A positive definite tracer mass fixer for high resolution weather and atmospheric composition forecasts

Recent high-resolution experiments with atmospheric composition forecasts using the IFS Bermejo & Conde tracer mass fixing scheme, revealed cases where the positive definiteness of the transport scheme was violated with negative values being introduced in the tracer. This occurs in near surface atmospheric levels in areas where the tracer substance is injected in the atmosphere, for example CO₂ and CH₄ anthropogenic emissions. Such emissions introduce a sharp increase in the concentration of the tracer field and, as a result of this, the asymptotic assumptions that guarantee monotonicity of the mass fixing scheme break down. In this report, the implemented in IFS Bermejo & Conde scheme is revisited and modified to guarantee positive definiteness. Alternative formulations are described which may have advantages for tracer fields which are localised and discontinuous in nature unlike long-lived carbon tracers which are well mixed with air and have large background values. The modified scheme is tested on idealised case studies, weather forecast cases and atmospheric composition forecasts of CO₂ and CH₄. Results from these tests re-enforce previous conclusions, showing clear improvements for simulations of CO₂ and CH₄ tracers and neutral results in terms of forecast skill when the mass fixer is applied to specific humidity field. The impact on water vapour transport of two quasi-monotone interpolation limiters available in IFS is also analysed and their aparent “moistening effect” is explained.