The influence of subgrid variability on vertical transport of a chemical species in a deep convective environment

The outflow from deep convective clouds is an important region for new particle formation, resulting partly from the rapid venting of gas-phase precursors out of the boundary layer (BL) by cloud updraught. Over the remote marine atmosphere, dimethyl sulfide (DMS) is one such precursor gas, important in the formation of sulfate aerosol. In modelling this transport out of the BL, global chemical transport models resolve scales larger than that of a typical convective cloud event. Consequently, the vertical transport of chemical species is parameterized. However, the inability to resolve certain subgrid variabilities in the BL, in both the dynamical and chemical fields, may also have consequences for estimates of vertical transport. In this study we use a 2-D cloud-resolving model (CRM) to examine the influence of such subgrid-scale features on the concentration and vertical transport of DMS in a deep convective environment. Two issues are highlighted. Firstly, deriving fluxes of DMS from the ocean surface using a mean surface wind representative of a global model reduces the domain-mean DMS concentration by approximately 50%. Emission of DMS from the sea surface is greater in the CRM because it resolves the localized high wind speeds associated with convective activity. Secondly, we find that the spatial pattern of DMS concentration in the boundary layer is positively correlated with the pattern of convective updraughts. Using a mean DMS concentration field reduces vertical transport to the upper troposphere by more than 50%. The explanation is that secondary convection occurs preferentially on the edges of spreading cold pools, where DMS concentrations are higher than the domain mean. The results presented here may also suggest a coupling between cloud and aerosol in which the clouds themselves create fields of aerosol that may then feedback on cloud microphysics and thus cloud development.