Abstract
Abstract. Recent studies with closed-path eddy covariance (EC) systems have indicated that the attenuation of fluctuations of water vapor concentration is dependent upon ambient relative humidity, presumably due to sorption/desorption of water molecules at the interior surface of the tube. Previous studies of EC-related tube attenuation effects have either not considered this issue at all or have only examined it superficially. Nonetheless, the attenuation of water vapor fluctuations is clearly much greater than might be expected from a passive tracer in turbulent tube flow. This study reexamines the turbulent tube flow issue for both passive and sorbing tracers with the intent of developing a physically-based semi-empirical model that describes the attenuation associated with water vapor fluctuations. Toward this end, we develop a new model of tube flow dynamics (radial profiles of the turbulent diffusivity and tube airstream velocity). We compare our new passive-tracer formulation with previous formulations in a systematic and unified way in order to assess how sensitive the passive-tracer results depend on fundamental modeling assumptions. We extend the passive tracer model to the vapor sorption/desorption case by formulating the model's wall boundary condition in terms of a physically-based semi-empirical model of the sorption/desorption vapor fluxes. Finally we synthesize all modeling and observational results into a single analytical expression that captures the effects of the mean ambient humidity and tube flow (Reynolds number) on tube attenuation.
Highlights
Eddy covariance technology (ECT) has been and continues to be critical to the quantification of exchange rates of CO2, H2O, and other trace gases between the atmosphere and the terrestrial biosphere
Though when they compared the model predictions with the observed attenuation, they found that the attenuation of water vapor fluctuations is significantly greater than might be expected for a passive tracer, and it is more strongly influenced by the flow Reynolds number than predicted as well
This (a) invalidates the assumption, on which both Lenschow and Raupach (1991) and Massman (1991) are based, that water vapor is a passive tracer, and (b) clearly indicates a need to carefully reexamine the previous models of tube attenuation effects for passive tracers and to develop a physically-based model that includes the effects of humidity on tube attenuation
Summary
Eddy covariance technology (ECT) has been and continues to be critical to the quantification of exchange rates of CO2, H2O, and other trace gases between the atmosphere and the terrestrial biosphere. Later Lenschow and Raupach (1991), using water vapor as the tracer, measured the attenuation of concentration fluctuations associated with turbulent tube flows They developed a model of these frequency-dependent tube attenuation effects, the basis of which was the modeling and observational results of Taylor (1954). More recent observations by Clement (2004), Amman et al (2006), and Ibrom et al (2007) have suggested that the attenuation of atmospheric water vapor fluctuations is strongly influenced by relative humidity, which leads to the very likely possibility that some of the greater-than-expected attenuation observed by Lenschow and Raupach (1991) resulted in part from humidity effects If so, this (a) invalidates the assumption, on which both Lenschow and Raupach (1991) and Massman (1991) are based, that water vapor is a passive tracer, and (b) clearly indicates a need to carefully reexamine the previous models of tube attenuation effects for passive tracers and to develop (if possible) a physically-based model that includes the effects of humidity on tube attenuation. The present study attempts a very different formulation for the wall boundary condition in the hope that (at least some of) the results are generally applicable to any trace gas that might adhere to the inside surface of a tube (e.g., H2O, O3, NH3, SO2, and many other polar molecules) and possibility to isotopes of such trace gases as well
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