A Lagrangian transport core for the simulation of stratospheric trace species in a chemistry climate model

Lagrangian transport schemes have proven to be useful tools for modelling stratospheric trace gas transport since they are less diusive than classical Eulerian schemes and therefore especially well suited for maintaining steep tracer gradients as observed in the atmosphere. Here, the implementation of the full-Lagrangian transport core of the Chemical Lagrangian Model of the Stratosphere (CLaMS) in the ECHAM/MESSy Atmospheric Chemistry model (EMAC) is presented. A ten-year time-slice simulation was performed to evaluate the coupled model system EMAC/CLaMS. Simulated zonal mean age of air distributions were compared to the age of air derived from airborne measurements, showing the expected characteristics of the stratospheric circulation. Climatologies of long-lived tracers (CFC-11 (CCl3F), CFC-12 (CCl2F2), CH4, N2O) were calculated using the standard ux-form semi-Lagrangian transport scheme (FFSL) in EMAC, as well as the new CLaMS Lagrangian transport scheme. The climatologies were compared both to each other and also to satellite measurements of trace gases. The dierences in the resulting tracer distributions are most pronounced in the regions of strong transport barriers, namely the edge of the tropical pipe, the tropopause, and the edge of the polar vortex. These regions were analysed in detail and show improved results using the Lagrangian transport scheme, with stronger gradients at the respective transport barriers. The analyses of various trace gases and age of air in the polar vortex regions shows that the CLaMS Lagrangian transport scheme produces a stronger, more realistic transport barrier at the edge of the polar vortex than the FFSL transport scheme of EMAC. Dierences in simulated age of air are in the range of up to one year in the Arctic polar vortex in late winter/early spring. The newly coupled model system EMAC/CLaMS thus constitutes a suitable tool for future model studies, e. g. for the simulation of polar ozone depletion, based on a sophisticated stratospheric tracer transport.