Comparison of Simulated and Daily Measured Air Concentrations of 210Pb and 7Be at Tahiti

Transport and deposition of terragenic 210Pb and cosmogenic 7Be are simulated by the GCM model, LMDZ, developed at the Laboratoire de Météorologie Dynamique in Paris. The model calculates over the globe, eulerian large-scale advection based upon finite-volume methods and parameterization of turbulent mixing and convection. Simulations with a horizontal resolution of 2° and 19 levels vertically, are carried out in a nudged mode for the 2004 year. Transport is based on mass conservation of the aerosol in the atmosphere, taking into account radioactive decay, dry and wet deposition. Time series of 7Be and 210Pb air concentrations at Tahiti station are reproduced with reasonable accuracy by the model, although the island is poorly described in the model. This resolution allows us to study the respective influence of convective and stratiform precipitation as well as dry deposition. Simulated wind fields are then compared with the ones calculated by the mesocale model WRF using 4 nested grids with resolution ranging from 50 km to 2 km. Finally, a variable mesh (by stretching grid coordinates) in LMDZ is used to increase resolution over Tahiti. Sensitivity tests show that data are much improved with resolution as fine as 0.5° by 0.5° in Tahiti region