Seminar: Response of organized convection over the Atlantic ITCZ to anthropogenic aerosol forcing / To what extent are tropical cyclones shaped

Deep convective systems in the tropics exhibit a broad spectrum of spatial organization patterns, spanning from isolated convective systems to large-scale structures like the Intertropical Convergence Zone (ITCZ) which extends over thousands of kilometres. The latter, comprising a myriad of tropical anvil clouds, exerts a significant influence on the average top-of-the-atmosphere (TOA) radiative flux in the tropics. While its influence on the global radiative budget is relatively well-known, there is a lack of certainty in the governing underlying processes, the highly complex interactions among themselves, and how they might evolve with changes in the levels of air pollution. We explore the impact of changing anthropogenic aerosol emissions on the ITCZ by performing limited-area simulations using the storm-resolving model, ICON, coupled to the interactive aerosol model, HAM-lite [Weiss et al., GMD (2025)]. We have configured the pre-existing model version with the Seifert-Beheng 2-moment cloud microphysics scheme, and a more explicit treatment of aerosol activation and wet deposition in order to simulate a more-detailed representation of aerosol-cloud interactions. We perform two sets of simulations – a ‘control’ case with present-day emissions, and a ‘clean’ case wherein the emissions of the carbonaceous and sulphate aerosols are reduced by a factor of 3 – with a focus on the Atlantic ITCZ during August 2024, coinciding with the tropical Atlantic field campaign, ORCESTRA. For a higher aerosol loading,  we see an increase in the number concentration and a decrease in the effective radii of cloud droplets which is a distinct signature of the Twomey effect. The vertical profiles of various hydrometer categories are used to infer changes in the structural characteristics of clouds. Diagnosis of the underlying cloud microphysical process rates enable us to quantify the various pathways of aerosol-cloud interactions and how they go on to influence the TOA radiative fluxes in this case study.