The active role of stratiform and anvil clouds through longwave radiative feedback

Our emphasis on distinct tropical convective cloud modes has evolved over the decades in step with advancements in our understanding of tropical convection, its governing dynamics, and its role in large-scale weather and climate. While an early undilute plume view of tropical convection emphasized the role of latent heating from deep cumulonimbi in tropical ascending motion, a later emphasis on shallow to congestus clouds came with our improved grasp of water vapor’s essential role as governor to convective cloud development and organization. Here I discuss the unique role of stratiform and anvil clouds in this context, which play a surprisingly active role in the mesoscale organization of deep convection. While stratiform and anvil clouds are the biproducts of deep convection, consuming remnant buoyancy from their parent cumulonimbi, their much larger spatial and temporal footprints cause radiative forcing that fosters the upscale growth of moist convection and its coupling with the larger-scale environment. These clouds are therefore uniquely capable of coupling convection with the larger scale owing to their very long inherent lifecycles, compared to the fundamental scales of deep convection. In this presentation, I first motivate these arguments through consideration of scales. I next present the results from numerical model experiments and observations of tropical convection supporting the argument that longwave forcing by stratiform and anvil clouds actively promote convective upscale development and intensification. This forcing acts by reducing downdraft mass flux in stratiform regions, which in turn yields more upward motion per unit precipitation in the overall convective system. This constitutes a destabilization of the moist convective system, fostering its intensification, compared to if cloud–radiative forcing was absent. The results of this study imply that stratiform and anvil clouds and their radiative forcing are essential elements to any realistic conceptual model of tropical moist convection, and hence, to the tropical hydrologic cycle.