Tropical Anvil Clouds: Observations of a Preferred State

Abstract:

Anvil clouds originating from deep convection play an important role in the tropical energy balance. We use combined radar-lidar observations and a radiative transfer model to examine how anvil clouds evolve in response to radiative heating. We find that high ice clouds with an optical depth between 1 and 2 are prevalent in tropical convective regions and are uniquely capable of persisting far from any active convection. These “modal anvils” experience strong radiative heating and are lofted with respect to their optically thick precursors, suggesting that radiatively driven mesoscale circulations play an important role in anvil cloud evolution. High ice crystal number concentrations are associated with stronger vertical gradients in radiative heating, which supports the hypotheses that ice concentrations are preserved by in-cloud convection. We also find distinct differences in the anvil cloud populations observed during day and night. Freshly detrained anvils thin more rapidly at night, when strong radiative cooling promotes cloud top subsidence. Diurnal differences in the altitude and geometric thickness of modal anvils suggest that they are shaped by the time of day at which the cloud was first detrained. These results underscore the importance of radiative heating and small-scale processes in determining the cloudiness of the tropical upper troposphere and the radiative effects of deep convection.