Analysis of periods with strong and coherent CO 2 advection over a forested hill

Horizontal  and  vertical  advective  fluxes  of  CO 2 measured  during  the  CarboEurope-IP  advectionexperiment (ADVEX) at the Wetzstein spruce forest site in Thuringia, Germany, were related to winddirection, stratification regime and friction velocity u * . Measurements of wind speed and directioncarried out at one of the slopes of the ridge revealed the existence of reverse flow below the canopy onthe downwind side. This uphill flow occurred concurrently with the advective fluxes measured at the topof the hill. Such result is in agreement with recent modeling works that support the existence ofadvection at low hills covered with a canopy. Another experimental evidence that suggest a link betweenadvection at this site with the flow over the hill came from the analysis of the horizontal gradient of CO 2inside the volume  formed by the ADVEX towers. It was observed that CO 2 accumulated  near thedownwind side of the crest for cross-ridge flows, what is consistent with another modeling work of thetransport of scalars across a low hill covered with a canopy.