Humidity-Dependent Sensitivity of Tropical Island Deep Convection to Aerosols

Using the aerosol--aware Weather Research and Forecasting (WRF) model in an idealized tropical island framework, we examine how boundary layer  moisture modulates the convective response to cloud condensation nuclei (CCN). Simulations across CCN concentrations of 30--2400 per cubic cm and varying relative humidity,  CAPE and surface–flux regimes, show that convection consistently weakens with increasing CCN, but only when the boundary layer is sufficiently moist. In such a setting, fewer CCN yield larger droplets that rapidly convert to rain and reduce evaporation at mid-levels (between 2 and 4km), both of which  warm and dry the layer and thereby weakening shallow convection. This limits vertical transport of moist static energy (MSE), allowing near-surface MSE and Convective Available Potential Energy (CAPE) to build up. As a result, subsequent deep convection in clean cases exhibits stronger updrafts, greater graupel production, and enhanced convective and mass fluxes. In contrast, humid but polluted environments yield numerous small drops which remain lofted and suppress warm rain, enhancing the evaporative source of vapor at mid-levels, strengthening  shallow convection, limiting CAPE growth, and ultimately producing weaker convection.However, when the boundary layer is dry, both CAPE and convection show little sensitivity to CCN concentration, highlighting the role of moisture preconditioning. Satellite composites of TRMM  convective intensity (measured by 40 dBZ echo top height) and MODIS droplet number (Nd) over tropical islands tentatively appear to support this mechanism. Higher Nd values are associated with lower CAPE and weaker convective vigor, with the strength of the trend being proportional to the near-surface relative humidity - consistent with simulations. Together, these results suggest that in a tropical island-setting, aerosols impact convection primarily when the boudary layer  is preconditioned with sufficient moisture, and that under these conditions increased aerosol loading tends to suppress rather than invigorate deep convection.