A first-order closure for covariances and fluxes of reactive species in the convective boundary layer

Covariances and fluxes of reactive species in the clear convective atmosphe ric boundary layer (CABL) are studied and parameterized. The covariances re sult from correlations between reactive species. These covariances may have a considerable influence on the modeled reaction rates in atmospheric chem istry models, but usually are neglected. To facilitate the representation o f covariance effects in large-scale atmospheric chemistry models, the autho rs have developed a new first-order closure for covariances, The closure is based on top-hat distributions, as is common in mass-flux schemes. In addi tion, the authors utilize an existing nonlocal first-order closure expressi on for the flux, which represents the combined effects of gradient mixing a nd nonlocal convective mixing. The authors shaw how the latter also include s the impact of chemistry on the nonlocal flux contribution. The impact of the closures is illustrated first for artificial, simple chemistry cases. T he results are evaluated using large-eddy simulation (LES). By comparing re sults for the entraining and solid-lid CABL it is established that the cova riance closure works satisfactorily away from the inversion. Subsequently, the closures are evaluated against LES for a photochemical case with 10 rea ctions involving six modeled species. The accuracy of the modeled covarianc es is found to be within a factor of 2, which is sufficient to improve the modeled concentrations.