. In contrast, a different, longwave-driven circulation dominates at night and leads to erosion of the cloud top and shorter lifetimes for anvil clouds.