Comparison of satellite ozone observations in coincident air masses in early November 1994

Ozone observed by seven satellite instruments, the Atmospheric Trace Molecu le Spectroscopy instrument (ATMOS), Stratospheric Aerosol and Gas Experimen t (SAGE) II, Polar Ozone and Aerosol Measurement (POAM) II instrument, Halo gen Occultation Experiment (HALOE), Microwave Limb Sounder (MLS), Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA), and Mil limeter-wave Atmospheric Sounder (MAS), during early November 1994 is mappe d in equivalent latitude/potential temperature (EqL/theta) space to facilit ate nearly global comparisons of measurements taken in similar air masses. Ozone from all instruments usually agrees to within 0.5 ppmv (similar to5%) in the upper stratosphere and similar to0.25 ppmv in the lower stratospher e; larger differences in the midstratosphere are primarily due to sampling differences. Individual profile comparisons, selected to match meteorologic al conditions, show remarkably good agreement between all instruments that sample similar latitudes, although some small differences do not appear to be related to sampling differences. In the Southern Hemisphere (SH) midstra tosphere, the instruments (ATMOS, SAGE II, and FOAM II) with observations c onfined to high latitudes measured low EqLs in air drawn up from low latitu des that had formed a "low-ozone pocket"; they measured much lower ozone at low EqL than those that sampled low latitudes. A low-ozone pocket had also formed in the Northern Hemisphere (NH) midstratosphere (a month earlier th an this phenomenon has previously been reported), also resulting in differe nces between instruments based on their sampling patterns. FOAM If sampled only high latitudes in the NH, where extravortex sampling did not include t ropical, high-ozone air, and thus measured lower ozone at a given EqL than other instruments. Ozone laminae appear in coincident profiles from multipl e instruments, confirming atmospheric origins for these features and agreem ent in some detail between ozone observed by several instruments; reverse t rajectory calculations indicate such laminae arise from filamentation in an d around the polar vortices. Both EqL/theta and profile comparisons indicat e overall excellent agreement in ozone observed by all seven instruments in early November 1994. When care is taken to compare similar air masses and to understand sampling effects, much useful information can be obtained fro m comparisons between instruments with very different observing patterns.