Available energy and energy balance closure at four coniferous forest sites across Europe

Abstract

The available energy (AE), driving the turbulent fluxes of sensible heat and latent heat at the earth surface, was estimated at four partly complex coniferous forest sites across Europe (Tharandt, Germany; Ritten/Renon, Italy; Wetzstein, Germany; Norunda, Sweden). Existing data of net radiation were used as well as storage change rates calculated from temperature and humidity measurements to finally calculate the AE of all forest sites with uncertainty bounds. Data of the advection experiments MORE II (Tharandt) and ADVEX (Renon, Wetzstein, Norunda) served as the main basis. On-site data for referencing and cross-checking of the available energy were limited. Applied cross checks for net radiation (modelling, referencing to nearby stations and ratio of net radiation to global radiation) did not reveal relevant uncertainties. Heat storage of sensible heat J H , latent heat J E , heat storage of biomass J veg and heat storage due to photosynthesis J C were of minor importance during day but of some importance during night, where J veg turned out to be the most important one. Comparisons of calculated storage terms (J E , J H ) at different towers of one site showed good agreement indicating that storage change calculated at a single point is representative for the whole canopy at sites with moderate heterogeneity. The uncertainty in AE was assessed on the basis of literature values and the results of the applied cross checks for net radiation. The absolute mean uncertainty of AE was estimated to be between 41 and 52 W m −2 (10-11 W m −2 for the sum of the storage terms J and soil heat flux G) during mid-day (approximately 12% of AE). At night, the absolute mean uncertainty of AE varied from 20 to about 30 W m −2 (approximately 6 W m −2 for J plus G) resulting in large relative uncertainties as AE itself is small. An inspection of the energy balance showed an improvement of closure when storage terms were included and that the imbalance cannot be attributed to the uncertainties in AE alone.

Contribution of advection to the carbon budget measured by eddy covariance at a steep mountain slope forest in Switzerland

“…Barr et al, 2006;Oncley et al, 2007). Our data, however, do not support this idea and indicate rather that the energy budget closure is not suitable to detect the occurrence of advection, as was also found within the ADVEX campaign (Feigenwinter et al, 2008;Moderow et al, 2009). One explanation might be that EC fluxes of water and sensible heat are more robust than CO 2 fluxes as stated by Lee and Hu (2002).…”

Energy balance closure over a cool temperate forest in steeply sloping topography during snowfall and snow-free periods

“…Previous studies reported that by considering the storage term, the increment rate of CR was 7 of the average value for 26 site-yr at a site with tall vegetation (Wilson et al, 2002) and 3-12 of the values in four coniferous forest sites (Moderow et al, 2009;Lindroth et al, 2010). Our result is similar to those previous findings.…”

“…where µ is the specific energy conversion due to photosynthesis (0.478 J µmol) and GPP is gross primary production (Moderow et al, 2009). GPP was estimated by measured net ecosystem exchange (NEE).…”

“…Saitoh et al (2010) provided complete description of GPP estimation. Some of the energy fixed by photosynthesis is steadily released by respiration and is already accounted for in other heat storage terms (Moderow et al, 2009 …”

“…Recent studies suspected and discussed that vertical and horizontal advections may contribute to the lack of energy balance closure and the estimation accuracy of nighttime CO2 flux (e.g., Foken, 2008). However, any evaluation of the importance of advection in relation to the energy balance will fail even over mountainous topography (Moderow et al, 2009). Therefore, at this study, we focus the energy imbalance without advection term.…”