ACME

Significant gaps exist in our understanding of aerosol-cloud interactions which are reflected in the uncertainties in weather forecasts and climate projections. One key aspect that is poorly understood is the role of the ice phase and the aerosol particles on which atmospheric ice crystals form, especially in mixed-phase clouds which are ubiquitous in the troposphere. Whether a cloud is predominantly composed of water or ice strongly influences cloud properties and feeds back into the cloud life cycle, precipitation formation, and radiative energy balance. Both the impacts of the aerosol particles on the formation of the ice crystals as well as on the ice crystal evolution, are suspected to play a dominant role in determining the properties of clouds. Recent studies suggest that marine aerosols (sea-spray, DMS-derived sulfate particles) are not only good CCN (cloud condensation nuclei) but have also good INP (ice-nucleating particle) ability. Depending on the marine environment, the number concentration of aerosol particles can vary by several orders of magnitude, as is the case over the Mediterranean Sea and the North Atlantic Ocean (areas that can be more or less affected by continental sources), or can be low in an open ocean such as the South Pacific Ocean (where the lack of measurements also induces important biases in numerical simulations). This variability has important consequences on both the droplet size distribution and the formation and evolution of ice crystals. The main objectives of ACME are to improve the understanding of the mechanisms of ice formation, the interaction between ice crystals and droplets as well as the role of the aerosol particles in the development and the evolution of clouds in contrasted marine environments (e.g. with different aerosol loadings and thermodynamical properties). To reach these objectives, ACME will exploit the measurements from different marine environments: over the Mediterranean Sea during the EXAEDRE campaign (autumn 2018, Corsica), over the North Atlantic Ocean during the HAIC campaign (May 2015, French Guiana), and over the South Pacific Ocean during the Sea2Cloud campaign (March 2020, New-Zealand), in order to better constrain the representation of the primary and secondary ice nucleation mechanisms in bin cloud-scale modelling (i.e. with the DEtailed SCAvenging Model (DESCAM) using sub-kilometre resolution), providing then a unique tool for aerosol-cloud interaction studies. ACME will improve our knowledge of primary and secondary ice formation, the interaction between ice crystals and droplets, and the role of aerosol particles in cloud development and evolution, in contrasted marine environments. ACME will also reveal the dominant ice formation mechanism in different types of clouds, and improve the phase partitioning of the mixed-phase clouds. ACME will furthermore contribute to the development of new parameterisations on ice formation processes under different atmospheric thermodynamical conditions for larger scale models.