Summertime impact of convective transport and lightning NO_x production over North America: modeling dependence on meteorological simulations

Abstract. Global-scale chemical transport model simulations indicate lightning NO_x dominates upper tropospheric O₃ production above Eastern North America during summertime but vary in their estimates. To improve our understanding, a regional-scale model (REAM) with higher resolution is applied. To examine the uncertainties in modeling the impact of convective transport and lightning NO_x production on upper tropospheric chemical tracer distributions, REAM simulations of chemical tracers are driven by two meteorological models, WRF and MM5, with different cumulus convective parameterizations. The model simulations are evaluated using INTEX-A aircraft measurements and satellite measurements of NO₂ columns and cloud top pressure, and we find that mid and upper tropospheric trace gas concentrations are affected strongly by convection and lightning NO_x production. WRF with the KF-eta convection scheme simulates larger convective updraft mass fluxes below 150 hPa than MM5 with the Grell scheme. The inclusion of the entrainment and detrainment processes leads to more outflow in the mid troposphere in WRF than MM5. The ratio of C₂H₆/C₃H₈ is found to be a sensitive parameter to convective outflow; the simulation by WRF-REAM is in closer agreement with INTEX-A measurements than MM5-REAM, implying that convective mass fluxes by WRF are more realistic. WRF also simulates lower cloud top heights (10–12 km) than MM5 (up to 16 km), and hence smaller amounts of estimated (intra-cloud) lightning NO_x and lower emission altitudes. WRF simulated cloud top heights are in better agreement with GOES satellite measurements than MM5. Simulated lightning NO_x production difference (due primarily to cloud top height difference) is mostly above 12 km. At 8–12 km, the models simulate a contribution of 60–75% of NO_x and up to 20 ppbv of O₃ from lightning, although the decrease of lightning NO_x effect from the Southeast to Northeast and eastern Canada is overestimated. The model differences and biases found in this study reflect some major uncertainties of upper tropospheric NO_x and O₃ simulations driven by those in meteorological simulations and lightning parameterizations.