Comparing CO2 storage and advection conditions at night at different carboeuroflux sites

Anemometer and CO2 concentration data from temporary campaigns performed atsix CARBOEUROFLUX forest sites were used to estimate the importance of non-turbulentfluxes  in  nighttime  conditions.  While  storage  was  observed  to  be  significant  only  duringperiods of both low turbulence and low advection, the advective fluxes strongly influence thenocturnal CO 2 balance, with the exception of almost flat and highly homogeneous sites. Onthe basis of the main factors determining the onset of advective fluxes, the ‘advection velocity’,which takes net radiation and local topography into account, was introduced as a criterion tocharacterise the conditions of storage enrichment/depletion. Comparative analyses of the sixsites showed several common features of the advective fluxes but also some substantial dif-ferences. In particular, all sites where advection occurs show the onset of a boundary layercharacterised  by  a  downslope  flow,  negative  vertical  velocities  and  negative  vertical  CO2concentration gradients during nighttime. As a consequence, vertical advection was observedto be positive at all sites, which corresponds to a removal of CO2 from the ecosystem. Themain differences between sites are the distance from the ridge, which influences the boundary-layer  depth,  and  the  sign  of  the  mean  horizontal  CO2 concentration  gradients,  which  isprobably determined by the source/sink distribution. As a consequence, both positive andnegative horizontal advective fluxes (corresponding respectively to CO 2 removal from theecosystem and to CO 2 supply to the ecosystem) were observed. Conclusive results on the importance of non-turbulent components in the mass balance require, however, further experimental investigations at sites with different topographies, slopes, different land covers, which would allow a more comprehensive analysis of the processes underlying the occurrence of advective fluxes. The quantification of these processes would help to better quantify nocturnal CO2 exchange rates.