Linking observed aerosol–cloud processes and kilometer-scale cloud-resolving simulations over the Amazon rainforest

At kilometer-scale resolution, convective systems start to be explicitly resolved in atmospheric models, albeit coarsely. This allows a more process-based analysis of certain aspects of aerosol–cloud interactions in tropical regions. Convective clouds are a ubiquitous feature above the Amazon rainforest and develop under strongly contrasting aerosol conditions, with particle number concentrations during the dry season often exceeding those in the wet season by an order of magnitude.

In this context, we explore aerosol and convective cloud processes over the Amazon rainforest by analyzing case studies that combine observations and km-scale cloud-resolving simulations with interactive aerosols in a limited-area configuration. Regional simulations are performed at approximately 1.6 km horizontal resolution using the Icosahedral Nonhydrostatic (ICON) model coupled to the one-moment aerosol scheme HAM-lite. The realism of the simulations is evaluated through comparison with a combination of ground-based, satellite, and aircraft observations.

For the wet season, we analyze a case study based on flight RF15, conducted with the German research aircraft HALO during the CAFE-Brazil (Chemistry of the Atmosphere: Field Experiment in Brazil; CAFE-BR) campaign in 2022–2023. Three simulations are presented for this case: a best-estimate factual simulation and two counterfactual sensitivity experiments representing background “green ocean” conditions and heavy aerosol loading associated with biomass burning during dry season periods.  

For the dry season, we also revisit two research flights from the ACRIDICON-CHUVA 2014 campaign, representing one clean and one polluted case, to further assess the representation of aerosol–cloud processes under different convective regimes. Combining these cases, we discuss the impact of changing aerosol environments on convective systems and draw conclusions relevant to a transition toward a post-fossil aerosol regime.