Accuracy of tropospheric and stratospheric water vapor measurements by the cryogenic frost point hygrometer: Instrumental details and observations

Abstract

[1] The cryogenic frost point hygrometer (CFH), currently built at the University of Colorado, is a new balloon borne hygrometer, which is capable of continuously measuring water vapor between the surface and the middle stratosphere. The design is loosely based on the old NOAA/CMDL frost point hygrometer, with improved accuracy and a number of significant new features that overcome some limitations of the older instrument. The measurement uncertainty of the new instrument depends on altitude and ranges between less than 4% in the tropical lower troposphere to no more than 10% in the middle stratosphere at 28 km. In the tropopause region the uncertainty is less than 9%. This instrument is used routinely at several sites for validation of satellite measurements and process studies in the upper troposphere and lower stratosphere region. It has proved to be particularly well suited for dehydration observations in the tropical upper troposphere, because the effects of cloud contamination have been significantly reduced. Results of this instrument are compared with the old NOAA/CMDL hygrometer, the Russian Fluorescent Lyman Alpha Stratospheric Hygrometer, the Vaisala RS92, the AURA/MLS satellite instrument, a cloud lidar, the NOAA/CSD frost point hygrometer and the Harvard Lyman-alpha hygrometer, both of the later instruments flown on board the NASA WB-57F high-altitude research aircraft. These comparisons demonstrate the level of accuracy of tropospheric and stratospheric water vapor measurements made by this instrument and point to areas where more research and development are needed.

Balloon-borne match measurements of midlatitude cirrus clouds

“…Ice supersaturated regions (ISSRs), i.e., clear air masses with ice supersaturation, are the potential formation regions for cirrus clouds (Gierens and Spichtinger, 2000;Spichtinger et al, 2003a, b). A variety of data sets (satellite, aircraft, radiosondes) and trajectory calculations serve to improve the understanding of ISSRs and cirrus formation (Ovarlez et al, 2000;Comstock et al, 2004;Jensen et al, 2005;Spichtinger et al, 2005;Gettelman et al, 2006;Vömel et al, 2007;Krämer et al, 2009). Although persistent ISSRs in the upper-tropospheric cloudfree air are well understood, very high supersaturations are difficult to comprehend inside thicker cirrus clouds, such as those described here, with high backscatter ratios of 30-200 (see Fig.…”

An update on the uncertainties of water vapor measurements using cryogenic frost point hygrometers

“…The work leading up to this paper has resulted in some instrument improvements over the work presented by Vömel et al (2007a), which are discussed here. The sequence of processing and data quality control steps from raw data to final data product is described in Appendix A.…”

“…The uncertainty due to controller stability has been considered the largest source of measurement uncertainty in balloon-borne frost point hygrometers (Vömel et al, 2007a) and was estimated at 0.5 K based on experience. In this section we revisit this subject and quantify the controller stability based on measurements.…”

“…Here we focus on the CFH, which has been described in detail elsewhere (Vömel et al, 2007a). This instrument uses the chilled-mirror principle, in which the water condensate is formed on a small temperature-controlled mirror, which is exposed to ambient air flowing across the mirror.…”

“…Cryogenic frost point hygrometers (CFHs) (Vömel et al, 2007a(Vömel et al, , 1995 are widely considered to be reference instruments for measurements of tropospheric and stratospheric water vapor due to their ability to relate the amount of water vapor to an easier temperature measurement. Frost point hygrometry makes use of the equilibrium thermodynamics of the vapor phase of water and its condensed phase.…”