Fabian Eder

Coherent Structures at a Forest Edge: Properties, Coupling and Impact of Secondary Circulations

Little is known about the influence of coherent structures on the exchange process, mainly in the case of forest edges. Thus, in the framework of the ExchanGE processes in mountainous Regions (EGER) project, measurements of atmospheric turbulence were taken at different heights between a forest and an adjacent clear cutting using sonic anemometers and high-frequency optical gas analyzers. From these turbulence data, dominant coherent structures were extracted using an already existing wavelet methodology, which was developed for homogeneous forest canopies. The aim of this study is to highlight differences in properties of coherent structures between a forest and a clear cutting. Distinct features of coherent exchange at the forest edge are presented and a careful investigation of vertical and horizontal coupling by coherent structures around the surface heterogeneity is made. Within the forest, coherent structures are less frequent but possess larger time scales, indicating that only the largest coherent motions can penetrate through the forest canopy. At the forest edge, there is no crown layer that can hinder the vertical exchange of coherent structures, because these exhibit similar time scales at all heights. In contradiction to that, no improved vertical coupling was detected at the forest edge. This is mainly because the structures captured by the applied routine contribute less to total turbulent fluxes at the edge than within the forest. Thus, coherent structures with time scales between 10 and 40 s are not the dominant exchange mechanism at the forest edge. With respect to the horizontal direction, a consistent picture of coherent transport could be derived: along the forest edge there is mainly good coupling by coherent structures, whereas perpendicular to the forest edge there is mainly decoupling. Finally, it was found that there is a systematic modulation of coherent structures directly at the forest edge: strong ejection motions appear in all time series during the daytime, whereas strong sweeps dominate at night. An effect of wind direction relative to the forest edge is excluded. Consequently, it is hypothesized that this might be an indication of a quasi-stationary secondary circulation above the clear cutting that develops due to differences in surface temperature and roughness. Such circulations might be a relevant turbulent transport mechanism for ecosystem-atmosphere exchange in heterogeneous landscapes.

Mesoscale Eddies Affect Near-Surface Turbulent Exchange: Evidence from Lidar and Tower Measurements

AbstractThe eddy-covariance technique tends to underestimate turbulent heat fluxes, which results in nonclosure of the surface energy balance. This study shows experimental evidence that mesoscale turbulent organized structures, which are inherently not captured by the standard eddy-covariance technique, can affect near-surface turbulent exchange. By using a combined setup of three Doppler wind lidars above a cropland-dominated area in Germany, low-frequency turbulent structures were detected in the surface layer down to a few meters above ground. In addition, data from two micrometeorological stations in the study area were analyzed with respect to energy balance closure. In accordance with several previous studies, the data confirm a strong friction velocity dependence of the energy balance residual. At both stations, the energy balance residual was found to be positively correlated with the vertical moisture gradient in the lower atmospheric boundary layer, but at only one station was it correlated wit...

Evaluation of Two Energy Balance Closure Parametrizations

A general lack of energy balance closure indicates that tower-based eddy-covariance (EC) measurements underestimate turbulent heat fluxes, which calls for robust correction schemes. Two parametrization approaches that can be found in the literature were tested using data from the Canadian Twin Otter research aircraft and from tower-based measurements of the German Terrestrial Environmental Observatories (TERENO) programme. Our analysis shows that the approach of Huang et al. (Boundary-Layer Meteorol 127:273–292, 2008), based on large-eddy simulation, is not applicable to typical near-surface flux measurements because it was developed for heights above the surface layer and over homogeneous terrain. The biggest shortcoming of this parametrization is that the grid resolution of the model was too coarse so that the surface layer, where EC measurements are usually made, is not properly resolved. The empirical approach of Panin and Bernhofer (Izvestiya Atmos Oceanic Phys 44:701–716, 2008) considers landscape-level roughness heterogeneities that induce secondary circulations and at least gives a qualitative estimate of the energy balance closure. However, it does not consider any feature of landscape-scale heterogeneity other than surface roughness, such as surface temperature, surface moisture or topography. The failures of both approaches might indicate that the influence of mesoscale structures is not a sufficient explanation for the energy balance closure problem. However, our analysis of different wind-direction sectors shows that the upwind landscape-scale heterogeneity indeed influences the energy balance closure determined from tower flux data. We also analyzed the aircraft measurements with respect to the partitioning of the “missing energy” between sensible and latent heat fluxes and we could confirm the assumption of scalar similarity only for Bowen ratios

Secondary circulations at a solitary forest surrounded by semi-arid shrubland and their impact on eddy-covariance measurements

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Large-eddy simulation of the energy balance closure problem: a case study for a field campaign with multi-scale observations

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Coherent structure patterns affect energy balance closure: evidence from virtual measurements for a field campaign

In 2011, a special experiment was conducted to investigate turbulent structures at the edge between the Waldstein–Weidenbrunnen forest site and the Kohlerloh clearing. A horizontal moving measuring system was used to detect significant gradients of the radiation fluxes, temperature, moisture, ozone, and carbon dioxide concentrations for different situations at day and night. In agreement with other studies, an increase of the turbulent fluxes and ejections at the forest edge could be found. This means that the energy balance closure was also better than that obtained directly at the Weidenbrunnen site. The vertical coupling by coherent structures was often—mainly at daytime—very good. In contrast, the horizontal coupling between the forest and the clearing at the edge was, in most cases, not apparent. For wind directions coming from the forest, the coherent structures did not touch down at the surface of the clearing. These investigations were made with a wavelet tool. A clear indication of secondary circulations between the forest and the clearing was not possible.