Controls on the Development and Circulation of Terminal versus Transient Congestus Clouds and Implications for Midlevel Aerosol Transport

Abstract

Cumulus congestus is the middle mode of tropical convection, with cloud tops around or exceeding the 08C level (∼5 km AGL). While some congestus are terminal, meaning capped by the 08C stable layer, others are transient and may develop into deep convection. Although this distinction impacts convective transport of water vapor and aerosols into the midtroposphere and the congestus to deep convection transition, there is still much to be understood about the processes causing congestus to overshoot the 08C level and continue growing. We simulate a field of tropical congestus using high-resolution idealized model simulations, identify and track the updrafts, and composite congestus properties. Terminal and transient congestus updrafts are characterized by a similar overturning circulation between the updraft and subsiding shell. However, transient congestus have stronger updrafts, and the downward branch of their corresponding circulations are constrained by the 08C level. The balance between buoyancy and perturbation pressure gradient accelerations predominantly determines the shape of the vertical velocity profile, though vertical advection through bulk and subplume fluctuations are also shown to be important near and above the 08C level. Our findings support previous results suggesting buoyancy as a control on congestus height. We find that congestus developing in more humid midlevel environments are more likely to be transient. Finally, we explore how congestus updrafts influence their near moisture and aerosol environments: terminal congestus return more aerosol to the atmosphere through evaporation along their edges, while transient congestus create stronger midlevel detrainment layers of aerosol and water vapor due to the trapping of the regenerated aerosol above the 08C level.