Andrei Serafimovich

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.

Description of the Waldstein Measuring Site

The chapter gives a brief overview of the measuring sites of the Bayreuth Center of Ecology and Environmental Research at the Waldstein mountain, Germany, northeast Bavaria, with a special focus on the Waldstein-Weidenbrunnen site (FLUXNET site DE-Bay) as well as the Pflanzgarten and Kohlerloh sites. The climate of the area, the structure of the forest stands with plant area index profiles, and the soil properties are described. There is a special focus on the tower installations, the permanent program, and the special experimental campaigns. Pictures of all sites and all relevant data are given, while the instrumentation is listed in Appendix A. Due to the storm event “Kyrill” in January 2007, the southern part of the forest was destroyed. A trace gas flux footprint analysis is presented for conditions after a large wind throw.

Upscaling surface energy fluxes over the North Slope of Alaska using airborne eddy-covariance measurements and environmental response functions

The goal of this study is to scale aircraft measured fluxes of sensible and latent heat to the North Slope of Alaska and develop high resolution flux maps. For this purpose we analyzed an eddy-covariance data set obtained by the research aircraft POLAR 5 as part of the AIRMETH-2012 campaign, and investigated the spatial patterning of energy fluxes. Environmental response functions between flux observations and corresponding biophysical and meteorological drivers were estimated using a combination of time-frequency decomposition, dispersion modeling and machine learning. The extracted relationships are then used to scale observational data across heterogeneous Arctic landscapes, thus improving the spatial coverage and representativeness of the energy fluxes. Maps of projected energy fluxes are used to asses energy partitioning in northern ecosystems and to determine dominant energy exchange processes of permafrost area.

Coherent Structures and Flux Coupling

This chapter summarizes the significant findings of the research on coherent structures contributed by investigations conducted at the Waldstein-Weidenbrunnen site from several field campaigns. The description of the quasi-online wavelet detection algorithm and of the coherent flux computation method using a triple decomposition is followed by a presentation of their application to define and diagnose vertical and horizontal couplings in forest canopies. It is demonstrated that these exchange regimes provide physically and biologically meaningful proxies for the communication of air and integration of the spatially separated sinks and sources as a result of the stratified canopy architecture. We continue by presenting two innovative applications of the coherent forest exchange that include the computation of daytime respiration fluxes directly from above-canopy eddy-covariance measurements and the explanation of stationary gradients in the sub-canopy CO2 field causing systematic advection as a result of the spatial heterogeneity of the forest architecture. Advantages and limitations of both are discussed. The chapter concludes by formulating directions for future research and indicating new observational techniques that may have the potential to improve understanding and quantifying the forest coherent exchange.

Long-Term Carbon and Water Vapour Fluxes

In this study we analyse eddy-covariance flux measurements of carbon dioxide and water vapour from 18 years at Waldstein–Weidenbrunnen (DE-Bay), a Norway spruce forest site in the Fichtelgebirge, Germany. Standard flux partitioning algorithms have been applied for separation of net ecosystem exchange NEE into gross primary production GPP and ecosystem respiration Reco, as well as gap-filling. The site has always been a carbon sink, and annual net uptake ( − NEE) shows a positive trend with values around 40 g C m−2 a−1 for 1997–1999 up to 615 ± 79 g C m−2 a−1 for 2011–2014. This is related to a strong increase in GPP, while Reco is slightly enhanced. Evapotranspiration increases coherently with NEE, while atmospheric demand, that is, potential evaporation, shows inter-annual variability, but no trend. Comparisons with studies from other warm-temperate coniferous forests show that our NEE estimates are at the upper range of the distribution, but still realistic. Also evapotranspiration estimates, evaluated in the Budyko framework, are in a similar range but with a large inter-annual variability. We identified instrumental problems and variability from different flux partitioning algorithms as a large source of uncertainty, but with only minor influence on the trends found. Warming and rising CO2-concentrations are consistent with the observed trend, but cannot be disentangled from site-specific changes such as the recovery from “Waldsterben” after liming and an increase in heterogeneity after a wind-throw, which likely plays the most important role in the observed dynamics. As such transitions from an “ideal” to a disturbed or heterogeneous site are likely more-often the case at FLUXNET stations built 10–20 years ago, a systematic bias in regional studies can only be avoided by taking each single site history into account.

Interaction Forest–Clearing

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.

Influence of Low-Level Jets and Gravity Waves on Turbulent Fluxes

Atmospheric waves and local wind phenomena in the atmospheric boundary layer are common forms of air motions observed above the forest canopy at night. Low-level jets with duration times of several hours and the gravity wave event were detected by SODAR-RASS and miniSODAR systems installed in the Fichtelgebirge Mountains in Germany.