Steering mechanisms of a persistent slope wind system on plot-scale vertical and horizontal transport of CO2 in and above an alpine forest

Extensive measurements of the 3D wind vector and temperature with a high temporal and spatial resolution were made during 200 days in summer 2005 at the CarboEurope site in Renon/Ritten, Italy. The site is situated some 12 kms north of Bolzano at 1735 m a.s.l. on a south exposed steep (11°) forested slope in the Italian Alps. The measurements were conducted in the frame of the ADVEX field activities of the CarboEurope advection group with the aim to investigate the influence of advection on the Net Ecosystem Exchange (NEE) of CO2. Four additional towers of 30 m height were installed in about 60 m diagonal distance of the central flux tower to form a 3D cube control volume. Wind components u, v, w, and temperature T were measured at 4 levels (1.5, 6, 12 and 30 m) at each tower by ultrasonic anemometers at 10 Hz. In addition, CO2 and H2O concentrations were measured at the same locations every 160 s by three multi valve systems (MVS) each connected to a Li-6262 Infrared Gas Analyzer (IRGA). Though in complex terrain, a persistent slope wind system established for most of the time at the site which allows figuring out the characteristics of how a scalar like CO2 is transported parallel to the slope and how this transport affects NEE in the diurnal course. The local slope wind system was sometimes superimposed by two different large scale synoptic situations. The “tramontana”, a persistent strong wind from the north, amplified the drainage flow during nighttime and suppressed the upslope flow above the forest canopy during daytime. Vice versa, we observed periods with continuing flow from the south, which supported the local daytime upslope flow and partly retarded the nighttime downslope flow. This leaded to periods of several hours with opposite flow directions in and above the canopy, as observed during the transition periods while the local wind system dominated. Animations show how the wind field and the scalar concentration field develop in the diurnal course and how the trunk space is coupled and/or decoupled with/from the roughness sublayer above the forest canopy, depending on the prevailing situation. In particular, vertical and horizontal mixing of CO2 was strongly dependent on the dominating wind field with essential impact on the horizontal advective flux of CO2. Taking into account these advective fluxes would significantly reduce the reported annual CO2 uptake of this forest. Process-related effects are expected to occur at flux tower sites with similar topography and vegetation.