Interactions of Radiation and Convection in Simulated Tropical Cloud Clusters

Abstract

A two-dimensional cumulus ensemble model is used to study the interactions of radiation and convection in tropical squall cloud clusters. The model includes cloud-scale and mesoscale dynamics, an improved bulk ice microphysics parameterization, and an advanced interactive radiative transfer scheme. The life cycle of a tropical squall line is simulated over a 12-h period using thermodynamic and kinematic initial conditions as well as large-scale advective forcing typical of a GATE Phase III squall cluster environment. The focus is on the interaction and feedback between longwave (or IR) radiation and cloud processes. It will be shown that clew-sky IR cooling enhances convection and, hence, surface precipitation. Simulation results reveal an increase of surface precipitation by ∼15% (∼1.7 mm) over a 12-b period due to this clear-sky cooling. With fully interactive IR radiative heating, direct destabilization of clouds via IR radiative top cooling and base warming generates more turbulence and contributes to the longevity and extent of the upper-tropospheric stratiform (anvil) clouds associated with deep convection. The greater extent of anvil clouds decreases the outgoing IR flux at the top of the atmosphere by as much as 20 W m⁻². With fully interactive IR radiative heating, the anvil cirrus reduces the IR cooling of the troposphere with respect to the clear-sky values. This cloud IR radiative forcing has a negative feedback on tropical deep convection, which will be referred to as “anvil cloud IR radiative feedback.” This feedback decreases surface precipitation by ∼10% (∼1.3 mm). It will also be shown that IR radiative processes modify the hydrometer profiles by affecting convection. On changing the cloud particle size distributions prescribed in radiation calculations, it is further demonstrated that the size distributions significantly influence the convective activity through their effects on the cloud IR radiative forcing. The impact of clear-air IR cooling and cloud radiative forcing on deep convection is further examined by using the cloud-work function, which is a generalized number of the moist convective instability in die large-scale environment. The clear-air IR cooling tends to increase the cloud-work function, but the cloud IR radiative forcing tends to reduce it, especially for the deposit clouds.