Abstract
Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u<sub>*</sub>-threshold filtering technique
Highlights
The eddy covariance method is being used to measure the exchange of carbon dioxide (CO2), water vapor and energy between the land and atmosphere at over 400 stations globally
During well-mixed, daytime conditions, turbulent fluxes and change in storage measured on a single tower in flat-to-moderate topography with homogeneous vegetation will equal net ecosystem exchange because horizontal and vertical advection in the layer below the instruments will be negligible
A common approach is to exclude nighttime flux measurements below a critical u∗ threshold, when turbulence levels are low, and to use the remaining data to develop relationships between one or more independent environmental variables and the turbulent fluxes plus changes in storage in the layer below the measurement height. This assumes that advection is insignificant above the u∗ threshold, but this study has shown that there is no u∗ unique threshold at our site and that there is a significant underestimation of nocturnal carbon efflux (NCE) for all but very high u∗ values
Summary
The eddy covariance method is being used to measure the exchange of carbon dioxide (CO2), water vapor and energy between the land and atmosphere at over 400 stations globally. Data are used to parameterize land-surface models (Wang et al, 2001) and to validate regional and continental carbon budgets (Baldocchi et al, 1996). The net exchange of CO2 between the underlying vegetation and the atmosphere is assumed to equal the sum of the turbulent flux and the rate of change in storage of CO2 in the layer of air below the measurement height, an assumption that is correct only in the absence of horizontal and vertical flux divergences. Such conditions are more obtained over uniform vegetation on landscapes with flat to moderate topography during the daytime, when the air flow
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