Secondary ice production affects tropical convective clouds under different aerosol conditions

Abstract. Secondary ice production (SIP) modulates tropical convection, but its interactions with aerosol loading remain uncertain. Using the UK Met Office Unified Model with detailed SIP parameterizations, this study investigates how multiple SIP mechanisms affect a Hector storm under weak instability and varying cloud condensation nuclei (CCN) concentrations. At moderate aerosol loading (N_d = 400 cm^–3), SIP substantially enhances the ice phase. Ice number increases by over 3 orders of magnitude at temperatures warmer than –10 ℃, and ice water content rises by up to ~120 % in the anvil. SIP also reduces outgoing longwave radiation (OLR) by 6.8 W m^–2 (–2.5 %), increases outgoing shortwave radiation (OSR) by 7.9 W m^–2 (+2 %), over a 110 km × 110 km region in 6 hours. For this case, precipitation becomes more organized near the convective core, with total accumulation increasing by ~25 %. Under polluted (N_d = 800 cm^–3) and clean (N_d = 200 cm^–3) conditions, SIP effects are weaker or spatially fragmented, indicating non-linear aerosol dependence. Sensitivity experiments show that, when combined with rime-splintering (RS), Mode 1 (droplet fragmentation during freezing) weakens at high CCN owing to reduced supercooled raindrop supply in the mixed-phase layer, while Mode 2 (drop–ice collisions with splashing) is inhibited at low CCN due to less encounters with large rimed ice particles. In contrast, RS and its combination with ice–ice collisional breakup show relatively limited aerosol sensitivity in this case.