AERO-CLO-WV will investigate aerosol-cloud-water vapour feedback mechanisms which have previously not been studied in depth and quantified. Thus the proposal is highly relevant to the call with detailed studies of processes and feedback mechanisms. It wil l complement the work in the scenario project NorClim in which AERO-CLO-WV participants have leading responsibilities. In the project we will quantify, in terms of radiative forcing, climate feedback due to anthropogenic aerosols through changes in the wa ter vapour concentration in cloud-free air. This will be done by using the CAM-Oslo atmospheric climate model. This forcing will be compared to compatible estimates of known indirect aerosol-cloud forcings due to anthropogenic aerosols which will be made in the project. The water vapour budget involves a complexity of microphysical and turbulent processes, and estimating water vapour changes due to anthropogenic aerosols is thus a very complicated task. In this project we will limit ourselves to investiga tions of certain selected cloud types, namely trade wind cumuli, due to their relative simplicity, and cold clouds due to their expected importance for the problem that we investigate. Due to the high complexity of processes and the small scales of clouds we find it necessary to make a range of process studies covering processes governing the fluxes of water between the individual hydrometeors and entrainment/detrainment processes. The process studies will be made with a Single Column version of CAM (SCAM ) and the WRF model. The results of the process studied will be used to validate the relevant processes in CAM-Oslo and as a basis for improved parameterisations. In order to validate results from the model simulations we will rely on two sources of obser vational data; satellite data with high spatial and temporal coverage and field campaign data which provide detailed information at high resolution within limited geographical areas.