The goal is to understand what factors affect the magnitude of the aerosol-cloud interactions in several different model systems. The indirect radiative effect of aerosols on clouds (ACI, or ERF_ACI according to the IPCC) is the largest uncertainty in climate forcing over the historical record. Sophisticated earth system models typically treat aerosols cloud interactions as a series of processes starting with aerosols and total Cloud Condensation Nuclei (CCN), to activation of aerosols as cloud droplets (Activation) to the loss process for cloud water, often through precipitation (Autoconversion). This experiment will test several different processes to see how ACI are sensitive to the process representations, and in what combination.

Each participating model will run a 3-parameter perturbed parameter experiment (PPE). This will consist of 39 pre-defined simulations that will be run for the years 2008 and 1850 + any required spin-up time. The 2008 simulations will be the priority but 1850 simulations are required to calculate the radiative forcing. This is a total of 78 years of simulation + spin-up. The pre-defined simulations will allow statistical modelling to be carried out for defined diagnostics producing sensitivity analyses that will be used to compare individual models following Lee, et al. 2011 and Carslaw et al. 2013. Participants are also requested to submit the results of the one-at-a-time high/low tests used to test the implementation of the perturbation for initial comparisons.