New Studies On SAGE II And HALOE Comparisons

Ozone is an important atmospheric constituent that shields the Earth's surface from harmful ultraviolet radiation, and also plays a critical role in radiation forcing, thus affecting climate change. Both the Stratospheric Aerosol and Gas Experiment II (SAGE II) and the Halogen Occultation Experiment (HALOE ) employ the solar occultation technique to obtain unique information about ozone vertical profiles, primarily in the stratosphere. Examination of the interconsistency between data records is critical in building confidence in the measurements provided by these experiments. A comparison of the ozone data sets from SAGE II using its version 6.1 processing code and HALOE using its version 19 processing code is discussed for the period 1991 to 2000. The analysis of weighted mean and weighted root mean square differences in near coincident measurements by the two experiments shows an approximately 1% difference over the altitudes 18 to 50 km. We also examine the time dependence of the bias between measurements of the two instruments. Our results indicate statistically insignificant slopes, less then 0.3% year^-1, in the time series of differences between the two measurement systems over 20 to 50 km, with few exceptions. Thus no long-term calibration problems were revealed in SAGE II or HALOE. We also give an estimate of the long-term changes in SAGE II and HALOE ozone data sets by fitting weighted monthly zonal averages to a model including linear, annual, semi-annual, Quasi-Biennial Oscillation (QBO), Solar Cycle and autoregressive noise terms. We fit the above mentioned linear regression model using the weighted least squares approach. The comparison of the trends in ozone data from these instruments shows trend differences on the order of less then 0.3% year^-1 in a majority of latitude bands at 25, 35, 45 and 55 km altitudes.