Energy balance across the eddy covariance sites

Abstract

Eddy covariance technique has become one of the most reliable approaches to study the interactions between atmosphere and earth’s surface. However, observations from single or multiple eddy covariance systems always lack energy balance closure (EBC) across all the FLUXNET research sites. Particularly, the measurements over the heterogeneous surface are questionable and the heterogeneity is closely related to the energy imbalance issue. This dissertation is broadly divided into three parts: the first part investigates the temporal and spatial variations of EBC across 150 FLUXNET research sites, the other two parts performs field measurements respectively over the wetland heterogeneous surface and the urban heterogeneous surface. Chapter 2 investigates the temporal and spatial variations of EBC across 150 FLUXNET research sites covering nine landscapes and five climate zones. Overall, the best EBC is observed over the sites covering with savannahs and grass, and the worst EBC is found in the wetland sites. And EBC generally decreases from Köppen climate zone A to E. The results from this part of study could be used to correct eddy covariance data and EBC related models; they also connect the cross-influenced relationships between various environmental variables and EBC to the stomata aperture and metabolism of the vegetation, which enhances our understanding of the potential link between EBC and vegetation physiology. Given that the worst EBC is observed over the wetland sites in Chapter 2, field measurements are firstly conducted in a sub-tropical wetland of Hong Kong in Chapter 3, to explore the overlooked influence of lateral energy fluxes in the subsurface (Q_Sub) on EBC. The results show that Q_Sub could not account for the energy imbalance here because their magnitudes are relatively small and fluctuate in out of phase with the energy budget residuals. This part of study enhances our understanding of energy exchanges between a terrestrial biotope and the surrounding water, which might further generate insights into the biochemical processes in wetlands. In Chapter 4, the eddy covariance tower is installed on a building roof in a highly-dense and compact urban area of Hong Kong to assess the surface energy fluxes and EBC over such complex urban surface. With a high density of high-rise buildings, as well as small fraction of vegetation, the measured energy fluxes are featured with smaller latent heat fluxes, larger sensible heat fluxes and the extremely higher anthropogenic heat fluxes comparing with other urban sites but with low and sparse buildings. EBC displays a significant difference among various wind directions, and generally shows a negative relationship with surface heterogeneity. This part of the study helps to fill a gap in our understanding of surface energy fluxes and EBC in highly-dense cities, and sheds insights into the future installation of eddy covariance tower in similar areas. Overall, the results of this dissertation improve our understanding of the interaction of energy and gas fluxes between the heterogeneous land surface and atmosphere, as well as facilitate the application of an eddy covariance system over the heterogeneous surface. They also benefit the optimization of models related to EBC and enhance the understanding of the relationship between EBC and different environmental variables.