Spatial variability of vertical and horizontal turbulent fluxes above a boreal forest

The exchange of momentum, energy and matter between ecosystems and the atmosphere is measured by the eddy covariance (EC) technique at numerous experimental sites in the frame of flux networks (e.g. FLUXNET). The measured annual sums of CO2 uptake are considered to be representative for larger areas and for the respective plant species, despite the awareness of non-ideal conditions for the application of the EC-method like insufficient fetch conditions, low turbulence during calm and stable nights, intermittent turbulence, etc.

The activities of the CarboEurope advection group ADVEX at the Norunda site in summer 2006 provided an excellent opportunity to investigate the spatial variability of turbulent fluxes and the horizontal flux divergence. This site represents a mixture of mature pine-spruce stands of 25 m height growing in flat landscape. The main tower here is 102 m tall. The surrounding of the tower is characterized by small inhomogeneties (large rocks, wet openings, roads, differences in stands).

The main aim of the ADVEX campaign was to measure the horizontal and vertical advective fluxes of CO2. Thus, the site was complemented with four additional towers, where the wind vectors, air temperatures and CO2 concentrations were measured at four heights (1.5, 6, 12, 30 m) by means of sonic anemometers and IR gas analysers. The four towers were installed in about 65 m diagonal distance from the central tower forming a 3D cube control volume. The central tower and two of the additional towers were equipped with EC systems at the 32-33 m level to estimate the spatial variability of turbulent fluxes. These three towers were in line in the SW to NE direction which was the dominant wind direction for the site. All the used gas analysers were of open-path type (LI-7500), two of the sonics were produced by Gill and the one in the central tower by Metek. The analysers were calibrated versus the same reference gas and dew point generator. During the campaign, there were several fair weather periods when the gas analysers functioned well. We will study whether horizontal flux variations larger than the measurement uncertainties can be detected, especially when the advective fluxes were considerable. The preliminary analysis, however, shows that the horizontal variations of turbulent fluxes were quite small.