PhD Thesis
Investigation of horizontal meteorological structures in comparison to turbulent structures at a forest edge
Jörg Hübner (07/2010-12/2014)
Support: Thomas Foken
Forest ecosystems play a key role in the Earth's carbon cycle, as their uptake of carbon dioxide is largest in the terrestrial biospheres (CO2 sink). Despite a vast number of ecological studies about forest ecosystem, there are still outstanding issues about prevailing meteorological conditions, turbulent and coherent structures and exchange processes of heterogeneous forest ecosystems. Therefore, this thesis is related to the joint research project EGER IOP3 (ExchanGE processes in mountainous Regions – Intensive Observation Period 3), aimed at the investigation of diurnal cycles of energy, matter and (non-)reactive trace compounds in the soil-vegetation-boundary-layer-system at a forest edge within a heterogeneous forest ecosystem in a very complex terrain in mid Europe.
Well established measurement techniques were used, such as the eddy-covariance method for determination of turbulent/coherent fluxes and for detection of different vertical and horizontal coupling regimes. SODAR/RASS systems were used to investigate boundary layer phenomena. Additionally, a novel, fully automatic Horizontal Mobile Measuring System (HMMS) was successfully developed, applied and assessed within this thesis. The HMMS was installed on a 150 m long transect perpendicular to the forest edge, to obtain higher information density about horizontal gradients of eight quantities (short/long-wave down/upwelling radiation, temperature, humidity, CO2 and O3 concentration). The experimental design, with 3D installation of the towers/masts, HMMS and profiling systems was ideal for the investigation of the research issues mentioned above.
By combination of all measurement techniques, significant differences could be observed along the transect forest – forest edge – clearing (three locations), with strong distinctions in the vicinity of the forest edge. The highest gradients in the HMMS measurements occurred near the forest edge. Furthermore, it could be observed that the turbulence influenced quantities in the HMMS measurements (temperature, humidity and trace gases) were mainly affected by the prevailing vertical structures at the forest edge, while the influence decreased the further away from the forest edge. These findings coincide with the findings for coherent structures, where only at the forest edge a significant daily variation could be observed for ejections and sweeps, with strong ejection motions during daytime (updraft) and strong sweep motions during nighttime (downdraft).
The thermal updraft during daytime could be attributed to the highest temperatures occurring near the forest edge (1.5 K warmer than at the clearing and within the forest) and the advective transport of energy towards the forest edge. The reverse was also apparent, where energy was transported off the forest edge towards the clearing. Nevertheless, highest energy and CO2 fluxes, as well as highest variation could be observed at the forest edge. This leads to a better energy balance closure at the forest edge (Residual Res = 17 %), compared to both other locations (Res = 25 – 30 %). These findings indicate the facilitation of quasi-stationary secondary circulations above the observation site, which could be confirmed to some extent by further experimental observations. During times of horizontal energy exchange, there was a horizontal coupled regime and an inflow of O3-depleted (decrease in concentration of approximately 20 ppb), and slightly colder air along the transect perpendicular to the forest edge, originating from the atmosphere above the site. Additionally, the ACASA model showed discrepancies between modelled and measured fluxes which could be attributed to horizontal advective flow and secondary circulations.
The investigation of boundary layer phenomena, such as low-level jets (LLJ) and strong winds showed considerable different impacts on exchange processes above the heterogeneous forest ecosystem. During LLJ there was a coupled regime (C/Cs), with good mixing and enhanced CO2 fluxes, while during strong wind situations wave motions (Wa; uncoupled regimes) occurred with no mixing and reduced CO2 fluxes. True, effects could be observed at every location, due to changes in vertical coupling regimes and coherent transport. However, coherent transport was found to be largest above the forest and decreasing towards the clearing, and thus the CO2 fluxes were also most affected above the forest and less affected towards the clearing. During LLJ events the CO2 fluxes were enhanced by approximately 100 % above the forest, 70 – 100 % at the forest edge, while above the clearing the fluxes were only marginally enhanced. During uncoupled situations (Wa) the fluxes were reduced by approximately 100 % above the forest and again, the changes were less at the forest edge and marginal above the clearing. The different CO2 fluxes led to significant gradients of CO2 concentration along the transect, resulting even to a drainage of CO2-enriched air out off the forest onto the clearing (during strong wind situation).
The continuous horizontal transect measurements of the HMMS in combination with tower measurements provided a new understanding of exchange processes of heterogeneous forest ecosystems. Such an overview of prevailing gradients, inflows, outflows, accumulations, depletions, as well as insights on coherent/turbulent transport, coupling regimes, secondary circulations and boundary layer phenomena over spatial and temporal terms would have not been possible if only static tower measurements were used.