Abstract
Abstract. We have extended ATTILA (Atmospheric Tracer Transport in a LAgrangian model), a Lagrangian tracer transport scheme, which is online coupled to the global ECHAM/MESSy Atmospheric Chemistry (EMAC) model, with a combination of newly developed and modified physical routines and new diagnostic and infrastructure submodels. The new physical routines comprise a parameterisation for Lagrangian convection, a formulation of diabatic vertical velocity, and the new grid-point submodel LGTMIX to calculate the mixing of compounds in Lagrangian representation. The new infrastructure routines simplify the transformation between grid-point (GP) and Lagrangian (LG) space in a parallel computing environment. The new submodel LGVFLUX is a useful diagnostic tool to calculate online vertical mass fluxes through horizontal surfaces. The submodel DRADON was extended to account for emissions and changes of 222Rn on Lagrangian parcels. To evaluate the new physical routines, two simulations in free-running mode with prescribed sea surface temperatures were performed with EMAC–ATTILA in T42L47MA resolution from 1950 to 2010. The results show an improvement of the tracer transport into and within the stratosphere when the diabatic vertical velocity is used for vertical advection in ATTILA instead of the standard kinematic vertical velocity. In particular, the age-of-air distribution is more in accordance with observations. The global tropospheric distribution of 222Rn, however, is simulated in agreement with available observations and with the results from EMAC in grid space for both Lagrangian systems. Additional sensitivity studies reveal an effect of inter-parcel mixing on the age of air in the tropopause region and the stratosphere, but there is no significant effect for the troposphere.
Highlights
Due to the increasing demand for including interactive tracers in climate simulations it is becoming necessary to use global models which meet the needs of a fast and exact tracer transport scheme
We described a comprehensively updated version of the LG tracer transport scheme ATTILA, including a new LG convection scheme and the option to use a diabatic instead of the standard kinematic vertical velocity
In this study we described and evaluated the updated LG tracer transport scheme ATTILA
Summary
Due to the increasing demand for including interactive tracers in climate simulations it is becoming necessary to use global models which meet the needs of a fast and exact tracer transport scheme. Describing the transport of tracers with an LG transport scheme has advantages compared to an Eulerian transport method: mass conservation (not in CLaMS) and the absence of numerical diffusion These advantages become most important if tracer distributions are inhomogeneous with strong vertical or horizontal gradients (Stenke and Grewe, 2005), which ought to be smoothed by physical and not by numerical diffusion processes. In the FLEXPART transport model (Stohl et al, 1998) the convection scheme relies on the ECMWF grid-scale temperature and humidity and provides a matrix for vertical convective particle displacement (Seibert et al, 2002; Forster et al, 2007) In the former (non-parallelised) version of ATTILA, convective tracer tendencies were calculated in grid-point space and transformed onto the parcels (Reithmeier and Sausen, 2002). We compare the LG simulation results with observations and with EMAC (GP) simulations, which were already evaluated by Jöckel et al (2016)
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