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 t his study. The observed storm track is characterized by the time-averaged e ddy kinetic energy in the ECMWF analyses for Bears 1985-1992. RCM runs nest ed within these analyses are used to verify the ability of the model to sim ulate the storm track with "perfect" boundary conditions. Comparison is als o made with 20 years of the CSIRO9 Mk2 global climate model (GCM) and the R CM nested within this GCM, for equilibrium simulations with both current an d doubled CO2 levels. Correlations of the meridional wind with a point with in the storm track over Australia are used to characterize the structure of the mean baroclinic wave within the storm track. The location of the storm tracks in both the RCM and the GCM are simulated very well. The main defic iency 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 i n the GCM and the RCM simulations is very similar to observed. Under double d CO2 conditions, both the upper-level jetstream and storm track intensify, and move equatorward, in response to the increasing upper-troposphere nort h-south temperature gradient. In the lower troposphere, the mean and eddy k inetic energy decrease to the southeast of Australia in response to the dec rease in the north-south temperature gradient there. As a response to the g enerally decreasing mid-tropospheric north-south temperature gradient, the mean baroclinic wave appears to become slightly more barotropic, as indicat ed by less tilt in the zonal direction.