Modulation of Tropical Convection-circulation Interaction by Aerosol Indirect Effects in Idealized Simulations of a Global Convection-permitting Model

Observations suggest tropical convection intensifies when aerosol concentrations enhance, but quantitative estimations of this effect remain highly uncertain. Leading theories for explaining the intensification are based on the dynamical response of convection to changes in cloud microphysics, neglecting possible changes in the environment. Here, we use a global convection-permitting model that explicitly simulates convective updraft cores within organized convective systems and the tropical large-scale overturning circulation to demonstrate how the enhanced aerosol concentration modulates tropical climates. We carried out a pair of idealized non-rotating radiative-convective equilibrium simulations with different background aerosol concentrations under the fixed sea surface temperature of 300K. The results show that convective self-aggregation takes place in both simulations. While the overturning deep circulation is weakened in the simulation with more pollution, the horizontal advection of energy over moist ascending patches plays a more prominent role in weakening convective self-aggregation, revealed by the budget analysis for the spatial variance of column-integrated frozen moist static energy. We found that convective systems in the polluted simulation are triggered and develop geographically closer to the edge of moist ascending patches, and more cloud drops are being activated at the mid-level atmosphere and exported from moist ascending patches to dry regions through the overturning shallow circulation. The spatial distribution of convection triggering is associated with the balance between the cold pool strength and the low-level inflow at the edges. Although the threshold column water vapor that heralds the increase in convective intensity occurs at a lower value (53 mm) in the polluted simulation than it does in the pristine simulation (57 mm), the less unstable environment over moist ascending patches in the polluted simulation limits the overall intensity of deep convection. Our results emphasize the importance of allowing atmospheric phenomena to evolve continuously across spatial and temporal scales in simulations when investigating the response of tropical convection to changes in cloud microphysics.