Implementation of a New Empirical Relationship between Aerosol and Cloud Droplet Number Concentrations to HadGEM2-AO

In the Hadley Centre Global Environmental Model version 2 - Atmosphere-Ocean (HadGEM2-AO), cloud droplet number concentration is estimated according to the empirically determined relationship between aerosol and cloud droplet number concentrations. However, the observational dataset used for establishing the relationship was obtained from limited regions of the earth; as such, it may not be representative of the entire Earth. In this study, we have re-established the relationship between aerosol and cloud droplet number concentrations, based on a composite of the observational dataset obtained from many different regions around the world, including the original dataset. The new relationship tends to provide a lower cloud droplet number concentration when the aerosol number concentration is < 600 cm-3 and the opposite when it is > 600 cm-3, compared to those from the original relationship. Experiments are performed under the same conditions as the Coupled Model Intercomparison Project phase 5 (CMIP5). The results from the complete historical run with the new relationship are expected to show a number of significant differences from the original historical run. With the new relationship, the cloud droplet effective radius became closer to the results retrieved from the observed data, and the total cloud amount changed significantly over the ITCZ region. Changes in the total precipitation rate tend to follow the changes in the total cloud amount. More detail will be discussed at the conference.