ATTILA: atmospheric tracer transport in a Lagrangian model

The model ATTILA has been developed to treat the global-scale transport of passive trace species in the atmosphere within the framework of a general circulation model (GCM). ATTILA runs online within the GCM ECHAM4 and advects the centroids of 80.000 to 190.000 constant mass air parcels. Each trace constituent is thereby represented by a mass mixing ratio in each parcel. ATTILA contains state-of-the-art parameterizations of convection. turbulent boundary layer mixing and inter-parcel transport. and provides all algorithm to map the tracer concentrations from the trajectories to the ECHAM model grid. The transport characteristics of ATTILA are evaluated against observations and the standard semi-Lagrangian transport scheme of ECHAM by two experiments. (1) We simulate the distribution of the short-lived tracer radon (Rn-222) in order to examine fast vertical transport over continents, and long-range transport from the continents to remote areas. (2) We simulate the distribution of radiocarbon (C-14) from nuclear weapon tests in order to examine tipper tropospheric and stratospheric transport characteristics. Contrary to the semi-Lagrangian scheme, ATTILA shows a greatly reduced meridional transport in the upper troposphere and lower stratosphere, and a reduced downward flux from the stratosphere to the troposphere, especially in mid-latitudes. Since ATTILA is a numerically non-diffusive scheme. it is able to maintain steep gradients, which compare better to the observations than the rather smooth gradients produced by the semi-Lagrangian scheme.