C​loud ​D​roplet ​N​umber ​C​oncentration – satellite retrievals Advanced by Atmospheric models for Assessing ​A​erosol-​C​loud ​I​nteractions

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Aerosol-cloud interactions imply an effective radiative forcing that is a key uncertainty when un­derstanding and interpreting observed climate change. Global data are needed to better quan­tify the relevant processes, but a key quantity – the cloud droplet number concentration (CDNC, Nd) - is not available from operational products. Building on preliminary work, CDNC4aci will work towards reliable retrievals of Nd from satellites in close observations – model interaction: newly-available cloud-resolving simulations will inform the retrieval development and refinement, and the data, in turn, will be used to improve understanding and quantification of aerosol-cloud inter­actions in the model and from statistical analysis. Specifically, the project will include multi-angle and polarimetric observations for better Nd data, it will revise retrieval approaches using model-informed cloud vertical stratification conditioned on cloud regime and thoroughly quantify and correct retrieval errors and biases and assess aerosol-cloud interaction processes from data. The project will assess the cloud-process information in the retrieved data using model sensitiv­ity analyses, it will make model and data comparable by forward-simulating measured polarized radiances and retrieval products, and assess aerosol-cloud interactions in a global model evalu­ated using the data and the process understanding in model-data assessment. The final goal is a consistent quantification of the aerosol-cloud forcing between model and data analysis.