Abstract
Abstract. Herein, we provide an assessment of the data quality of Stratospheric Aerosol and Gas Experiment (SAGE III) Version 4 aerosol extinction coefficient and water vapor data products. The evaluation is based on comparisons with data from four instruments: SAGE II, the Polar Ozone and Aerosol Measurement (POAM III), the Halogen Occultation Experiment (HALOE), and the Microwave Limb Sounder (MLS). Since only about half of the SAGE III channels have a direct comparison with measurements by other instruments, we have employed some empirical techniques to evaluate measurements at some wavelengths. We find that the aerosol extinction coefficient measurements at 449, 520, 755, 869, and 1021 nm are reliable with accuracies and precisions on the order of 10% in the mission's primary aerosol target range of 15 to 25 km. We also believe this to be true of the aerosol measurements at 1545 nm though we cannot exclude some positive bias below 15 km. We recommend use of the 385 nm measurements above 16 km where the accuracy is on par with other aerosol channels. The 601 nm measurement is much noisier (~20%) than other channels and we suggest caution in the use of these data. We believe that the 676 nm data are clearly defective particularly above 20 km (accuracy as poor as 50%) and the precision is also low (~30%). We suggest excluding this channel under most circumstances. The SAGE III Version 4 water vapor data product appears to be high quality and is recommended for science applications in the stratosphere below 45 km. In this altitude range, the mean differences with all four corroborative data sets are no bigger than 15% and often less than 10% with exceptional agreement with POAM III and MLS. Above 45 km, it seems likely that SAGE III water vapor values are increasingly too large and should be used cautiously or avoided. We believe that SAGE III meets its preflight goal of 15% accuracy and 10% precision between 15 and 45 km. SAGE III water vapor data does not appear to be affected by aerosol loading in the stratosphere.
Highlights
The Stratospheric Aerosol and Gas Experiment (SAGE III) continues the long SAGE series of solar occultation instruments (SAGE, 1979–1981; SAGE II, 1984–2005) that produce high vertical resolution ozone and multi-wavelength stratospheric aerosol extinction coefficient profiles
Lidar is possibly useful for heavily loaded periods but for the SAGE III lifetime, stratospheric aerosol levels were at historic lows and corresponding lidar backscatter ratios were often less than 1.04 and converting from a backscatter measurement to extinction is not trivial considering the level of precision/accuracy required for validation
For SAGE III/POAM III comparisons, we find an agreement better than 20% above 20 km (POAM III greater than SAGE III) that increases at lower altitudes to 50% at 15 km and the standard deviation is fairly constant at about 50%
Summary
The Stratospheric Aerosol and Gas Experiment (SAGE III) continues the long SAGE series of solar occultation instruments (SAGE, 1979–1981; SAGE II, 1984–2005) that produce high vertical resolution ozone and multi-wavelength stratospheric aerosol extinction coefficient profiles. An evaluation of SAGE III Version 3 lunar occultation ozone, NO2, and NO3 data products can be found in Wang et al (2009). In June 2008, Version 4 SAGE III data products were released including the initial release of a water vapor data product. We evaluate the aerosol extinction coefficients and water vapor data products through comparison with independent data sets from other spaceborne instruments and, in the case of the aerosol product, internal self-consistency. Lunar occultation does not include aerosol or water vapor data products and is not discussed in this paper
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