Philip Stier

Abstract:

Deep convective clouds (DCCs) transport heat, water, and momentum, affecting Earth’s radiative balance and water cycle. Aerosol particles influence these properties as cloud condensation nuclei, but how anthropogenic aerosols modify mixed-phase cloud microphysics and radiative effects remains unclear. Here, we investigate two sea-breeze-driven convection cases over Houston using high-resolution ICON-HAM-lite simulations, exploring present-day, cleaner, and more polluted conditions by scaling anthropogenic aerosol concentrations and emissions by factors of 3 and 0.3. We ran 10-member ensembles of 24-hour simulations with perturbed initial temperatures. As previous studies have shown, increased aerosols raise cloud droplet numbers and suppress warm rain formation. We further show that aerosols shift microphysics pathways by reducing riming and enhancing evaporation, ice nucleation, and depositional growth. These changes maintain more condensate in cloud ice and snow while reducing transfer to graupel and hail. Consequently, pollution strengthens both longwave warming and shortwave cooling, with shortwave cooling dominating in active convection. The principal responses are consistent across both cases, but their vertical and radiative expression varies with storm organisation: more organised storms produce larger anvils and more sustained cooling, while isolated storms have more heterogeneous cloud structures. In the most aerosol-rich simulations, the net radiative effect is also less robust because of greater ensemble spread during late-stage anvil dissipation. Overall, these results highlight the importance of competing mixed-phase microphysical processes when assessing aerosol effects on deep-convective clouds.