Simulation of extratropical synoptic-scale stratosphere-troposphere exchange using a coupled chemistry GCM: Sensitivity to horizontal resolution

The transport of ozone between the stratosphere and troposphere has been studied with a coupled chemistry GCM (ECHAM4) for two horizontal resolutions (T30: 3.75 degrees x 3.75 degrees and T63: 1.875 degrees x 1.875 degrees). The meteorological conditions pertain to the first half of March 1996, when a large amplification of waves in the upper troposphere resulted in the development of two upper-level subtropical cyclonic vortices in the Northern Hemisphere over the Atlantic Ocean. The focus is on the sensitivity of the model simulations to the horizontal model resolution. A finer, more detailed structure was obtained for the calculated meteorological fields (especially the "streamers" of high potential vorticity (PV) and low specific humidity associated with synoptic disturbances) when the higher resolution was applied. In quantitative terms, main differences were found in the upper-tropospheric PV values and the jet stream maxima (higher at T63). Overall, the higher-resolution results were in excellent agreement with European Centre for Medium-Range Weather Forecasts analysis fields (at T106 resolution: 1.125 degrees x 1.125 degrees). Backward trajectory analysis provided complementary information on the history of air masses associated with vertical transports in the vicinity of the vortices. Modeled ozone concentrations show a pronounced increase in the upper troposphere, and their spatial and temporal evolution is in good agreement with the development of the cutoff lows (in both resolutions). However, the calculated concentrations increase with increasing resolution, while a stronger vertical mixing is evident at higher resolution, particularly in areas associated with tropopause folds. The finer resolution improved (overall) the simulation of vertical ozone profiles, but not very significantly. Finally, refinement of the horizontal resolution from T30 to T63 leads to a similar to 12% increase in the amount of stratospheric ozone transferred into the troposphere during the particular synoptic event.