Simplified expressions for adjusting higher-order turbulent statistics obtained from open path gas analyzers |
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Authors: | M Detto G G Katul |
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Institution: | (1) Dipartimento di Ingegneria Idraulica, Ambientale e del Rilevamento, Politecnico di Milano, Grugliasco, P.zza L. Da Vinci 32, Italy;(2) Nicholas School of the Environment and Earth Sciences, Duke University, Box 90328, Durham, NC, USA;(3) Department of Civil and Environmental Engineering, Pratt School of Engineering, Duke University, Durham, NC, USA |
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Abstract: | When density fluctuations of scalars such as CO2 are measured with open-path gas analyzers, the measured vertical turbulent flux must be adjusted to take into account fluctuations
induced by ‘external effects’ such as temperature and water vapour. These adjustments are needed to separate the effects of
surface fluxes responsible for ‘natural’ fluctuations in CO2 concentration from these external effects. Analogous to vertical fluxes, simplified expressions for separating the ‘external
effects’ from higher-order scalar density turbulence statistics are derived. The level of complexity in terms of input to
these expressions are analogous to that of the Webb–Pearman–Leuning (WPL), and are shown to be consistent with the conservation
of dry air. It is demonstrated that both higher-order turbulent moments such as the scalar variances, the mixed velocity-scalar
covariances, and the two-scalar covariance require significant adjustments due to ‘external effects’. The impact of these
adjustments on the turbulent CO2 spectra, probability density function, and dimensionless similarity functions derived from flux-variance relationships are
also discussed. |
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Keywords: | Carbon flux Flux correction Higher order statistics Open path gas analyzer |
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