Aerosol Modulation of Warm Sector Extreme Rainfall over Coastal South China in Its Multi-Scale Structures: Progressive Impact Chain Extension from Microphysics to Convective Organization

Abstract. How aerosols modulate warm-sector extreme rainfall over South China by altering mesoscale convective system (MCS) organization remains poorly understood. We investigate a coastal warm-sector extreme-rainfall event on 10–11 May 2022 using five WRF v4.3.3 sensitivity experiments with different water-friendly aerosol concentrations. Multiscale responses are diagnosed using a two-dimensional discrete cosine transform, hydrometeor mass budgets, and buoyancy analysis. All experiments reproduce the inland frontal and coastal warm-sector rainbands, while aerosol sensitivity is concentrated in coastal MCS continuity, convective-scale hydrometeor activity, and rainwater production. As aerosol loading increases from low to moderate levels, condensational growth and cloud-water collection by raindrops intensify, establishing a more persistent warm-rain water source. Increased supercooled cloud water further promotes snow and graupel riming and melting, strengthening cold-rain processes. Meanwhile, low-level inflow, ascent, and hydrometeor columns become better aligned, favoring sustained MCS regeneration and a more continuous coastal rainband. At higher aerosol loading, smaller mean cloud-droplet sizes suppress warm-rain production, while evaporative cooling and low-level cold anomalies intensify, coinciding with greater spatial separation of newly triggered convection from the primary rainband and increased rainband fragmentation. Although cold-rain compensation occurs in a few intense cores late in the highest-concentration experiment, it does not restore regionally averaged rainwater generation or rainband continuity. These results reveal a non-monotonic aerosol sensitivity in which precipitation depends not only on local microphysical enhancement but also on whether microphysical and dynamical responses remain spatially coupled to the MCS regeneration zone within this coastal warm-sector convective system over South China.