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Validation of SAGE III Water Vapor

The Stratospheric Aerosol and Gas Experiment III on the International Space Station (SAGE III/ISS) was launched on 19 February 2017 and began routine operation in June 2017. In this study, the vertical profiles of water vapor (WV) from SAGE III/ISS beta version 5.1 have been evaluated by inter-comparison with other satellite and balloon based instruments and methods for flagging and filtering the data have been produced.

The figure below shows median absolute differences of WV between SAGE III/ISS and MLS or ACE-FTS for coincident profiles. SAGE data shows drier values below 30 km as compared to ACE-FTS. Above 30 km comparison of SAGE with ACE-FTS version 3.6 and version 4.1 differ significantly. ACE-FTS version 3.6 has been used further in this study because it has been more extensively validated.

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In order to study the effects of geographic variations in the comparisons, the data has been plotted as a function of altitude versus latitude in the figure below. SAGE III/ISS is dry-biased relative to ACE-FTS in the stratosphere, with a decrease in this bias at higher altitudes. SAGE III/ISS shows wetter values at low latitudes and drier values at high latitudes, in comparison to ACE-FTS. This could be due to the low number of matching profiles in the tropics (N = 74, 30°S–30°N) in comparison to high latitudes (N = 658 for |lat| > 30°).

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In general, WV data from SAGE III/ISS v5.1 from 20 to 40 km, exhibit a bias of 0.0 to −0.5 ppmv (∼10%) relative to ACE-FTS v3.6, and this also depends on the altitude. Even though SAGE III/ISS WV is a beta version, it shows good agreement with independent data and shows similarity to previous SAGE instruments, and therefore the data are suitable for extending stratospheric WV data records.

Reference:

Davis, S. M., Damadeo, R., Flittner, D., Rosenlof, K. H., Park, M., Randel, W. J., et al.: Validation of SAGE III/ISS solar water vapor data with correlative satellite and balloon-borne measurements. Journal of Geophysical Research: Atmospheres, 126, e2020JD033803. https://doi.org/10.1029/2020JD033803, 2021.