Vortrag, Flux Measurements in Difficult Conditions, a Specialist Workshop, Boulder, CO, U.S.A.: 26.01.2006 - 28.01.2006
Abstract:
During the past decade, the eddy covariance technique has become the most important method for measuring trace gas exchange between ecosystems and the atmosphere. Over an ideal flat and homogeneous terrain the vertical turbulent transport is the only important flow. Therefore, the general approach in the calculations of the net ecosystem CO2 exchange (NEE) includes only two components: the turbulent CO2 exchange between the ecosystem and the atmosphere and the CO2 storage below the canopy. During situations of low turbulent transport it is expected that the sources within the fetch area are balanced by a storage increase. However, in non-uniform terrain the spatial discontinuity of atmospheric properties results in horizontal or vertical gradients of temperature, humidity or trace gas concentration which can cause a non-turbulent mass exchange usually called ‘advection’. Advection can be caused by thermal circulation systems (e.g. ‘land and sea breezes’ near shorelines or ‘katabatic/anabatic airstreams’ along mountain slopes) or by wind speed and pressure differences induced by a complex topography. In particular during night, advection can severely influence the ecosystem CO2 exchange. However, a scientific sound calculation of the night time fluxes is crucial for the confidence of the eddy covariance technique because an underestimation of ecosystem respiration results in an overestimation of the net ecosystem production (NEP) of a site. At two sites of the Jena Cluster within the CarboEurope network several approaches to estimate advection are currently used. Basic problem at both sites was the fact that the annual NEE estimated from the EC measurements was not in accordance with process models or inventory data. At Hainich Forest, Thuringia, Germany, where the tower is located at a gentle slope, EC seems to overestimate the carbon sink compared to other methods. Potential losses by advection were estimated in two ways: (1) by comparing night time eddy covariance fluxes to an independent value of total ecosystem respiration by means of bottom-up modelling of the underlying processes and (2) by a direct measurement of a horizontal CO2-gradient and horizontal wind speed at 2 m height. Both approaches resulted in strong indications that katabatic fluxes along the hill slope during evening reduce the measured apparent ecosystem respiration rate. (see poster of Kutsch et al.) In contrast, at the Wetzstein Forest, Thuringia, Germany, the apparent values of night time respiration were higher than the values predicted by the process analysis. This flux tower is located at a crest over a complex landscape. We suppose that the heterogeneous topography causes advectional fluxes due to pressure differences within the wind field. (see poster of Zeri et al.)