Evaluation of the new Australian climate model ACCESS

The Australian Community Climate and Earth System Simulator (ACCESS, see www.accessimulator.org.au) is a global coupled atmosphere-ocean climate model being developed by the Centre for Australian Weather and Climate Research (CAWCR, a partnership of the Bureau of Meteorology and CSIRO) in collaboration with the university community and the U.K. Met Office (UKMO). The atmospheric component is based on the UKMO's Unified Model (UM), and a high-resolution global version is now being used for weather prediction. A climate version of intermediate resolution (145 latitudes, 192 longitudes and 38 vertical levels) has been coupled to the GFDL MOM-4 ocean model, the CICE sea-ice code, and the CABLE land surface model using the OASIS coupler. This study focuses on the evaluation of simulations by both atmospheric and coupled model versions.

Inspired by the applications-driven approach to model evaluation developed by the Hadley Centre (UK), a comprehensive evaluation package in the python programming language is being developed. Its framework allows interoperability of model evaluation tools from the numerical weather, seasonal, and climate prediction communities. During each model run a standard set of scores are generated for each variable and observational data set. The final component is a set of common post-processing scripts to generate standard evaluation metrics such as the CPI (Murphy et al, 2004) and the targeted metric scheme from the UKMO (Banks et al, 2008). The evaluation has the aim of supporting both the development of the ACCESS model and its use in the next climate model intercomparison project, CMIP5.

Comparing climatological means of a range of variables from the new simulations indicates improved skill over previous models from both the Hadley Centre and CSIRO. The simulation of rainfall over Australia is of particular interest, and evaluation is a challenge given recent observational trends. Relationships between rainfall, temperatures and large-scale weather patterns have been examined. Daily rainfall is being further assessed in terms of the parameters of a gamma distribution model.