Surface layer CO₂ budget and advective contributions to measurements of net ecosystem-atmosphere exchange of CO₂

The aim of this case study is to examine the surface layer CO₂ budget and possible impacts of advection on the 20 km scale on estimates of net ecosystem-atmosphere exchange (NEE) of CO₂ under windy conditions. The advection terms in the CO₂ mixing ratio conservation equation are computed using data from two flux towers 20 km apart in a forested area in Wisconsin, USA. The data are analyzed when the winds blow continuously from the southeast (SE) or from the northwest (NW) under windy conditions for 5 h or longer during the months of May through September of 1999 and 2000. Compared to the vertical turbulent flux, the contribution of horizontal flux divergence is negligible on the scale of 20 km at all times. The contributions of both horizontal and vertical advection terms to NEE estimates are negligible in the day. The mean nocturnal vertical and horizontal components of the advective fluxes have opposite signs and are of the same order of magnitude. Considering only one of the two advection terms in NEE calculation would be inappropriate. On the spatial scale specified, the contribution of horizontal advection is negative and can be 10% of NEE at night under sustained SE winds. The contribution of vertical advection is positive and is about 20% of NEE for the same wind direction. The contribution of nocturnal advection is negligible for most cases with sustained NW winds. The evaluation of the advective effects is still incomplete only from this dataset and significant contribution of advection on the scales smaller than 20 km is not ruled out because the 20-km scale may not be the primary scale of the heterogeneous distributions of land cover and soils. Nevertheless, the analyses suggest that significant errors due to the neglect of the impacts of the land cover and soil heterogeneity at larger scales (than local) are possible in one-dimensional eddy-covariance NEE measurements under windy conditions; these measurements are usually selected to describe nighttime NEE at this site and, therefore, the likely errors may deserve attention. The analyses also suggest that impacts of both advection terms within the surface layer on NEE estimates from the atmospheric boundary budget method could be negligible in the day but significant at night. More comprehensive experiments are needed to completely assess the advection issue.