Sensitivity of Marine Cloud Brightening over the Great Barrier Reef to Spatial Variability in Aerosol Forcing: A Case Study using convection-permitting model

Abstract. The Great Barrier Reef (GBR) is increasingly threatened by mass thermal coral bleaching events under climate change. Marine cloud brightening (MCB) has been proposed as a potential adaptation strategy to reduce thermal stress by enhancing cloud reflectivity through aerosol injection. This study evaluates the sensitivity of cloud–aerosol interactions to aerosol emission intensity and spatial configuration over the GBR using convection-permitting Weather Research and Forecasting (WRF) model simulations. A control simulation representing a non- to weakly-precipitating shallow trade-cumulus regime is compared with three MCB sensitivity experiments: a densely distributed (20 km apart), moderate-intensity emission scenario (EXP20), a sparsely distributed (100 km spacing), high-intensity scenario (EXP100), and an intermediate configuration (EXP40). Results show that enhanced aerosol emissions substantially increase near-surface aerosol concentrations, with dispersion strongly governed by source spacing and prevailing trade winds. The EXP20 configuration produces more homogeneous and widespread aerosol enhancements, whereas EXP100 generates localized peaks that are rapidly scavenged, resulting in smaller domain-mean increases despite identical total emissions. Over a 24-hour period, domain-averaged cloud droplet number concentration (CDNC), optical depth, and cloud albedo exhibit strong sensitivity to aerosol loading, while cloud water path (CWP) and cloud fraction show limited responses. These findings indicate a dominant Twomey effect in this cloud regime, with only weak evidence of the Albrecht effect. Nonlinear CWP responses are noted under varying conditions of mid-level humidity, wind shear, and lower-tropospheric stability. Overall, the results highlight the importance of aerosol source configuration and background atmospheric state in shaping MCB effectiveness over the GBR.