Surface energy balance closure by the eddy-covariance method above three boreal forest stands and implications for the measurement of the CO2 flux

Closure of the surface energy balance provides an objective criterion for evaluating eddy-covariance (EC) flux measurements. This study analyses 5 years of EC carbon dioxide, water vapor, and sensible heat flux measurements from three mature boreal forest stands in central Saskatchewan, Canada. The EC sensible and latent heat fluxes, H and; E, underestimated the surface available energy by 11% (aspen), 15% (black spruce), and 14% (jack pine). At all sites, the energy-closure fraction CF responded similarly to the friction velocity u(*), atmospheric stability, and time of day. At night, CF increased from similar to 0.3 at very low-u(*) to an asymptotic maximum of similar to 0.9 at u(*) above 0.35 m s(-1). During unstable-daytime periods, CF varied linearly from similar to 0.7 at low-u(*) to similar to 1.0 at high-u(*) The energy imbalance pattern was similar among sites and may be characteristic of the continental, boreal forest. EC measurements of net ecosystem exchange F-NEE have no objective, diagnostic parameter that is equivalent to CF. We, therefore derived an analogous F-NEE "closure function" CFNEE by normalising measured F-NEE against estimates from an empirical model that was tuned to the high-u(*) data. CF and CFNEE responded similarly to u(*), atmospheric stability, and time of day. We discuss two implications for EC flux data post-processing. The results uphold the common practice of rejecting EC measurements during low-u(*) periods. They also lend support to the application of energy-closure adjustments to H, lambda E, and F-NEE. Crown Copyright (c) 2006 Published by Elsevier B.V. All rights reserved.