Measurements of water vapor distributions by the improved stratospheric and mesospheric sounder: Retrieval and validation

The Improved Stratospheric and Mesospheric Sounder (ISAMS) measured emissions from the 6.3-mu m band of water vapor using the pressure modulation technique, The particular form of the technique employed by ISAMS produces two signals: a pressure-modulated signal and a wideband signal. Water vapor retrievals from the wideband signal are presented here since data from the former are still under investigation, The data are also restricted to nighttime because of the significant nonthermal emission processes which govern the daytime water vapor emissions at this wavelength, In this paper the retrieval method for the wideband water vapor retrievals is discussed in detail and an internal error budget is derived based on results for version 9 of the ISAMS data set. Comparisons are also made with available coincident measurements during the ISAMS period of observations from September 1991 to July 1992, including data from other instruments on the Upper Atmosphere Research Satellite.