# Energy Balance Closure Using Eddy Covariance Above Two Different Land Surfaces and Implications for CO_{2} Flux Measurements

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## Abstract

Components of the surface energy balance of a mature boreal jack pine forest and a jack pine clearcut were analysed to determine the causes of the imbalance that is commonly observed in micrometeorological measurements. At the clearcut site (HJP02), a significant portion of the imbalance was caused by: (i) the overestimation of net radiation (*R* _{ n }) due to the inclusion of the tower in the field of view of the downward facing radiometers, and (ii) the underestimation of the latent heat flux (λ*E*) due to the damping of high frequency fluctuations in the water vapour mixing ratio by the sample tube of the closed-path infrared gas analyzer. Loss of low-frequency covariance induced by insufficient averaging time as well as systematic advection of fluxes away from the eddy-covariance (EC) tower were discounted as significant issues. Spatial and temporal distributions of the total surface-layer heat flux (*T*), i.e. the sum of sensible heat flux (*H*) and λ*E*, were well behaved and differences between the relative magnitudes of the turbulent fluxes for several investigated energy balance closure (*C*) classes were observed. Therefore, it can be assumed that micrometeorological processes that affected all turbulent fluxes similarly did not cause the variation in *C*. Turbulent fluxes measured at the clearcut site should not be forced to close the energy balance. However, at the mature forest site (OJP), loss of low-frequency covariance contributed significantly to the systematic imbalance when a 30-min averaging time was used, but the application of averaging times that were long enough to capture all of the low-frequency covariance was inadequate to resolve all of the high-frequency covariance. Although we found qualitative similarity between *T* and the net ecosystem exchange (NEE) of carbon dioxide (CO_{2}), forcing *T* to closure while retaining the Bowen ratio and applying the same factor to CO_{2} fluxes (*F* _{ C }) cannot be generally recommended since it remains uncertain to what extent long wavelength contributions affect the relationship between *T*, *F* _{ C } and *C*.

## Keywords

Available energy flux Eddy covariance Energy balance closure High-frequency loss Latent heat flux Low-frequency loss Sampling-tube attenuation Sensible heat flux Turbulent flux cospectra## Preview

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