Developing scenarios of climate change for Southeastern Australia: an example using regional climate model output

Regional climate models nested in global climate models are now being used in Australia to produce high resolution climate change scenarios for use in impact assessments. This paper describes the development of such scenarios for the State of Victoria in Southeastern Australia using the CSIRO Division of Atmospheric Research Limited Area Model (DARLAM) at a horizontal resolution of 60 km nested in the slab ocean version of the CSIRO global climate model (GCM). Rainfall and temperature over Victoria are substantially better simulated in DARLAM than they are in the host GCM, although some poor features of the simulation of the seasonal cycle of rainfall have not been improved with the increase in resolution. There is evidence to suggest that some of these errors are due to errors in the simulation of atmospheric circulation provided by the host GCM. Under 2 x CO2 conditions, DARLAM simulates patterns of rainfall and temperature change which differ significantly from those provided by the host GCM and which appear more physically plausible. In particular, winter rainfall in southern Victoria increases in the DARLAM simulation, but decreases in the host GCM, Although the DARLAM scenario could be considered the best single scenario available for Victoria at the time of its production, it is noted that different, but equally plausible, scenarios could be obtained by nesting DARLAM in other GCMs, or by increasing the simulation sample size. Our assessment is that the rainfall changes simulated by DARLAM in summer may be most sensitive to such changes. In general, it is concluded that there is great potential to increase the realism of climate change scenarios in the Victorian region through careful use of nested regional modelling.