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The Brewer–Dobson circulation (BDC) is a stratospheric circulation characterized by upwelling of tropospheric air in the tropics, poleward flow in the stratosphere, and downwelling at mid and high latitudes, with important implications for chemical tracer distributions, stratospheric heat and momentum budgets, and mass exchange with the troposphere. As the photochemical losses of nitrous oxide (N2O) are well known, model differences in its rate of change are due to transport processes that can be separated into the mean residual advection and the isentropic mixing terms in the transformed Eulerian mean (TEM) framework. Here, the climatological impact of the stratospheric BDC on the long-lived tracer N2O is evaluated through a comparison of its TEM budget in the Whole Atmosphere Community Climate Model (WACCM), in a chemical reanalysis of the Aura Microwave Limb Sounder version 2 (BRAM2) and in a chemistry transport model (CTM) driven by four modern reanalyses: the European Centre for Medium-Range Weather Forecasts Interim reanalysis ERA-Interim; , the Japanese 55-year Reanalysis JRA-55; , and the Modern-Era Retrospective analysis for Research and Applications version 1 MERRA; and version 2 MERRA-2;. The effects of stratospheric transport on the N2O rate of change, as depicted in this study, have not been compared before across this variety of datasets and have never been investigated in a modern chemical reanalysis. We focus on the seasonal means and climatological annual cycles of the two main contributions to the N2O TEM budget: the vertical residual advection and the horizontal mixing terms. The N2O mixing ratio in the CTM experiments has a spread of approximately ∼20% in the middle stratosphere, reflecting the large diversity in the mean age of air obtained with the same CTM experiments in a previous study. In all datasets, the TEM budget is closed well; the agreement between the vertical advection terms is qualitatively very good in the Northern Hemisphere, and it is good in the Southern Hemisphere except above the Antarctic region. The datasets do not agree as well with respect to the horizontal mixing term, especially in the Northern Hemisphere where horizontal mixing has a smaller contribution in WACCM than in the reanalyses. WACCM is investigated through three model realizations and a sensitivity test using the previous version of the gravity wave parameterization. The internal variability of the horizontal mixing in WACCM is large in the polar regions and is comparable to the differences between the dynamical reanalyses. The sensitivity test has a relatively small impact on the horizontal mixing term, but it significantly changes the vertical advection term and produces a less realistic N2O annual cycle above the Antarctic. In this region, all reanalyses show a large wintertime N2O decrease, which is mainly due to horizontal mixing. This is not seen with WACCM, where the horizontal mixing term barely contributes to the TEM budget. While we must use caution in the interpretation of the differences in this region (where the reanalyses show large residuals of the TEM budget), they could be due to the fact that the polar jet is stronger and is not tilted equatorward in WACCM compared with the reanalyses. We also compare the interannual variability in the horizontal mixing and the vertical advection terms between the different datasets. As expected, the horizontal mixing term presents a large variability during austral fall and boreal winter in the polar regions. In the tropics, the interannual variability of the vertical advection term is much smaller in WACCM and JRA-55 than in the other experiments. The large residual in the reanalyses and the disagreement between WACCM and the reanalyses in the Antarctic region highlight the need for further investigations on the modeling of transport in this region of the stratosphere. [ABSTRACT FROM AUTHOR]

The stratospheric circulation is investigated over the period 2005-2015 using WACCM4 (Whole Atmosphere Community Climate Model version 4), together with BASCOE CTM (Chemistry-Transport Model; Chabrillat et al., 2018) and a reanalysis of stratospheric composition observed by MLS: BRAM2 (BASCOE Reanalysis of AURA MLS release 2).We use long-lived tracers (N2O, H2O) and the Transformed Eulerian Mean framework (Abalos et al., 2017) to analyse the tracers budget including the mean analysis increment for BRAM2. We focus on the annual and seasonal June-July-August mean of the advection and eddy mixing terms and on the impact of the Quasi-Biennial Oscillation (QBO).For both tracers, the annual mean pattern is in general agreement among the three datasets but larger differences are found in JJA. Eddy mixing in WACCM (free dynamics and free chemistry) is smaller and has less variability than in the CTM (constrained dynamics and free chemistry) and in BRAM2 (constrained dynamics and chemistry). In addition, the SH polar transport barrier in WACCM is weaker with respect to the CTM and BRAM2.The next step of our research is to perform this analysis with the newer version of WACCM (version 6) as well as additional BASCOE CTM runs using different reanalysis products or short-term forecasts. Multi-decadal changes in the terms of the budget, and their space dependence, will be investigated as well.ReferencesChabrillat, S., et al.: Comparison of mean age of air in five reanalyses using the BASCOE transport model, Atmos. Chem. Phys., 18(19), 14715–14735, doi:10.5194/acp-18-14715-2018, 2018.Abalos, M., W.J. Randel, D.E. Kinnison, and R.R. Garcia, 2017: Using the Artificial Tracer e90 to Examine Present and Future UTLS Tracer Transport in WACCM. J. Atmos. Sci., 74, 3383–3403, https://doi.org/10.1175/JAS-D-17-0135.1 [ABSTRACT FROM AUTHOR]

The Copernicus Atmospheric Monitoring Service (CAMS) provides global forecasts and analyses of trace gases using the Integrated Forecast System (IFS) as operated by ECMWF. The operational chemistry module of the IFS relies on a tropospheric chemistry scheme using a modified version of the CB05 reaction mechanism, complemented with a linear ozone parameterization for the stratosphere. In recent years various updates to this module have been developed. In particular, a stratospheric chemistry module based on the BASCOE chemistry has been coupled to this module. Also two alternative modules have been implemented, based on MOCAGE and MOZART chemistry. Any of the modules have been reviewed thoroughly, resulting in updates in parameterizations for reaction rates, heterogeneous chemistry, photolysis and numerical solver. Also the option to use an online parameterization for dry deposition is currently supported. As a result, these three, largely independent, modules for atmospheric chemistry are currently available for sensitivity testing, and may as such provide a mini-ensemble of global atmospheric trace gases. Also, with mature chemistry modules within the IFS, the modeling of secondary organic and inorganic aerosol formation is receiving more attention. An example is the development of a secondary organic aerosol module depending on precursor trace gases originating from aromatics emissions.In this contribution we provide an overview of the current status of the atmospheric chemistry modeling capabilities in the operational, pre-operational, as well as experimental versions of the IFS. We focus on the range of available chemistry modules, their recent revisions, and their relevance within the scope of ECMWF's integrated chemistry-aerosol-meteorology modeling. [ABSTRACT FROM AUTHOR]