INTERACTIONS OF RADIATION AND CONVECTION IN SIMULATED TROPICAL CLOUD CLUSTERS

A two-dimensional cumulus ensemble model is used to study the interact ions of radiation and convection in tropical squall cloud clusters. Th e model includes cloud-scale and mesoscale dynamics, an improved bulk ice microphysics parameterization, and an advanced interactive radiati ve transfer scheme. The life cycle of a tropical squall line is simula ted over a 12-h period using thermodynamic and kinematic initial condi tions as well as large-scale advective forcing typical of a GATE Phase III squall cluster environment. The focus is on the interaction and f eedback between longwave (or IR) radiation and cloud processes. It wil l be shown that clear-sky IR cooling enhances convection and, hence, s urface precipitation. Simulation results reveal an increase of surface precipitation by similar to 15% (similar to 1.7 mm) over a 12-h perio d due to this clear-sky cooling. With fully interactive IR radiative h eating, direct destabilization of clouds via IR radiative top cooling and base warming generates more turbulence and contributes to the long evity and extent of the upper-tropospheric stratiform (anvil) clouds a ssociated with deep convection. The greater extent of anvil clouds dec reases 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 ci rrus reduces the IR cooling of the troposphere with respect to the cle ar-sky values. This cloud IR radiative forcing has a negative feedback on tropical deep convection, which will be referred to as ''anvil clo ud IR radiative feedback.'' This feedback decreases surface precipitat ion by similar to 10% (similar to 1.3 mm). It will also be shown that IR radiative processes modify the hydrometeor profiles by affecting co nvection. On changing the cloud particle size distributions prescribed in radiation calculations, it is further demonstrated that the size d istributions significantly influence the convective activity through t heir effects on the cloud IR radiative forcing. The impact of clear-ai r IR cooling and cloud radiative forcing on deep convection is further examined by using the cloud-work function, which is a generalized mea sure of the moist convective instability in the large-scale environmen t. 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 deepest clouds.