Evaluating multi-year, multi-site data on the energy balance closure of eddy-covariance flux measurements at cropland sites in southwest Germany

Abstract

<p><strong>Abstract.</strong> The energy balance of eddy covariance (EC) measurements is typically not closed. This so-called energy balance closure (EBC) problem has been a long-standing issue in micrometeorology. It is a main challenge in evaluating and interpreting EC flux data. EBC is crucial for validating and improving regional and global climate models. To investigate the reasons behind EBC more closely for agro-ecosystems, we analysed EC measurements from two climatically contrasting regions (Kraichgau (KR) and Swabian Jura (SJ)) in southwest Germany. Data were taken at six fully equipped EC sites from 2010 to 2017. The gap in EBC was quantified by ordinary linear regression, by the energy balance ratio (EBR) between turbulent fluxes and available energy, and by the residual energy term. In order to examine potential reasons for differences in EBC, we compared the EBC under varying environmental conditions and investigated a wide range of possible controls. Statistical analyses were conducted for the whole data set to test the effects of different regions, years, sites and crops on EBC. We also investigated whether EBC was a function of buoyancy, friction velocity, or atmospheric stability. The time-variable footprints of all EC stations were estimated based on data measured in 2015, complemented by micro-topographic analyses along the prevailing wind direction. The lowest mean annual energy balance gap was 17 % in KR and 13 % in SJ. Highest EBRs were commonly measured for winds originating from the prevailing wind direction. The variation of EBC was higher in winter than in summer. The measurement site exerted a statistically significant effect on EBC, but not crop or region. The spread of EBR distinctly narrowed under unstable atmospheric conditions, strong buoyancy, and high friction velocities. Smaller footprint areas led to better EBC due to increasing homogeneity. Flow distortions of winds that first travelled past the back head of the anemometer affected EBC negatively.</p>

Altered energy partitioning across terrestrial ecosystems in the European drought year 2018

“…Eddy-Covariance measurements are known for a gap in the energy balance closure (EBC): the sum of H and λET is frequently about 15 to 30% smaller than Rn -Sl -Ec [58,64]. Current theory suggests a number of different reasons including underestimation of the turbulent heat fluxes due to surface heterogeneity or incomplete correction of spectral losses, or unaccounted energy storage [64][65][66][67][68]. However, there is no consensus yet on the application of a correction, its distribution between H and λET and its implications for Ec [69,70].…”