An Integrated clear air turbulence scheme for the FLEXPART model

Atmospheric turbulence above the planetary boundary layer (PBL) plays a critical role in the vertical and horizontal mixing of aerosols and trace gases. In the troposphere, such turbulence is highly intermittent and primarily associated with jet stream boundaries and planetary-scale waves, while in the stratosphere it is strongly modulated by the quasi-biennial oscillation. Owing to the long residence times of air masses in the stratosphere, vertical mixing across the tropopause and within the stratosphere is a key process controlling stratospheric composition. Accurate representation of stratospheric transport is also essential to understand the dispersion and lifetime of sulphur aerosols injected for potential solar radiation management applications.

Lagrangian atmospheric transport models commonly represent turbulent mixing using spatially and temporally constant diffusion coefficients, despite the inherently intermittent nature of turbulence in the free atmosphere. In this study, we implement a time- and space-dependent turbulent mixing scheme in the FLEXPART model, based on local diffusion coefficients derived from the Richardson number. This parameterization is consistent with the scheme used natively in the IFS model to represent turbulent exchange above the PBL.

Using a suite of sensitivity experiments, we investigate the impact of intermittent turbulent mixing on the distribution of trace gases in both the troposphere and stratosphere. Our approach provides a unified representation of turbulence from the boundary layer to the uppermost model levels, enabling a more physically consistent treatment of atmospheric mixing across dynamical regimes.