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Energy balance closure in a boreal old-growth jack pine stand and clearcut, and implications for eddy covariance CO₂ flux measurements Kidston, Joe

Abstract

The accuracy of the components of the surface energy balance were analysed, in an attempt to determine the cause of the imbalance that is commonly observed in energy balance closure studies when eddy covariance (EC) measurements of the sum of sensible (H) and latent (LE) heat fluxes, which we call the total surface layer heat flux (7), are compared with the net radiation (R„) minus the soil heat flux. We utilise data from a mature boreal jack pine forest, and a jack pine clearcut, both part of the boreal ecosystem research monitoring sites (BERMS) study area in central Saskatchewan, Canada, to elucidate differences in the cause of the imbalance when EC measurements are made over tall vegetation compared to when they are made close to a solid surface. We demonstrate that at the clearcut site (HJP02), a significant portion of the imbalance was caused by both the overestimation of R„ due to instrumentation issues and the inclusion of the tower in the field of view of the downward facing radiometers, and underestimation of LE due to attenuation of H2O variance in the sample tube of the closed-path infrared gas analyser. Loss of low-frequency covariance due to insufficient averaging time, and systematic advection of fluxes away from the EC tower, were discounted as significant issues; the latter through comparison of measurements from a second, roving EC system with the long term system. That the spatial and temporal distributions of T were well behaved, and furthermore that there was not similarity of the relative magnitudes of the turbulent fluxes during half-hours when different closure was achieved despite similar climatic conditions, implied that the variation in closure was not caused by micrometeorological processes that impacted all turbulent fluxes similarly, and therefore the turbulent fluxes should not be forced for closure. Conversely at the mature forest site (O JP), loss of low-frequency covariance when a 30-min. averaging time was used contributed significantly to the systematic imbalance. However, stationarity became a significant issue when longer averaging times were used; averaging times that were long enough to capture all of the low-frequency covariance were too long to resolve all of the high-frequency covariance. This, combined with analysis of the 30-min. flux data that showed similarity between T and net ecosystematmosphere exchange of CO2 (NEE), implied that where EC measurements are made over tall canopies, T should be forced for energy balance closure while retaining the measured Bowen ratio, and the same factor should be applied to NEE.

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