Cloud amount simulated by the coupled model MRI-CGCM2.3

The coupled general circulation model (CGCM) used in this study is the MRI-CGCM2.3. The atmospheric part of this model has a horizontal spectral truncation of T42 corresponding to about a 270-km horizontal grid spacing. The horizontal grid spacing of oceanic part is 2.5 degrees in longitude and 2 degrees in latitude. The model treats clouds with a diagnostic cloud scheme. Cloud fraction is calculated from relative humidity, but we introduced the critical relative humidity Rc required to form a cloud. We have assumed different vertical profiles of Rc for convective clouds and layer clouds. Furthermore, different profiles are applied for clouds over ocean and clouds over land, which can roughly express an effect from difference of condensation nuclei between ocean and land. As a part of contributions to the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC AR4), we have conducted 5-member ensemble simulations of the 20th Century climate from year 1850 to 2000. The forcing agents of the experiment include greenhouse gases (CO2, CH4, N2O and CFCs), sulfate aerosol direct effects, volcanoes and solar forcing. We have carefully tuned the cloud scheme such that the simulated climatological cloud radiative forcing (CRF) agree with the estimation from the Earth Radiation Budget Experiment (ERBE). However, simulated low-level cloud was underestimated compared with observations of International Satellite Cloud Climatology Project (ISCCP, 1983-2001), especially over the oceans.