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

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 (?E) 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 (?E) 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.