Methodological Development of the Conditional Sampling Method. Part II: Quality Control Criteria of Relaxed Eddy Accumulation Flux Measurements |
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Authors: | A?K?Fotiadi F?Lohou A?Druilhet Email author" target="_blank">D?Ser?aEmail author F?Said P?Laville A?Brut |
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Institution: | (1) Laboratoire d’Aérologie, UMR CNRS-UPS 5560, 14 Avenue Edouard Belin, 31400 Toulouse, France;(2) Unité EGC, INRA, 78850 Thiverval-Grignon, France;(3) CNRM-Météo-France, URA CNRS, 1357 Toulouse, France |
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Abstract: | Determination of biosphere–atmosphere exchanges requires accurate quantification of the turbulent fluxes of energy and of
a wide variety of trace gases. Relaxed Eddy Accumulation (REA) is a method that has received increasing attention in recent
years, because it does not require any rapid sensor for the scalar measurements as the Eddy Correlation method (EC) does.
As in all micrometeorological studies, REA measurements in the atmospheric surface layer are valid under some restrictive
conditions so as to be representative of a specific ecosystem. These conditions are the homogeneity of the underlying surface,
stationary and horizontally homogeneous turbulence. For most experiments these conditions are not fully satisfied. Data uncertainties
can also be related to not fulfilling the method principles or to the technical characteristics of the REA system itself.
In order to assess REA measurement quality, a methodological approach of data analysis is developed in this study. This methodological
analysis is based on the establishment of criteria for data quality control. A set of them, deduced from the mean and turbulent
flow, are called ‘Dynamic criteria’ and are designated to control the stationarity and homogeneity of the w function and the validation of Taylor’s hypothesis. A second set (‘REA operational criteria’) is designed to check the sampling
process and, more precisely, the homogeneity of the negative and positive selection process throughout the sampling period.
A third set of criteria (‘Chemical scalar criteria’) concerns the scalar measurements. Results of the criteria application
to data measured at two different experimental sites are also presented. Cut-off limits of criteria are defined based on their
statistical distribution and shown to be specific for each site. Strictness of each criterion, defined by the percentage of
flagged samples, is analysed in conjunction with the meteorological conditions and atmospheric stability. It is found that
flagged samples mainly correspond to neutral and stable nocturnal conditions. During daytime, nearly free convection conditions
can also yield poor quality data. |
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Keywords: | Atmospheric stability Data quality control criteria Relaxed eddy accumulation method Turbulent functions |
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