Overview of the Stratospheric Aerosol and Gas Experiment II water vapor observations: Method, validation, and data characteristics

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Abstract
Water vapor observations obtained from the Stratospheric Aerosol and Gas Experiment II (SAGE II) solar occultation instrument for the troposphere and stratosphere are presented and compared with correlative in situ measurement techniques and other satellite data. The SAGE II instrument produces water vapor values from cloud top to approximately 1 mbar, except in regions of high aerosol content such as occurs in the low to middle stratosphere after volcanic eruptions. Details of the analysis procedure, instrumental errors, and data characteristics are discussed. Various features of the data set for the first 5 years after launch (1985–1989) are identified. These include an increase in middle and upper tropospheric water vapor during northern hemisphere summer and autumn, thus at times of warmest sea surface temperature; minimum water vapor values of 2.5–3 ppmv in the tropical lower stratosphere, with lower values during northern hemisphere winter and spring; slowly increasing water vapor values with altitude in the stratosphere, reaching 5–6 ppmv or greater near the stratopause; extratropical values with minimum profile amounts occurring above the conventionally defined tropopause; and higher extratropical than tropical water vapor values throughout the stratosphere except in locations of possible polar stratospheric clouds. SAGE II data will be useful for studying individual water vapor profiles, tropospheric response to climate perturbations, tropospheric‐stratospheric exchange (due to its inherent high vertical resolution), and stratospheric transports. It should also aid in the preparation, for the first time on a global scale, of climatologies of the stratosphere and the upper level cloud‐free troposphere, for use in radiative, dynamical, and chemical studies.