[1] The radiative effects from increased concentrations of well-mixed greenhouse gases (WMGHGs) represent the most significant and best understood anthropogenic forcing of the climate system. The most comprehensive tools for simulating past and future climates influenced by WMGHGs are fully coupled atmosphere-ocean general circulation models (AOGCMs). Because of the importance of WMGHGs as forcing agents it is essential that AOGCMs compute the radiative forcing by these gases as accurately as possible. We present the results of a radiative transfer model intercomparison between the forcings computed by the radiative parameterizations of AOGCMs and by benchmark line-by-line (LBL) codes. The comparison is focused on forcing by CO 2 , CH 4 , N 2 O, CFC-11, CFC-12, and the increased H 2 O expected in warmer climates. The models included in the intercomparison include several LBL codes and most of the global models submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In general, the LBL models are in excellent agreement with each other. However, in many cases, there are substantial discrepancies among the AOGCMs and between the AOGCMs and LBL codes. In some cases this is because the AOGCMs neglect particular absorbers, in particular the near-infrared effects of CH 4 and N 2 O, while in others it is due to the methods for modeling the radiative processes. The biases in the AOGCM forcings are generally largest at the surface level. We quantify these differences and discuss the implications for interpreting variations in forcing and response across the multimodel ensemble of AOGCM simulations assembled for the IPCC AR4.Citation: Collins, W. D., et al., (2006), Radiative forcing by well-mixed greenhouse gases: Estimates from climate models in the

Baiu-Changma-Meiyu is a rainy period in early summer over East Asia (Japan, Korea and China) and its variability and change is one of the major focus in climate change projections in these areas. We analyze the changes in intensity and duration of Baiu-Changma-Meiyu rain by global warming using daily precipitation data of fifteen coupled atmosphere-ocean general circulation model (AOGCM) simulations under the SRES A1B scenario at the end of the twenty-first century. It is revealed that a delay in early summer rain withdrawal over the region extending from Taiwan, Ryukyu Islands to the south of Japan is contrasted with an earlier withdrawal over the Yangtze Basin, although the latter is not significant due to inconsistent sign of changes among the models. Higher mean sea-level pressure anomalies in the tropical western Pacific in the future may be related to these late withdrawals. Changes in onset dates are relatively less compared to those in withdrawal dates.

Five-year integrations with the Non-hydrostatic Regional Climate Model (NHRCM) are conducted to evaluate the reproducibility of the regional climate. NHRCM, with a grid interval of 10 km, is nested in the Regional Analysis data set, and a 4 km grid interval NHRCM is nested in it. NHRCM reproduces the monthly precipitation, seasonal change, and regional features well when compared to AMeDAS observations. The model also simulates the frequency of heavy precipitation well. Annual mean temperature of NHRCM exhibits a +0.8°C bias compared to the AMeDAS observation. NHRCM also reproduces inter-annual variation of surface temperature well, especially in summer.

A new version of the Meteorological Research Institute (MRI) coupled general circulation model MRI-CGCM2 (MRI2.3) is developed and compared with the previous version (MRI2.0). The cloud scheme includes diagnostic function for cloud amount separately specified for convective and layer clouds, which is one of the major modifications contributing to the improved model performance. MRI2.3 exhibits better agreement with the observations in many aspects of present-day climate simulations, including the global energy budget, meridional distributions of shortwave and longwave radiation at the top of the atmosphere, and geographical distributions of surface air temperature and precipitation. The effective climate sensitivity of each version is evaluated based on an experiment with a transient (1%/year) increase of carbon dioxide concentration. The effective climate sensitivity of MRI2.3 (2.9 K) is about twice that of MRI2.0 (1.4 K). The change in the cloud-forcing response, particularly for shortwave cloud forcing, is essential for increasing climate sensitivity. A difference in tropical low-level clouds over the subsidence regions contributes significantly to the difference in cloud-forcing changes in response to a climate change. Analyses based on circulation regimes, defined by the vertical velocity at the mid-troposphere, suggest that the cloud-forcing response in the tropics is controlled more by thermodynamic characteristics, such as changes of the stability in the lower troposphere, rather than by large-scale circulation changes, such as a change in the subsidence strength.

Changes in indices of extremes between the presentday climate and a future warmer climate are projected over Japan using a global 20-km-mesh atmospheric model. Comparisons with observed data show that the indices on temperature extremes are represented well in the model, while less intense precipitation biases are found. In the future climate simulation around 2090, the number of frost days decreases by 20 45 days with larger decrease along the Sea of Japan than the other area. Growing season length increases about a month. Changes in the temperature extremes are not uniform over Japan, showing usefulness of projections using a high-resolution model. Although changes in precipitation extremes are small and not significant over a large part of Japan, statistically significant increase in indices of heavy precipitation is found in western part of Japan and Hokkaido.