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Models and Experiments

CORDEX South Asia Models

The models used for CORDEX South Asia (listed in Table 1) describe the atmosphere and its coupling with the land surface. This ensemble includes four RCMs and a variable grid atmospheric GCM. The dynamics and physics set up for each model differ, but the regional domain at 50-km horizontal resolution is common to all RCMs as specified by the CORDEX experiment protocol (http://wcrp-cordex., covering West and South Asia region, and we present results within the interior model domain over the Indian sub-continent (e.g. Fig. 1). This ensemble consists of five 140-yr transient downscaled regional climate change simulations during the time period 1950-2100 and taking 6-hourly lateral and monthly ocean surface boundary conditions from four AOGCMs that participated in the CMIP5 RCP4.5 scenario experiments (listed in Table 1). It may be noted that two RCM experiments viz. CCLM4 (MPI) and REMO (MPI) were driven with the same MPI-ESM-LR AOGCM. Also RegCM411 RCM outputs are only available for the historical 1950-2005 period. The latest version of this RCM (RegCM445) includes an updated physics, in particular a more comprehensive representation of the land surface processes. The temporal evolution of the greenhouse gas concentrations is prescribed in the RCMs similar to that used by the CMIP5 AOGCMs, based on the observed values for the historical period and based on the RCP4.5 scenario for the future projection period. The remaining anthropogenic and natural forcings such as ozone and aerosols are kept constant. The regional land cover and land use changes are also not included in these downscaled climate simulations over South Asia.

The comparison of model results to the reference climate in the period 19762005 is presented to demonstrate how large biases the state-of-the-art RCMs shows when forced by lateral boundary conditions from AOGCMs. The quality of the simulations over India is assessed using the monthly mean 2-m air temperature and

a Summer monsoon

Fig. 1 a Summer monsoon (JJAS) season mean 2-m air temperature (°C; APHRODITE) for 1976-2005 and biases of 2-m air temperature (°C) in the CORDEX South Asia simulations driven by CMIP5 AOGCM historical experiments: b multi-model ensemble mean (ENSM) and c-h six different CORDEX RCMs listed in Table 1. Stippling denotes areas where the 30-year mean differences are not statistically significant at the 1 % level using Student’s t test the rain gauge-based global land precipitation dataset available at 0.5° spatial and monthly temporal resolution from the Asian Precipitation—Highly-Resolved Observational Data Integration Towards Evaluation of Water Resources (APHRODITE 1951-2005; Yatagai et al. 2012). This dataset is constructed to represent grid box average observations, allowing direct comparison to model results. The monthly model outputs from the CORDEX RCMs and the driving CMIP5 AOGCMs are bi-linearly interpolated to the APHRODITE spatial resolution. The seasonal averages for the Indian summer monsoon (June-September; JJAS) and post-monsoon (October-December; OND) months are computed. The annual averages (ANN) are also calculated. These seasonal and annual averages are computed for each year in 30-year periods both for the baseline period (1976-2005) and for a future time period (2031-2060). The Student’s t test at the 1 % significance level is used to determine the robustness for the climate change signal in the future period and for differences between the simulated and reference climate in the baseline period.

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