Role of aerosol–cloud–radiation interactions in modulating summertime quasi-biweekly rainfall intensity over South China

Abstract. Persistent heavy rainfall events over densely populated South China are closely linked to the intensification of quasi-biweekly (8–30-day) oscillations. This study examines whether and how aerosols influence quasi-biweekly oscillations using observational analyses and model experiments. Statistical analysis reveals a significant phase-leading relationship between increased aerosol loadings, quantified by aerosol optical depth, and subsequent enhancement of 8–30-day rainfall anomalies. At the 8–30-day timescale, aerosols primarily influence rainfall intensity through cloud microphysical processes, with radiative effects playing a secondary role. Approximately four days before enhanced rainfall events, positive aerosol anomalies contribute to increased low-level cloud water content, leading to condensational latent heat release. This low-level latent heating strengthens low-level moisture convergence and ascending motion, which uplifts cloud droplets above the freezing level. Subsequently, additional latent heat release from mixed-phase processes (freezing/deposition) further intensifies vertical motion, amplifying precipitation anomalies. Once deep convection develops, clouds absorb longwave radiation, sustaining precipitation intensification. Sensitivity experiments using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) confirm these mechanisms, demonstrating that anthropogenic aerosol enhancement intensifies precipitation anomalies through both aerosol-cloud microphysical interactions and longwave cloud-radiative effects, with the former being more dominant. These findings highlight the need to improve aerosol-cloud microphysical parameterizations in operational models to enhance the accuracy of extended-range heavy rainfall predictions in South China.