The contribution of mixing in Lagrangian photochemical predictions of polar ozone loss over the Arctic in summer 1997

Measurements from the Halogen Occultation Experiment, together with assimil ated winds, temperatures, and diabatic heating rates from the NASA Goddard data assimilation office, are used in the NASA Langley Research Center traj ectory-photochemical model to compute photochemistry along three-dimensiona l air parcel trajectories for the Northern Hemisphere for the period March through September 1997. These calculations provide a global perspective for the interpretation of constituent measurements made fram the ER-2 platform during the Photochemistry of Ozone Loss in the Arctic Region in Summer air craft campaign. An important component of the model is a parameterization o f sub-grid-scale diffusive mixing. The parameterization uses an "n-member m ixing" approach which includes an efficiency factor that enhances the mixin g in regions where strain dominates the large-scale flow. Model predictions of O-3 and CH4 are compared with in situ measurements made from the ER-2. Comparison of the in situ data with model predictions, conducted with and w ithout diffusive mixing, illustrates the contribution that irreversible mix ing makes in establishing observed tracer-tracer correlations. Comparisons made for an ER-2 flight in late April 1997 show that irreversible mixing wa s important in establishing observed tracer-tracer correlations during spri ng 1997. Comparisons made in late June 1997, when filaments of very low N2O and CH4 were observed, indicate that remnants of air from the polar vortex survived unmixed in the low stratosphere 6 weeks after the breakup of the polar vortex in May. The results demonstrate that the sub-grid-scale mixing parameterization used in the model is effective not only for strong mixing conditions in late winter and early spring, but also for relatively weak m ixing conditions that prevail in summer.