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Published December 2, 2009 | Published
Journal Article Open

Validation of the Harvard Lyman-α in situ water vapor instrument: Implications for the mechanisms that control stratospheric water vapor


Building on previously published details of the laboratory calibrations of the Harvard Lyman-α photofragment fluorescence hygrometer (HWV) on the NASA ER-2 and WB-57 aircraft, we describe here the validation process for HWV, which includes laboratory calibrations and intercomparisons with other Harvard water vapor instruments at water vapor mixing ratios from 0 to 10 ppmv, followed by in-flight intercomparisons with the same Harvard hygrometers. The observed agreement exhibited in the laboratory and during intercomparisons helps corroborate the accuracy of HWV. In light of the validated accuracy of HWV, we present and evaluate a series of intercomparisons with satellite and balloon borne water vapor instruments made from the upper troposphere to the lower stratosphere in the tropics and midlatitudes. Whether on the NASA ER-2 or WB-57 aircraft, HWV has consistently measured about 1–1.5 ppmv higher than the balloon-borne NOAA/ESRL/GMD frost point hygrometer (CMDL), the NOAA Cryogenic Frost point Hygrometer (CFH), and the Microwave Limb Sounder (MLS) on the Aura satellite in regions of the atmosphere where water vapor is <10 ppmv. Comparisons in the tropics with the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite show large variable differences near the tropopause that converge to ~10% above 460 K, with HWV higher. Results we show from the Aqua Validation and Intercomparison Experiment (AquaVIT) at the AIDA chamber in Karlsruhe do not reflect the observed in-flight differences. We illustrate that the interpretation of the results of comparisons between modeled and measured representations of the seasonal cycle of water entering the lower tropical stratosphere is dictated by which data set is used.

Additional Information

© 2009 American Geophysical Union. Received 5 May 2009; accepted 28 August 2009; published 2 December 2009. We greatly appreciate the efforts of the pilots and crews of the NASA ER-2 and WB-57 aircraft in making the missions we participated in so successful. We thank the AIDA support team and especially hosts Ottmar Mo¨hler and Harald Saathoff for their hospitality and organizational efforts, and David Fahey and Ru-Shan Gao for serving as referees along with Ottmar Mo¨hler during AquaVIT. We also thank the PIs for data we use in the intercomparisons: Robert Herman (JLH), Holger Vo¨mel (CFH), Cornelius Schiller (FISH), and Volker Ebert (AIDA TDL). Thanks also go to the MLS and HALOE PIs and instrument teams for all the hard work and perseverance over many years and for providing easy access to their data. Thanks go to Holger Vo¨mel for access to flight frostpoint data and helpful comments on the manuscript. Thanks also go to Andrew Dessler for supplying his model data used in Figure 19. Helpful comments from Stephan Fueglistaler, Eric Jensen, Leonard Pfister, and Ellis Remsberg and the three manuscript referees are gratefully acknowledged. Elliot Weinstock is grateful for the encouragement of David Fahey regarding the writing of this manuscript as well as his and Rushan Gao's challenging questions about HWV. The Harvard data in this manuscript would not exist without continuous support from the NASA Upper Atmospheric Research Program, most recently NASA grant NNG05GJ81G.

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Published - Weinstock2009p6660J_Geophys_Res-Atmos.pdf


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