Implementation of new empirical parameterization for cloud droplet number concentrations to HadGEM2-AO

Abstract:

In the Hadley Centre Global Environmental Model version 2 - Atmosphere-Ocean (HadGEM2-AO), cloud droplet number concentration (CDNC) 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 CDNC when the aerosol number concentration is < 680 cm^-3 and the opposite when it is > 680 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. Significant change of total cloud amount by the new relationship was shown mainly in the ITCZ due to the change of wind circulation pattern over North Pacific. The surface temperature became closer to the results retrieved from observational data especially over the northern hemisphere. In this study, despite of the little change in CDNC, the results showed significant differences, thus parameterization of CDNC is very important in climate model. The change in CDNC can affects to temperature and dynamics as well as variables directly associated with the cloud and those effects interact with each other, therefore the climate changes is not simply predictable by the changes in CDNC. It implies that accurate estimation of climate change in climate modeling is highly challenging. More detail will be discussed at the conference.