How much additional improvements in measurements and data filtering can help to close the energy balance at eddy covariance stations

The non-closed energy balance is still an unsolved problem of eddy covariance measurements: when the net radiation budget is compared with all the different energy exchange components measured at the flux station there is a general systematic imbalance, highlighting that one or more  energy components are either underestimated or not measured. Different hypotheses have been stated and many analyses into potential causes have been performed in the past years, ranging from methodological problems (water vapour flux underestimation due to condensation or unaccounted spectral damping in the analyser tube, underestimation of the vertical wind speed component due to transducers shadowing in the sonic anemometers), to components not fully monitored (e.g. the heat stored in the vegetation) different sensor footprints and field of views, to large scale motions particularly relevant in heterogeneous and fragmented landscapes. All these aspects are important and probably the issue of the energy balance non-closure is due to a combination of all the factors.

The ICOS Ecosystem network consists of a set of eddy covariance stations equipped with high-level and quality standardized instrumentation, whose data are processed centrally. The availability of these data (freely accessible through the ICOS Carbon Portal) allows for a systematic analysis of the importance of the different measured energy components (soil heat flux and the soil storage above the soil heat flux plates, air mass storage of sensible and latent heat measured with sensors along vertical profiles) in relation to various data quality filtering steps applied (raw data screening, low turbulence conditions etc.). The results show that improving the measurement and the quality control of the energy components, leads to an average 10% increase of the energy balance closure. This is not yet sufficient to obtain a perfect closure of the balance and further investigation is needed but helps to identify the magnitude of the real imbalance to be explained. Analysis of data collected under different environments and conditions helps also to identify and better understand the main possible causes of the energy balance non-closure.