Radon global simulations with the multiscale chemistry and transport model MOCAGE

We present an evaluation of the representation of subgrid scale transport in the new multiscale global chemistry and transport model MOCAGE. The approach is an off-line computation of vertical mass fluxes due to convective and turbulent processes, using only large-scale variables archived in meteorological analyses. Radon is a naturally emitted as with a radioactive half-life of 3.8 days and is a useful tracer of tropospheric transport processes. A 1-yr (1999) simulation of atmospheric radon concentration has been performed, using 6-hourly meteorological analyses for the forcings. Two different mass flux convection schemes have been tested: a simplified version of the Tiedtke (1989) scheme and Kain-Fritsch-Bechtold (Bechtold et al., 2001). We compare model outputs with observations at different time and space scales, showing good overall results. A new interpretation is given to the more contrasted results obtained in Antarctica, as for other models. The state-of-the-art representation of synoptic scale activity around Antarctica is markedly worse than in other parts of the world, both due to oversimplifications of the seasonal evolution of the extent of sea ice, and to the scarcity of observations. Twelve-hourly simulated concentrations are evaluated at two sites for 1999. At Amsterdam Island results are satisfactory: correlation between observed and modelled concentrations is of the order of 0.5. The model reproduces well "radonic storm" events. At the coastal site of Mace Head in Ireland, simulations are available at two different horizontal resolutions. The correlation between observations and the model is of the order of 0.7. This result is mainly determined by the synoptic scale context, even though local-scale circulations such as breezes interfere on occasions. Finally, it appears that the off-line approach in MOCAGE for subgrid transport is a practical one for chemistry and transport multiscale modelling.