Global mass fixer algorithms for conservative tracer transport in the ECMWF model

TitleGlobal mass fixer algorithms for conservative tracer transport in the ECMWF model
Publication TypeMiscellaneous
Year of Publication2013
AuthorsDiamantakis, M, Flemming, J
Secondary TitleTechnical Memorandum

Various mass fixer algorithms have been implemented in the ECMWF Integrated Forecasting System (IFS). These algorithms are applied on tracer fields to maintain a globally mass conserving solution during the semi-Lagrangian advection step. In this report, the mass conservation properties of IFS are discussed and a description of the implemented mass fixer algorithms is given. Emphasis has been placed in implementing schemes that, despite being primarily global in nature, adjust the solution mostly in regions where the advected field has large gradients and therefore interpolation (transport) error is assumed larger. The algorithms have been tested in weather forecast, idealised and atmospheric dispersion cases. Results show that, in the context of 10-day weather forecasts, when these fixers are applied on specific humidity and cloud fields the impact on predictive skill is mostly neutral. Global mass tracer conservation is achieved without deteriorating the solution accuracy. However, for longer forecast timescales or for forecasts in which correlated species are transported, there are indications that the mass fixer approach may improve IFS forecasts. The above results are valid for hydrostatic regimes. Further testing is needed to quantify more accurately the impact of mass fixers on non-hydrostatic scales as well as on IFS based prediction systems for atmospheric composition and climate forecasts. Based on current results we make recommendations on which options to use for specific cases.