Integration of Aerosol–Cloud Interactions With a Land‐Atmosphere Coupled Model in the Gray Zone

Abstract

Clouds strongly impact net surface radiation, creating differential heating patterns, and trigger land breezes that in turn organize cloud development. These dial processes determine the location of persistent ecohydrological regimes at local scales. Aerosol–cloud interactions (ACI) directly impact the diurnal cycle of cloud optical depth, and therefore the surface energy budget; yet they are generally not explicitly represented in studies of land‐atmosphere interactions. To address this gap, a 3D Large‐Eddy Simulation model was modified to describe ACI in the gray zone. The goal is to simulate realistic cumulus cloud dynamics consistent with the spatial dynamics of land surface moisture and temperature, and associated boundary layer processes. The model is tested and evaluated using simulations at hectometer resolution with prescribed diurnal surface heating corresponding to idealized wet (low Bowen ratio [LBR]) and dry (high Bowen ratio [HBR]) surface boundary conditions. Expected cloud sensitivity is well captured: compared to LBR, HBR clouds have a significantly higher cloud base (about 1 km), up to 50% reduction in total cloud fraction, and cloudiness ends earlier by about 2 hr due to enhanced turbulence and lower moisture availability. Analyses of the evolution of 3D cloud drop size distributions show that the model captures the cloud life cycle, including aerosol activation, condensational growth, enhanced collision‐coalescence, and aerosol regeneration. The results show that resolved eddy‐scale (sub‐km) within‐cloud circulations modulate cloud microphysical structure by locally altering supersaturation at minute timescales. Successive aerosol regeneration yields coarser cloud condensation nuclei, producing a coarse‐aerosol mode, and impacting mid‐day cloud fraction and cloudiness persistence.