Quality analysis applied on eddy covariance measurements at complex forest sites using footprint modelling

Abstract

SummaryMeasuring turbulent fluxes with the eddy covariance method has become a widely accepted and powerful tool for the determination of long term data sets for the exchange of momentum, sensible and latent heat, and trace gases such as CO 2 between the atmosphere and the underlying surface. Several flux networks developed continuous measurements above complex terrain, e.g. AmeriFlux and EUROFLUX, with a strong focus on the net exchange of CO 2 between the atmosphere and the underlying surface. Under many conditions basic assumptions for the eddy covariance method in its simplified form, such as stationarity of the flow, homogeneity of the surface and fully developed turbulence of the flow field, are not fulfilled. To deal with non-ideal conditions which are common at many FLUXNET sites, quality tests have been developed to check if these basic theoretical assumptions are valid.In the framework of the CARBOEUROFLUX project, we combined quality tests described by Foken and Wichura (1996) with the analytical footprint model of Schmid (1997). The aim was to identify suitable wind sectors and meteorological conditions for flux measurements. These tools were used on data of 18 participating sites. Quality tests were applied on the fluxes of momentum, sensible and latent heat, and on the CO 2 -flux, respectively. The influence of the topography on the vertical wind component was also checked. At many sites the land use around the flux towers is not homogeneous or the fetch may not be large enough. So the relative contribution of the land use type intended to be measured was also investigated. Thus the developed tool allows comparative investigations of the measured turbulent fluxes at different sites if using the same technique and algorithms for the determination of the fluxes as well as analyses of potential problems caused by influences of the surrounding land use patterns.

The Turbulence Characteristics of the Atmospheric Surface Layer on the North Slope of Mt. Everest Region in the Spring of 2005

“…The third part of the program performs a planar fit coordinate rotation (Wilczak et al 2001), correction of spectral loss (Moore 1986), and conversion of buoyancy into sensible heat flux (Schotanus et al 1983;Liu et al 2001). Lastly, a post-field quality control (Rebmann et al 2005), a steady state test (Foken and Wichura 1996) and a test for integral turbulence characteristics (Foken and Wichura 1996;Thomas and Foken 2002) were performed.…”

“…These tests resulted in combined quality flags for each half-hourly measurement ranging from 1 to 5 and the quality flags were obtained from the combination of individual flags for the integral turbulence characteristics and stationarity tests (see Table 1) (Rebmann et al 2005). Classes 1-2 were used in fundamental research, whereas classes 3-4 could still be utilized for the determination of monthly or annual sums of fluxes in continuously running measuring programs, e.g., FLUXNET (Baldocchi et al 2001).…”

Towards a standardized processing of Net Ecosystem Exchange measured with eddy covariance technique: algorithms and uncertainty estimation

“…Many improvements in the Eddy measurements treatments were presented and applied over the last 10 years, often detailed information about the data processing methods were not available and important variables like the CO 2 storage under the canopy were not measured. For this reason it is very important to have a set of tools to process all the datasets available with a standardized method with the aim to improve their quality, particularly if the data are used for interannual analysis or site intercomparisons, and where raw data are not available and for this reason it is impossible to use others criteria recently proposed (Rebmann et al, 2005;Ruppert et al, 2006).…”

Assessing eddy-covariance flux tower location bias across the Fluxnet-Canada Research Network based on remote sensing and footprint modelling

“…Networks of EC systems have been set up at local to continental scales based loosely on the idea that individual nodes are representative of a large area (Baldocchi, 2008;Chasmer et al, 2008). However, utilisation and application of EC measurements (especially for spatial up-scaling of these measurements to landscape and regional scales) can be problematic due to difficulties/uncertainties in assessing/interpreting the associated measuring biases of EC data arising from the sensor's views over heterogeneous natural vegetation from different wind sectors (Schmid, 1997(Schmid, , 2002Soegaard et al, 2003;Gockede et al, 2004;Rebmann et al, 2005;Chen et al, 2009a). The adoption of the EC technique to estimate surface exchange is based on the assumption that the contributing area of the fluxes is topographically flat and the vegetation extends uniformly within the footprint area (Foken and Wichura, 1996;Baldocchi, 2003Baldocchi, , 2008Finnigan et al, 2003).…”