STRATOSPHERIC AEROSOL AND GAS EXPERIMENT-II UMKEHR OZONE PROFILE COMPARISONS

This study compares 1789 pairs of ozone profiles derived from 1384 Umk ehr observations at 14 different stations and 1163 Stratospheric Aeros ol and Gas Experiment (SAGE) II profiles coincident within 1000 km and 14 hours between October 1984 and April 1989. The comparison indicate s the following significant percentage differences (SAGE II-Umkehr)/Um kehr with 2 x standard errors of the mean: Umkehr layer 4, (18.3 +/- 0 .8)%; layer 5, (-1.6 +/- 0.4)%; layer 6, (-6.2 +/- 0.5)%; layer 7, (0. 8 +/- 0.6)%; layer 8, (7.7 +/- 0.6)%; and layer 9, (12.0 +/- 1.1)%. Di fferences in layers 4 and 6 are due at least in part, to inaccurate Um kehr climatologies. Average SAGE II/Umkehr differences in layers 5 thr ough 9 at individual stations are generally less than 10%. While the U mkehr retrievals are known to be sensitive to aerosol interference, th e mean layer 8 correction during the period of this study is estimated to be only 2% with large station-to-station variability. The correcti on in lower layers is smaller. We have chosen to ignore the small Umke hr aerosol correction in this study. The mean difference would decreas e if Umkehr profiles were: corrected for a priori profile effects calc ulated by DeLuisi et al. (1989a). However, using the newer Bass and Pa ur (1985) ozone absorption cross sections would tend to increase the d ifferences at most levels. The profile of mean differences is similar to previously observed differences between Umkehr and solar backscatte red ultraviolet (SBUV) observations. Comparing SAGE II/Umkehr differen ces to SAGE I/Umkehr differences at Seven common stations shows a bias of -4% at the ozone peak (layer 4). This bias increases with altitude to 8% in layer 8 and 15% in layer 9, with SAGE II ozone partial press ures higher than or equal to those of SAGE I (version 6.1) relative to Umkehr in all layers above 4. A systematic upward reference-altitude shift between 0.25 and 0.50 km for SAGE I, similar to the quoted uncer tainty, would increase SAGE I ozone 4% to 8% in layer 8 and would resu lt in similar SAGE and Umkehr ozone trends during the 1980. Cross corr elations of numerous variables associated with the Umkehr and SAGE II data sets show a minimum correlation between SAGE II and Umkehr ozone partial pressures in layers 5, 8, and 9. This correlation is a result similar to the one previously noted in other comparisons against Umkeh r data. We discuss these minimum correlations in relationship to the s easonal cycle in ozone and synoptic scale variations at midlatitudes b ased on model results. Substantial differences between SAGE II and Umk ehr exist in both the mean and the variability of ozone in layers 8 an d 9. Substantial differences also exist in layer 6 where the Umkehr al gorithm does not retrieve the low atone values periodically observed b y SAGE II during winter.