Comparison of horizontal and vertical advective CO2 fluxes at three forest sites

Extensive field measurements have been performed at three CarboEurope-Integrated Pro-ject  forest  sites  with  different  topography  (Renon/Ritten,  Italian  Alps,  Italy;  Wetzstein,Thuringia, Germany; Norunda, Uppland, Sweden) to evaluate the relevant terms of thecarbon balance by measuring CO 2 concentrations [CO 2 ] and the wind field in a 3D multi-tower cube setup. The same experimental setup (geometry and instrumentation) and thesame methodology were applied to all the three experiments. It is shown that all sites areaffected by advection in different ways and strengths. Everywhere, vertical advection (F VA )occurred only at night. During the day, F VA disappeared because of turbulent mixing, leadingto a uniform vertical profile of [CO 2 ]. Mean F VA was nearly zero at the hilly site (Wetzstein)and at the flat site (Norunda). However, large, momentary positive or negative contributionsoccurred at the flat site, whereas vertical non-turbulent fluxes were generally very small atthe hilly site. At the slope site (Renon), F VA was always positive at night because of thepermanently negative mean vertical wind component resulting from downslope winds.Horizontal advection also occurred mainly at night. It was positive at the slope site andnegative at the flat site in the mean diurnal course. The size of the averaged non-turbulentadvective fluxes was of the same order of magnitude as the turbulent flux measured byeddy-covariance  technique,  but  the  scatter  was  very  high.  This  implies  that  it  is  notadvisable to use directly measured quantities of the non-turbulent advective fluxes forthe estimation of net ecosystem exchange (NEE) on e.g. an hourly basis. However, situations with and without advection were closely related to local or synoptic meteorological conditions. Thus, it is possible to separate advection affected NEE estimates from fluxes which are representative of the source term. However, the development of a robust correction scheme for advection requires a more detailed site-specific analysis of single events for the identification of the relevant processes. This paper presents mean characteristics of the advective CO2 fluxes in a first site-to-site comparison and evaluates the main problems for future research.