Comparison between archived and off-line diagnosed convective mass fluxes in the chemistry transport model TM3

The 40-year reanalysis data set ERA-40 from the European Centre for Medium-Range Weather Forecasts includes, unlike ERA-15, archived convective mass fluxes. These convective fluxes are useful for off-line chemistry transport modeling. The impact of using these archived convective mass fluxes (based on a convective parameterization described in Gregory et al. [2000] ) instead of off-line diagnosed mass fluxes (based on a convective parameterization described in Tiedtke [1989] ) was investigated with the chemistry transport model TM3. At first sight the two types of mass fluxes look similar. However, some differences can be noted: the archived updrafts extend higher than the off-line diagnosed ones; they are also less intense below 500 hPa over sea. The archived downdrafts are much weaker than the off-line diagnosed downdrafts. With archived convective mass fluxes, we found slightly higher ²²²Rn concentrations in the boundary layer, lower ²²²Rn values in the free troposphere and significantly higher ²²²Rn values in the upper troposphere and lower stratosphere. The effect on tropospheric chemistry of using archived mass fluxes instead of diagnosed ones is an increase of NO x and O3 in the free troposphere, but a decrease in the upper troposphere. The differences amount to up to 20% for O3 in the zonal and seasonal mean. Our results thus underline the sensitivity of tropospheric ozone chemistry to the description of convective transport. Comparison with ²²²Rn observations shows that the archived convective mass fluxes give better agreement in the tropical upper troposphere. More comparisons to free tropospheric observations of ²²²Rn or another tracer of convective transport will be needed to unambiguously identify either of the convective data sets as optimal for use in chemistry transport models.