Storm tracks in regional climate simulations: verification and changes with doubled CO2

The ability of the CSIRO Atmospheric Research’s regional climate model (RCM) to simulate the mean July storm track over Australia is investigated in this study. The observed storm track is characterized by the time-averaged eddy kinetic energy in the ECMWF analyses for years 1985–1992. RCM runs nested within these analyses are used to verify the ability of the model to simulate the storm track with “perfect” boundary conditions. Comparison is also made with 20 years of the CSIRO9 Mk2 global climate model (GCM) and the RCM nested within this GCM, for equilibrium simulations with both current and doubled CO₂ levels. Correlations of the meridional wind with a point within the storm track over Australia are used to characterize the structure of the mean baroclinic wave within the storm track. The location of the stormtracks in both the RCM and the GCM are simulated very well. The main deficiency is the lack of intensity of the upper-level storm track, but the RCM is more realistic than the GCM, indicating that the intensity of the storm track is partly related to horizontal resolution. The mean wave structure in the GCM and the RCM simulations is very similar to observed. Underdoubled CO₂ conditions, both the upper-level jetstream and storm track intensify, and move equatorward, in response to the increasing upper-troposphere north–south temperature gradient. In the lower troposphere, the mean and eddy kinetic energy decrease to the southeast of Australia in response to the decrease in the north–south temperature gradient there. As a response to the generally decreasing mid-tropospheric north–south temperature gradient, the mean baroclinic wave appears to become slightly more barotropic, as indicated by less tilt inthe zonal direction.