An analogue-based method to downscale surface air temperature: application for Australia

 A statistical model (SM) has been developed to downscale large-scale predictors given by general circulation models (GCMs); emphasis has been put on local surface air temperature in two areas of interest: the south-west corner (SWC) of Western Australia and the Murray-Darling Basin (MDB) in southeastern Australia. This is a complementary approach to the dynamical modelling of climate change using high-resolution nested regional models. The analogue technique was chosen for this study as it has proven successful in the past for mid-latitude climate and, in particular, for forecasting in Australia. Furthermore, the analogue technique is successful in reproducing spells of anomalous events. The development and validation datasets used for both predictors and predictants cover the 1970–1993 period. Predictors are extracted from a dataset of operational analyses for the Australian region. Several predictors have been assessed alone and combined. Mean sea level pressure and temperature at 850 hPa have been identified as the most useful combination. Predictants have come from quality controlled stations with daily temperature extremes for the 1970–1993 period. Twenty two stations in the SWC and 29 in the MDB have been selected. The sensitivity of the SM has been tested to several internal parameters. The number of atmospheric predictors and the geographical domain on which large-scale fields are used are key factors that maximise the skill of the SM. Several metrics have been tested taking into account the state of the predictors on the day or, in order to describe the evolution of the atmosphere, over several days. This latter has been particularly useful in improving the representation of anomalous spells as it partially incorporates the auto-correlation of surface temperature. The correlation obtained between the observed local temperature series and the reconstructed series ranges between 0.5 and 0.8. Best results are obtained in summer and for maximum temperature. The reproduction of spells is satisfactory for most stations. The SM is then applied to large-scale fields obtained from a GCM forced by observed sea surface temperature; the improvement gained when using the SM instead of relying on the surface temperature calculated by the GCM is shown. Received: 14 December 1999 / Accepted: 10 January 2001