Editorial Type:
Article
Article Type:
Research Article

The Psychrometric Constant Is Not Constant: A Novel Approach to Enhance the Accuracy and Precision of Latent Energy Fluxes through Automated Water Vapor Calibrations

H. W. Loescher Science Office, National Ecological Observatory Network, and Institute of Alpine and Arctic Research, University of Colorado at Boulder, Boulder, Colorado

Search for other papers by H. W. Loescher in
Current site
Google Scholar
PubMed Close
,
C. V. Hanson College of Forestry, Forest Science, Oregon State University, Corvallis, Oregon

Search for other papers by C. V. Hanson in
Current site
Google Scholar
PubMed Close
, and
T. W. Ocheltree Stable Isotope Mass Spectrometry Laboratory, Division of Biology, Kansas State University, Manhattan, Kansas

Search for other papers by T. W. Ocheltree in
Current site
Google Scholar
PubMed Close
Print Publication:
01 Oct 2009
DOI:
https://doi.org/10.1175/2009JHM1148.1
Page(s):
1271–1284
Restricted access

Abstract

Numerous agencies, programs, and national networks are focused on improving understanding of water and energy fluxes across temporal and spatial scales and on enhancing confidence to synthesize data across multiple sites. Enhancing the accuracy and precision in the surface energy balance and the latent energy (λE) flux lies, in part, with being able to uniformly calibrate water vapor measurements at and among sites to traceable standards. This paper examines (i) the traceable physical controls on field applications of chilled-mirror hygrometers and (ii) an automated means to accurately and precisely calibrate infrared gas analyzers for water vapor concentrations and eddy covariance (λE) data. The environmental physics and gas handling were examined in a theoretical and applied manner that found that chilled-mirror technologies can be a robust measure of dewpoint temperatures and ambient water vapor only if the unit conversions are accounted for between inlet and body temperatures. Psychrometers were also examined and a functional relationship (exponential) was developed for the psychrometric constant against the wet-bulb temperature depression (Tdb − Twb), across a wider range of temperature depressions than previously reported. These empirical estimates of the psychrometer constant for small temperature depressions are much lower than other reported values—that is, ∼0.000 52 K−1 for a wet-bulb temperature depression (Tdb − Twb) of 4.3 K.

Corresponding author address: Hank Loescher, Science Office, National Ecological Observatory Network, 5340 Airport Blvd., Boulder, CO 80301. Email: hloescher@neoninc.org

Abstract

Numerous agencies, programs, and national networks are focused on improving understanding of water and energy fluxes across temporal and spatial scales and on enhancing confidence to synthesize data across multiple sites. Enhancing the accuracy and precision in the surface energy balance and the latent energy (λE) flux lies, in part, with being able to uniformly calibrate water vapor measurements at and among sites to traceable standards. This paper examines (i) the traceable physical controls on field applications of chilled-mirror hygrometers and (ii) an automated means to accurately and precisely calibrate infrared gas analyzers for water vapor concentrations and eddy covariance (λE) data. The environmental physics and gas handling were examined in a theoretical and applied manner that found that chilled-mirror technologies can be a robust measure of dewpoint temperatures and ambient water vapor only if the unit conversions are accounted for between inlet and body temperatures. Psychrometers were also examined and a functional relationship (exponential) was developed for the psychrometric constant against the wet-bulb temperature depression (Tdb − Twb), across a wider range of temperature depressions than previously reported. These empirical estimates of the psychrometer constant for small temperature depressions are much lower than other reported values—that is, ∼0.000 52 K−1 for a wet-bulb temperature depression (Tdb − Twb) of 4.3 K.

Corresponding author address: Hank Loescher, Science Office, National Ecological Observatory Network, 5340 Airport Blvd., Boulder, CO 80301. Email: hloescher@neoninc.org

Save
Cover Journal of Hydrometeorology
Journal of Hydrometeorology

  • Allan, D. W., 1966: Statistics of atomic frequency standards. Proc. IEEE, 54 , 221–231.

  • Baldocchi, D. D., 2003: Assessing the eddy covariance technique for evaluating carbon dioxide exchange rates of ecosystems: Past, present and future. Global Change Biol., 9 , 479–492.

    • Crossref
    • Search Google Scholar
    • Export Citation

  • Bolin, B., and Coauthors, 1995: IPCC Second Assessment—Climate Change 1995. IPCC, 64 pp.

  • Buck, A. L., 1981: New equations for computing vapor pressure and enhancement factor. J. Appl. Meteor., 20 , 1527–1532.

  • Campbell, G. S., and Norman J. M. , 1998: An Introduction to Environmental Physics. 2nd ed. Springer-Verlag, 286 pp.

  • Fan, J., 1987: Determination of the psychrometers coefficient A of the WMO reference psychrometers by comparison with a standard gravimetric hygrometer. J. Atmos. Oceanic Technol., 4 , 239–244.

    • Crossref
    • Search Google Scholar
    • Export Citation

  • Foken, T., and Wichura B. , 1996: Tools for quality assessment of surface-based flux measurements. Agric. For. Meteor., 78 , 83–105.

  • Hasewaga, S., and Little J. W. , 1977: The NBS two-pressure humidity generator, mark 2. J. Res. Nat. Bur. Stand., 81A , 81–88.

  • Houghton, J. T., Ding Y. , Griggs D. J. , Noguer M. , van der Linden P. J. , Dai X. , Maskell K. , and Johnson C. A. , Eds.,. 2001: Climate Change 2001: The Scientific Basis. Cambridge University Press, 881 pp.

    • Search Google Scholar
    • Export Citation

  • Jones, H., 1991: Plants and Microclimate: A Quantitative Approach to Environmental Plant Physiology. Cambridge University Press, 428 pp.

    • Search Google Scholar
    • Export Citation

  • Kaimal, J. C., and Wyngaard J. C. , 1990: The Kansas and Minnesota experiments. Bound.-Layer Meteor., 50 , 31–47.

  • Katsaros, K. B., 1991: An intelligent chilled mirror humidity instrument. J. Atmos. Oceanic Technol., 8 , 585–596.

  • Leuning, R., 2004: Measurements of trace gas fluxes in the atmosphere using eddy covariance: WPL corrections revisited. Handbook of Micrometeorology: A Guide for Surface Flux Measurement and Analysis, X. Lee, W. Massman, and B. Law, Eds., Kulwer Academic Publishers, 119–132.

    • Search Google Scholar
    • Export Citation

  • List, R. J., 1951: Smithsonian Meteorological Tables. Smithsonian Institution, 527 pp.

  • Loescher, H. W., Law B. E. , Mahrt L. , Hollinger D. , Campbell J. L. , and Wofsy S. C. , 2006: Uncertainties in, and interpretation of, carbon flux estimates using the eddy covariance technique. J. Geophys. Res., 111 , D21S90. doi:10.1029/2005JD006932.

    • Search Google Scholar
    • Export Citation

  • Loescher, H. W., and Coauthors, 2007: Enhancing water cycle measurements for future hydrologic research. Bull. Amer. Meteor. Soc., 88 , 669–676.

  • Massman, W. J., 2000: A simple method for estimating frequency response corrections for eddy covariance systems. Agric. For. Meteor., 104 , 185–198.

    • Crossref
    • Search Google Scholar
    • Export Citation

  • Massman, W. J., 2004: Concerning the measurement of atmospheric trace gas fluxes with open- and closed-path eddy covariance system: The WPL terms and spectral attenuation. Handbook of Micrometeorology: A Guide for Surface Flux Measurement and Analysis, X. Lee, W. Massman, and B. Law, Eds., Kulwer Academic Publishers, 133–160.

    • Search Google Scholar
    • Export Citation

  • Massman, W. J., and Clement R. , 2004: Uncertainty in eddy covariance flux estimates resulting from spectral attenuation. Handbook of Micrometeorology: A Guide for Surface Flux Measurement and Analysis, X. Lee, W. Massman, and B. Law, Eds., Kulwer Academic Publishers, 67–100.

    • Search Google Scholar
    • Export Citation

  • Monteith, J. L., and Unsworth M. H. , 1990: Principles of Environmental Physics. Edward Arnold Publishers, 291 pp.

  • Ocheltree, T. W., and Loescher H. W. , 2007: Design of the AmeriFlux portable eddy covariance system and uncertainty analysis of carbon measurements. J. Atmos. Oceanic Technol., 24 , 1389–1406.

    • Crossref
    • Search Google Scholar
    • Export Citation

  • Richardson, S. J., Knuteson R. O. , and Tobin D. C. , 1999: A chilled mirror dew point hygrometer for field use. Proc. Ninth ARM Science Team Meeting, San Antonio, TX, U.S. Department of Energy, 5 pp.

    • Search Google Scholar
    • Export Citation

  • Richardson, S. J., Splitt M. E. , and Lesht B. M. , 2000: Enhancement of ARM surface meteorological observations during the fall 1996 water vapor intensive observation period. J. Atmos. Oceanic Technol., 17 , 312–322.

    • Crossref
    • Search Google Scholar
    • Export Citation

  • Ritsche, M. T., 2005: Chilled mirror dew point hygrometer handbook. U.S. Department of Energy, ARM TR 032, 12 pp. [Available online at http://www.arm.gov/publications/tech_reports/handbooks/cm_handbook.pdf].

    • Search Google Scholar
    • Export Citation

  • Sarmiento, J. L., and Coauthors, 1999: A U.S. carbon cycle science plan. Report of the NACP Committee of the U.S. Interagency Carbon Cycle Science Program, U.S. Global Change Research Program, 78 pp.

    • Search Google Scholar
    • Export Citation

  • Schurer, K., 1981: Confirmation of a lower psychrometer constant. J. Phys. E: Sci. Instrum., 14E .Abstract 1153, doi:10.1088/0022-3735/14/10/014.

    • Search Google Scholar
    • Export Citation

  • Shen, S. H., and Leclerc M. Y. , 1997: Modeling the turbulence structure in the canopy layer. Agric. For. Meteor., 87 , 3–25.

  • Sierra, C. A., Loescher H. W. , Harmon M. E. , Richardson A. D. , Hollinger D. Y. , and Perakis S. S. , 2009: Interannual variation of carbon fluxes from a tropical, a temperate, and a boreal evergreen forest: The role of forest dynamics and climate. Ecology, in press.

    • Search Google Scholar
    • Export Citation

  • Tarbell, S. A., and Weller R. A. , 1991: Development of an intelligent chilled mirror dew point sensor. Preprints, Seventh Symp. on Meteorological Observations and Instrumentation, New Orleans, LA, Amer. Meteor. Soc., 297–302.

    • Search Google Scholar
    • Export Citation

  • Visscher, G. J. W., 1995: Standard psychrometers: A matter of (p)references. Meas. Sci. Technol., 6 , 1451–1461.

  • Waterfield, R., 2000: The First Philosophers: The Presocratics and Sophists. Oxford University Press, 290 pp.

  • Webb, E. K., Pearman G. I. , and Leuning R. , 1980: Correction of flux measurements for density effects due to heat and water vapor transfer. Quart. J. Roy. Meteor. Soc., 106 , 85–100.

    • Crossref
    • Search Google Scholar
    • Export Citation

  • Wofsy, S. C., and Harris R. C. , 2002: The North American Carbon Program (NACP). Report of the NACP Committee of the U.S. Interagency Carbon Cycle Science Program, U.S. Global Change Research Program, 56 pp.

    • Search Google Scholar
    • Export Citation

  • Wright, M. R., 1981: Empedocles: The Extant Fragments. Yale University Press, 161 pp.

  • Wylie, R. G., and Lalas T. , 1981: Detailed determination of the psychrometer coefficient for the wet cylinder in a transverse airstream and an analysis of its accuracy. Tech. Paper 7, CSIRO Division of Applied Physics, 24 pp.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 912 265 16
PDF Downloads 647 185 12