Aerosol impacts on drizzle rate and probability in warm clouds: ground-based observations and forecast model evaluation

Aerosol effects on cloud microphysics and warm-rain formation remain one of the largest sources of uncertainty in future climate projections. While simulations show diverse sensitivity of warm clouds to aerosol perturbations, it remains challenging to narrow down the processes responsible using observations, particularly from satellites due to their limitations in retrieving coincident and collocated aerosol and cloud properties. To advance our understanding of aerosol-cloud-precipitation interactions and to further improve their parametrisation, we exploit the synergy between in-situ aerosol and cloud remote-sensing instruments from Atmospheric Radiation Measurement (ARM) Mobile Facility deployments in Germany and the Azores. Drizzle rate at cloud base has been estimated by combining radar and lidar signals. We will show how precipitation intensity and probability respond to both the liquid water path of the cloud and to aerosol perturbations, thereby providing key information to examine the strength of the aerosol effect on warm rain in current climate models. Using this unique ground-based dataset, we will also for the first time evaluate the prognostic aerosol scheme (MURK) and its interaction with clouds and precipitation in the UK Met Office Unified Model, and the extent to which the degree of agreement is dependent on factors in the model such as the parameterization of the raindrop size distribution.