Parametrization of segregation effects due to convective boundary-layer mixing in atmospheric chemistry models

Abstract

We study chemical species that are not well-mixed in the convective boundary layer. This segregation occurs when the chemical lifetime of species is relatively short compared to the turbulent mixing time. Segregation in the convective boundary layer can have a significant effect on the evolution of the mean chemical concentrations. The case of reacting bottom-up and top-down diffusing species is considered. In meso-scale or global chemistry models segregation effects in the convective boundary layer have to be treated as sub-grid scale phenomena. Mass-flux schemes are particularly suitable for parametrizing sub-grib scale segregation effects. In this paper the results of local first-order closure, non-local first-order closure and mass-flux schemes for modelling transport of quickly reacting species in the convective boundary layer are compared to large-eddy simulation results. We find that the local first-order closure scheme generates too high gradients for the simulated case and therefore tends to segregate bottom-up and top-down diffusing reactive species in the vertical, which is an erroneous effect of the same order of magnitude (but with opposite sign) as the neglect of horizontal segregation due to turbulent convective mixing. For the non-local first-order closure scheme the gradients are closer to the large-eddy simulation results. However, the boundary-layer averaged concentrations are modelled worse due to the neglect of segregation effects. The mass-flux scheme incorporates both non-local transport effects and segregation effects and gives the best overall performance.