Plasticity, Intraspecific Variability and Local Adaptation to Climatic Extreme Events of Ecotypes/Provenances of Key Plant Species
- Climate change, and especially an increase of magnitude and frequency of climatic extreme events such as drought periods or heatwaves, will alter growing conditions for plants in the future. Persistent ecosystems, with long-living organisms, such as forest or permanent grassland will be particularly impacted by this development. The velocity of these changes is likely to occur at a pace, which species may not be able to keep track with by natural dispersal or genetic adaptation. Agriculture, forestry and ecosystem management must develop counteracting practices to secure the persistence and functioning of these ecosystems and thus their provision of goods and services. Therefore it is important to develop a better understanding how species and ecosystems may respond to future climatic stressors. Impact assessments, e.g. via climatic envelope modelling are prone to misinterpretations of the adaptive capacity of species, as they do not incorporate the intraspecific genetic and phenotypic differences that exist within the populations accross the distribution range of a species.
Yet, intraspecific variation may exhibit potential tools for the development of climate change adaptation strategies. Here, I focus on key ecosystems in Central Europe. In particular the selective use of plant provenances or ecotypes may help to make ecosystems climate-resilient without a potentially more problematic introduction of exotic species. Especially provenances from warmer, drought-prone regions, with a current climate similar to the projected one for Central Europe recently came into focus as potential substitutes for local provenances, as they might have developed local adaptations to climate conditions at their location of origin. Insights about the response of these provenances to changing averages and extreme event regimes are crucial for a reasonable use of within-species diversity in climate change adaptation.
First, the concept of assisted colonization or migration of species or ecotypes and the role it can play as an adaptation strategy in agriculture, forestry or nature conservation is introduced (Manuscript1). It is suggested that a focus should be laid on keystone species that ensure ecosystem persistence and functioning as they govern the habitat structure and microclimate of a site. The assisted colonization of pre-adapted ecotypes of keystone species from climates similar to future projections for the target site is proposed.
Furthermore, provenances of selected grassland and forest key-species were exposed to drought and warming in two experiments in Bayreuth and Landau, and their ecological responses were analysed. Results suggest that local adaptations to climatic stressors exist. However, the magnitude and direction of responses strongly depend on species and climatic variables. For grassland species, e.g. differences in drought sensitivity could be demonstrated in some cases (Manuscript 4). Fagus sylvatica exhibited differences between the provenances in response to drought conditions, as well (Manuscript 3). It seems that marginal provenances, from the dry margins of the distribution range, show less increment reduction due to the drought treatment. Yet, under more favourable conditions of water supply these provenances did not yield the same high increment rates than more central provenances, indicating a trade-off between stability under stress and yield under non-stress conditions. A pine species that is generally considered to be rather drought-resistant, Pinus nigra, which is a potential substitute for climate-threatened conifers on dry sites in Central Europe, did not show any differences in response to drought and warming (Manuscript 2), maybe due to a weak selective pressure as a result of high drought-resistance across the whole distribution range. The impacts of drought on increment became not visible before the second year after the treatment, stressing the need for more long-time experiments in climate impact research.
Even in a generally warmer environment, cold extremes in winter or spring are expected still to prevail in the future. Therefore, the provenances of the selected species were tested for their cold-hardiness and late frost resistance (Manuscripts 5-7). Growth of the grassland species and F. sylvatica were negatively impacted by a late frost event and differences in late-frost sensitivity between provenances or ecotypes were identified. The (sub-) mediterranean species P. nigra showed differences between provenances in their winter cold hardiness. Correlations between performance under cold stress and winter conditions or late frost proneness of the places of origin could be established for almost all species. However, preceding climate experience, such as the warming or drought treatment of the plants altered their reaction to cold extremes compared to the control treatment, indicating the complexity of the interactive impacts of climate factors on ecosystem and plant performance.
The uncertainty of climate projections and the multitude of changing climatic stressors, though, make the prospect of an easy and rapid success in the search for single “best-adapted” provenances very questionable. In economics the portfolio effect shows that a diversification of investments decreases the risk of a total loss of profits. Hence, in a modelling procedure based on the increment data from the above mentioned experiment it was tested if a “portfolio investment” in several provenances in one stand decreases the risk of yield losses (Manuscript 8). Results indicate that the higher the number of provenances the higher the chance for a “best-performer” to be included in the set. So the likelihood of higher yields, under different climatic conditions increases, yet the risk of low yields stays stable.
Generally, it seems that the selective use of plant species and ecotypes in climate change adaptation can be a feasible tool to maintain ecosystem functionality and productivity. However, the uncertain projections, the multitude of climatic stressors and their interplay with other environmental factors and the potential impacts of assisted colonization of ecotypes on the genetic diversity within species and populations require further research.
Rhizodeposition and its effects on C fluxes in the soil
- Organic compounds released from living roots (rhizodeposits) are easily available sources of energy for microorganisms strongly affecting soil organic matter (SOM) dynamics. Although, rhizodeposition is a key driver of microbially mediated processes in the soils, it still remains the most uncertain component of the terrestrial carbon (C) cycle.
The input of C through rhizodeposition occurs in temporal and spatial hotspots. The objective of Study 1 was to determine the dynamics of hotspots of recently assimilated C in ryegrass roots. Shoots were 14CO2 pulse labeled and the allocation patterns at increasing time intervals were visualized by phosphor imaging. We could show a quick translocation of assimilated C to the roots. 14C hotspots were detected at the root tips already 6 hours after labeling. The hotspots remained active for at least 2 days. Eleven days after assimilation the hotspots at the tips had disappeared, and the 14C distribution was much more even than after 6 hours and 2 days.
Through the availability of rhizodeposits, hotspots create preferred habitats for microbes. Rhizodeposits are an important source of C and energy for microorganisms stimulating their growth and activity. Thereby, roots can influence the rate of native SOM decomposition in the rhizosphere. This rhizosphere priming effect (RPE) was reported to be plant-species specific. Therefore, we hypothesized that also plant inter-species interactions affect the RPE.
In Study 2, we used continuous 13CO2 labeling to investigate the RPE of monocultures and mixtures of typical agricultural crops. The RPE was consistently positive for all cultures with an increase of 43% - 136% above the unplanted soil. Of particular interest was the result that plant inter-species interactions between sunflower and wheat significantly reduced the RPE in contrast to mixtures which included soybean as a legume. It was argued that the RPE of the sunflower-wheat mixture was reduced through a more severe competition for nitrogen (N), whereas, due to the N-rich rhizodeposits of the legume and its lower demand for soil mineral N the RPE of the legume containing mixtures remained unaffected.
Besides potential plant-specific differences in the quality and quantity of rhizodeposits, also photosynthesis could control root exudation because of the fast transport of recently assimilated C to belowground pools. Taking both factors into account, in Studies 3 and 4 the effect of limited photosynthesis on the distribution of recently assimilated C, of stored C and of N was investigated. Based on 13C, 14C and 15N labeling of a legume and a non-legume we could demonstrate that high C and N demands of regrowing shoots after clipping led to a remobilization of stored C and N to the shoots. Additionally, recently assimilated C was retained in the regrowing shoots.
Shading, in contrast, did not induce a remobilization of stored C, since recently assimilated C obviously covered the demand of the shoots with lower growth rates. For both treatments lower amounts of recently assimilated C were observed in the belowground pools emphasizing the importance of the tight coupling of assimilation and belowground processes. Furthermore, different responses of clipping and shading of the legume and the non-legume could be detected for root-derived CO2.
The quantitative importance of rhizodeposition at field scale was determined in Study 5. We proposed a new approach for an improved quantification of rhizodeposition under field conditions taking into account the decomposed fraction of rhizodeposits. Based on a 14CO2 pulse labeling experiment under controlled conditions a rhizodeposition-to-root ratio was calculated and was applied to the root biomass of the field. The root biomass C of maize, sampled in July 2009, was 298±64 kg C ha-1. Gross rhizodeposition was 166±53 kg C ha-1.
With aging of SOM, the availability of C for microbial decomposition declines. In Study 6 the availability of younger relative to older C sources was assessed. The natural isotope abundances of 13C and 12C of SOM and CO2 were analyzed after a C3 to C4 vegetation change. The contribution of younger C, originating from the belowground C input by maize in the previous year, and that of older C sources, derived from the former C3 vegetation, to SOM and CO2 was determined. Comparing the proportions of younger and older C in SOM with that in CO2, we found that younger C was 7 times more available for microbial decomposition than older C pools.
In summary, this thesis extends the understanding of factors affecting rhizodeposition and of processes occurring at the soil-root interface. Furthermore, it presents a new method to quantify gross rhizodeposition at field scale. Although, we could gain insight in temporal changes of the availability of C pools for microbes, the ecological importance of C fluxes in the rhizosphere requires future research on this topic with regard to spatial and temporal predictions.
Kinetische Untersuchungen der Halogen-Aktivierung einer simulierten Salzpfanne in einer Smogkammer
- Reaktive Halogenverbindungen, insbesondere solche von Br und Cl, spielen eine wichtige Rolle beim atmosphärischen Abbau von Ozon und Kohlenwasserstoffen. Auch beeinflussen sie die Radikalzusammensetzung in der Troposphäre und haben somit eine Auswirkung auf klimarelevante Prozesse. Ziel dieser Arbeit war die Untersuchung der Halogenfreisetzung aus einer künstlichen Salzpfanne bei unterschiedlichen umweltrelevanten Bedingungen, wie relative Feuchte (RF) und Konzentrationen von Stickoxiden und Kohlenwasserstoffen, in einer Smogkammer. Diese Methode wurde in Rahmen dieser Arbeit entwickelt, um diese bislang wenig untersuchte Quelle von Halogenen zu untersuchen. Als Modell für die Salzpfanne wurde eine umweltrelevante Mischung von NaCl mit NaBr verwendet. Kinetische Berechnungen lieferten die zeitlichen Verläufe der Cl-Atome und OH-Radikale aus dem Verbrauch der zugesetzten Kohlenwasserstoffe und der Br-Atome aus dem Ozonverbrauch und UV-Absorptionsmessungen die Verläufe der BrO-Radikale. Während Salzpfannen-Experimenten bei einer mittleren RF von 37 % erreichte die Cl-Konzentration ein Maximum von 5 x104 cm-3. Ein rascherer Ozonabbau wurde bei größerer RF und zugleich höherer BrO-Konzentration beobachtet. Bei den Experimenten in Anwesenheit von sekundären organischen Aerosolen (SOA) verlangsamte sich der Ozonabbau um einen Faktor 16. Das BrO-Mischungsverhältnis erreichte in diesem Fall maximal 0.15 ppb im Vergleich zu Experimenten ohne SOA, bei denen das BrO Mischungsverhältnis 6 ppb erreichte. Nach Einwirkung der aus der Salzpfanne freigesetzten Halogene auf das SOA wurde ein erneutes Partikelwachstum beobachtet. Dies könnte eine Hauptsenke für Halogene in der Atmosphäre darstellen und einen neuen Weg zur Partikelbildung.
Heterogeneous chemistry of HONO and surface exchange
- Nitrous acid (HONO) is an important precursor of OH radicals, which are the key oxidizing species in the atmosphere and are therefore called the detergent of the atmosphere. Despite the importance of HONO for atmospheric chemistry and about 30 years of detailed research the exact formation mechanisms of both day- and night-time formation remain unclear. The main formation pathways discussed to date are heterogeneous reactions with nitrogen dioxide as the HONO precursor or microbiological activity in soil. As the ground surface is a major source of HONO, the vertical distribution of HONO is very sensitive to the extent of vertical mixing. Additionally, some uncertainty in comparing laboratory and field measurements might be caused by the not yet clarified role of relative humidity and surface wetness on HONO formation and deposition, respectively. These influences have been investigated in field measurements in the present study. For HONO measurements, LOng Path Absorption Photometers (LOPAPs) have been deployed.
During the Diel Oxidant Mechanism In relation to Nitrogen Oxides (DOMINO) campaign, HONO and other reactive trace gases were measured above a pine forest in south west Spain. In line with all recently published work, this study also found a substantial daytime formation of HONO. This so-called additional daytime source or unknown source was found to be slightly correlated (r² = 0.16) with actinic flux. Normalizing this unknown source to nitrogen dioxide mixing ratios improved the correlation (r² = 0.38), which indicates an influence of nitrogen dioxide availability. The coefficient of determination improved further to 0.47 by restricting the data to clear days and rejecting data from advection events. Thus, a fraction of the unknown source might be explained by light-induced nitrogen dioxide conversion but other factors have to be taken into account. Two processes of light-induced nitrogen dioxide conversion, proposed by recent laboratory studies, were shown to be negligible for the semirural conditions during our study. HONO photolysis was found to be the most important primary OH-radical source during DOMINO, contributing 20 % more OH than ozone photolysis integrated over the day.
Vertical exchange of HONO was studied at the “Waldstein-Weidenbrunnen” field site of the University of Bayreuth in the Fichtelgebirge Mountains in south east Germany. The simultaneous HONO measurements in and above a forest canopy highlighted the importance of turbulent exchange for the vertical distribution of HONO mixing ratios. The so-called coupling regimes of the forest (with the air layers above) were found to be a very useful micrometeorological concept to study vertical differences of mixing ratios in a forest. They denote which parts of the forest are coupled to the air layer above the canopy and thus take part in turbulent exchange of energy and matter. With this coupling tool it was possible to explain vertical mixing ratio differences by different sources and sinks and the magnitude of the difference by the intensity of vertical exchange. Studying the vertical mixing ratio differences of HONO, an unexpected result was that during late morning and around noon they were close to zero. As the lifetime of HONO below canopy of about 250 to 300 min was a factor of 25 to 30 longer than that above canopy of about 10 min, large mixing ratio differences would have been expected. The lack of these differences could be explained by efficient vertical mixing, which was indicated by a full coupling of the forest or a coupling by sweeps and only intermittent decoupling of the subcanopy during these periods. Around sunset, the whole forest became decoupled from the air layers above. This caused a steep increase in mixing ratio differences up to about 170 ppt due to a faster increase below canopy, indicating local formation below the canopy.
HONO and RH are correlated due to their diurnal cycles which are mainly caused by radiation. This diurnal contribution has to be removed from the respective signals in order to extract correlations on other timescales. Singular System Analysis, a tool for time series analysis, has been applied successfully to remove diurnal variations and long-term trends from the HONO and RH time series. Correlations of the higher frequency contributions of the remaining signals were poor but slightly positive. The HONO mixing ratios increase exponentially with RH from about 25 % RH to about 70 % RH. No clear correlation was found between around 70 and 95 % RH. Above 95 % RH, HONO mixing ratios decreased due to HONO uptake in droplets and liquid films. These features are in line with previously proposed mechanisms for interactions of water and HONO on surfaces.
The study highlighted the need to assess turbulent transport and surface properties in addition to chemistry for understanding the heterogeneous reactions and processes forming HONO.
Documentation of the Atmospheric Boundary Layer Experiment, Nam Tso, Tibet, 08th of July – 08th of August 2012
- no abstract
Applicability of weight-shift microlight aircraft for measuring the turbulent exchange above complex terrain
- The possibility to reliably observe the exchange of heat and moisture between the land surface and the atmosphere is vital to our understanding of the regional and global cycling of energy and water. While ground-based flux measurements can be made continuously for long periods, they only represent a small landscape unit. On the other hand, aircraft-based measurements have the ability to directly measure the exchange over large areas. Especially over heterogeneous landscapes the spatio-temporal characteristics of both approaches complement each other. However, complex terrestrial ecosystems are sparsely investigated to date, in particular over topographically structured terrain. This can be attributed to; (i) limitations in the description of boundary layer processes over non-homogenous terrain, and (ii) a lack of applicable measurement platforms and techniques to study these processes. In pursue of a resolution strategy, this dissertation investigates the applicability of weight-shift microlight aircraft (WSMA) to gain new insights in the spatial variability of heat and moisture exchange over complex terrain.
WSMA are comparatively cheap in procurement and maintenance, and their unique structure provides exceptional transportability and climb rate. These structural features qualify the WSMA for terrain-following flight over complex and inaccessible terrain, but potentially influence measurements aboard the aircraft. In this dissertation a WSMA with a scientific payload enabling fast measurements of the 3D wind, temperature, water vapor concentration, position, and the radiative flux is used to;
(i) Quantify the WSMA wind measurement uncertainty. A novel time-domain procedure is developed, which improves the accuracy of the WSMA wind measurement by 63% for the horizontal- and 72% for the vertical wind components. The resulting precisions are ±0.09 m s−1 and ±0.04 m s−1, and the agreement with ground-based measurements is in the order of ±0.4 m s−1 and ±0.3 m s−1 (root mean square deviation), respectively.
(ii) Quantify the WSMA eddy-covariance flux measurement uncertainty. From uncertainty propagation the smallest resolvable changes in friction velocity (0.02 m s−1), and sensible- (5 W m−2) and latent (3 W m−2) heat flux are estimated. In comparison to tower measurements, the WSMA observes higher fluxes (17–21%). The differences are not statistically significant, and can be explained by the tower setup and non-propagating eddies.
(iii) Spatially resolve and regionalize the heat and moisture exchange above a complex landscape. Wavelet decomposition of the turbulence data is used to yield a flux observation each 90 m along the flight path. For each flux observation the biophysical surface properties in the flux footprint are determined. An environmental response function between the flux observations and biophysical and meteorological drivers is then inferred using a machine learning technique. This function is used to produce regional maps of the heat and moisture exchange to an accuracy of ≤18% and a precision of ≤5% for individual land covers.
Hence this dissertation provides the necessary basis for using WSMA to investigate the mechanisms of turbulent exchange over heterogeneous and topographically structured terrain. Moreover, the developed algorithms are generally applicable to (i) partitioning flux uncertainty and environmental variability, (ii) extrapolating flux measurements, (iii) assessing the spatial representativeness of long-term tower flux measurements, and (iv) designing, constraining and evaluating flux algorithms for remote sensing and numerical modeling applications.
Tibet Plateau Atmosphere-Ecology-Glaciology Cluster Joint Kobresia Ecosystem Experiment: Documentation of the second Intensive Observation Period, Summer 2012 in KEMA, Tibet
- Experiment documentation of the second joined Kobresia ecosystem experiment conducted by the Atmosphere-Ecology-Glaciology Cluster within DFG SPP 1372 (Tibetan Plateau)in Kema, Tibet, China. The report provides background information about the field side, conducted measurements and participants.
Interactions between hydrology and biogeochemistry within riparian wetlands
- Interactions between hydrology and biogeochemistry at various spatio-temporal scales are important control mechanisms within terrestrial and aquatic ecosystems and exist among different compartments and transition interfaces. Understanding the fundamental mechanistic couplings between hydrological and biogeochemical processes and how these couplings feed back into ecosystem services and functions is an interdisciplinary challenge that must be addressed especially in the context of humanly mediated climate change. Riparian wetlands, as a transition zone between terrestrial and aquatic ecosystems, occupy large fractions of terrestrial ecosystems and provide important ecohydrological services. Due to their anoxic environments, riparian wetlands are able to store significant amounts of carbon as peat and act as an effective nutrient sink e.g. for sulfur, phosphorous and nitrogen. Riparian wetlands are characterized by highly dynamical interactions between hydrologically controlled transport mechanisms and biogeochemically controlled substrate availability, which governs nutrient cycling as well as the sink and source functions of wetlands. Generally, these interactions and their potential implications on ecosystem functions are only poorly understood. The representation of the tight couplings between hydrology and biogeochemistry in mechanistic models is a very challenging task because they have revealed a complexity which is often beyond the capabilities of current models. The objective of this thesis is to investigate interactions between hydrology and biogeochemistry in riparian wetlands and to understand their potential implications for internal biogeochemical process distributions and solute mobilization. Additionally, one major focus of the thesis is the attempt to represent such fundamental couplings in a process-based, hydrological/biogeochemical modeling approach. To this end, this thesis uses a combination of field and virtual experiments, as well as catchment-scale numerical modeling, performed for the Lehstenbach catchment, which was exemplarily chosen as main study site.
Results from the virtual experiments show very complex small-scale hydrological dynamics within the riparian areas. Here, runoff generation processes are strongly influenced by the spatial structure of the wetland-typical micro-topography (hummocks and hollows). Surface flow is episodically generated by a highly dynamical, threshold-controlled process where extended surface flow networks drain large fractions of the wetland's area. During intensive rainstorm events these surface flow networks, which contribute to stream discharge due to a fill and spill mechanism, dominate runoff generation. These fast flow components are characterized by very low residence times (minutes to hours) and once they are activated, the surface flow networks are able to rapidly mobilize large amounts of solutes, like nitrate or dissolved organic carbon (DOC), out of the wetlands by bypassing deeper anoxic layers. The importance of fast flow components for the catchment-scale mobilization of DOC was further confirmed by field investigations and catchment-scale numerical modeling. High frequency measurements of DOC in runoff of the Lehstenbach catchment revealed that DOC export is subject to substantial short term variations at an hourly to daily timescale. During intense rainstorms, DOC concentrations are up to ten times higher (up to 40 mg/L) compared to low flow conditions (~3-5 mg/L). Short term variations together with the dramatic rise of DOC concentrations in runoff during rainstorms can be explained by the episodically activation of fast flow components in the wetland areas. At the catchment-scale, application of a hydraulic mixing-cell (HMC) methodology in combination with numerical modeling has revealed that fast flow components like saturated overland flow are exclusively generated in the wetland areas during intensive rainstorm events. On an annual basis, exemplarily for the hydrological year 2001, the HMC analysis quantified the relative contribution of saturated overland flow related to the total discharge with 19.5%, which highlights the importance of riparian wetlands for catchment-scale runoff generation.
Virtual experiments, additionally show that distinct shifts between surface and subsurface flow dominance, as a result of small-scale micro-topographic driven runoff generation in the wetlands, are responsible for very complex three-dimensional subsurface flow patterns showing a wide range of subsurface residence times. To investigate how these micro-topography induced subsurface flow patterns, together with the non-uniform hydrological and biogeogeochemical boundary conditions, affect the internal re-distribution and transformation of redox-sensitive species (like nitrate, sulfate or iron) a coupled hydrological/biogeogeochemical model was developed. In the model, wetland-typical biogeochemical processes are represented in a sequential stream tube approach where redox-sensitive processes are implemented as kinetic reactions. Simulations show the formation of local hot spots for redox-sensitive processes within the subsurface as a result of the complex subsurface flow paths and the transport-limited availability of electron acceptors and donors. Formation of hot spots was simulated for all key reduction processes including iron(III)-/sulfate reduction and denitrification as well as for the corresponding re-oxidation processes. These results offer a new perspective on hydrologically controlled biogeochemical transformation processes in riparian wetlands, which provides a dynamic framework to explain process heterogeneity in wetland soils and variability in process rates over space and time.
Findings from this thesis clearly prove how useful interdisciplinary approaches are in understanding processes and mechanisms in ecosystems and how important functions of ecosystems are affected by couplings among those. However, a lot of knowledge gaps still exist in understanding the nature of dependency between water and nutrient cycles across scales and how these interacting cycles feed back into humanly-mediated climate change in ecosystems. Development of new interdisciplinary methodologies and frameworks as well as an integrated way of thinking across the boundaries of the different environmental disciplines is necessary to address the grand challenges associated with climate change.
Free convection and turbulent fluxes over complex terrain
- The impact of complex terrain on the land-atmosphere exchange is investigated in this thesis. Here, free convection, a very effective vertical transport mechanism as turbulence is predominantly driven by buoyant forces, is explicitly addressed. Recently, it was shown for certain situations over complex terrain that free convective injections of surface layer air masses into the atmospheric boundary layer (ABL) can alter the ABL properties significantly. This study aims at the general identification and description of such situations of near-ground free convection conditions (FCCs) over complex terrain. For this purpose, data obtained during the COPS (Convective and Orographically induced Precipitation Study) field campaign in summer 2007 were used. Within this project, several surface flux measurement stations were installed, mainly in valleys and on mountaintops of the Black Forest, southwestern Germany. Turbulent fluxes were calculated with the eddy-covariance (EC) method and were used to detect FCCs with the help of a stability parameter. The flux measurements were further combined with ABL profiling measurements (Sodar/RASS) and a large-eddy simulation (LES) model in order to investigate the impact of FCCs on ABL properties. The effect of complex terrain on the energy balance closure and on spatial and temporal flux differences was also studied with these flux data.
FCCs were detected on about 25% of the days during the three month COPS experiment. In situations of weak synoptic forcing, thermally driven orographic (e.g. valley winds) or local wind systems developed over the complex terrain due to heating differences. During the adaption of these wind systems to changing heating differences (e.g. during the reversal of the valley wind from down- to up-valley winds in the morning), the horizontal wind vanished. If, at the same time, the buoyancy flux was positive and enhanced, buoyant forces exceeded the usually prevailing shear forces in the surface layer and FCCs were detected. Moreover, it was demonstrated that FCCs are not restricted to the COPS region. Also, a data set of Nam Co station on the Tibetan Plateau showed FCCs during the reversal of a thermally driven land-lake breeze. However, at this high-altitude site, FCCs were more often detected in the afternoon compared to the COPS region due to the frequent change of heating differences during cloud cover periods.
The Sodar/RASS as well as the LES model showed the presence of coherent updraft
structures in the developing early-morning convective boundary layer (CBL) in the Kinzig valley (Black Forest) during FCCs. Spectral analysis of the EC data in these situations indicated the existence of large-eddy turbulent scales – typical for thermal updrafts in the CBL – already close to the ground. An ensemble and time mean analysis of the simulated flow field in the valley further confirmed that the Sodar/RASS was located preferably in an updraft region during FCCs. In a CBL over flat homogeneous terrain, the locations of convective structures would occur randomly. However, over the complex orography of the Kinzig valley, the updraft structures were found to develop in
quasi-stationary patterns at specific locations relative to the surrounding mountain ridges. The model further showed that the flux through the valley boundary layer is mainly determined by the flux within these coherent updrafts. In combination with the Sodar/RASS observations, the model also showed that these updrafts deeply penetrated into the stably stratified valley boundary layer up to approximately the height of the surrounding mountains leading to an effective upward counter-gradient transport of surface layer air mass properties during FCCs.
The analysis of the turbulent fluxes at the different COPS sites showed that the flux values were strongly determined by varying land surface characteristics. Also an increase of the Bowen ratio with increasing altitude could be detected. These findings are in accordance with former studies in this area. As expected, the energy balance was found to be unclosed on average during the entire COPS period, with values of the residual typical for heterogeneous landscapes. However, regarding only the periods with FCCs, no residual occurred on average. This is due to the fact that the landscape
heterogeneity is of minor importance in case of the more vertical oriented exchange regime during FCCs, so that missing advective flux components became strongly reduced in these situations. Moreover, it was found that in comparable periods with no FCCs, flux components were missing with exactly the proportions of the buoyancy flux ratio, thus suggesting a correction of the energy balance according to the buoyancy flux ratio approach. These results support recent publications on the energy balance closure
Site-specific modelling of turbulent fluxes on the Tibetan Plateau
- The Tibetan Plateau attracts attention in recent decades due to its influence on the East-Asian Monsoon and regional hydrology. Therefore estimates of the regional energy and water balance have come into the focus, utilising remote sensing and regional model approaches, but such attempts require surface-specific flux data of high quality for validation. Eddy-covariance measurements are qualified for this task, but these are scarce on the Tibetan Plateau, incomplete due to quality filtering and potentially biased due to the well-known closure gap of the observed energy balance as well as small-scale heterogeneity. This thesis is related to the infrastructural EU project CEOP-AEGIS, aiming at a standardised processing of eddy-covariance data – including correction of the energy balance closure and gap-filling – on the Tibetan Plateau.
In a pre-analysis step, particular issues about data quality of turbulent fluxes (sensible heat flux and latent heat flux/evapotranspiration) at Tibetan Plateau sites have been addressed. One of them is the degradation of data quality due to the frequent occurrence of near-ground free convective conditions. Another issue arises from coordinate rotation for non-omnidirectional sonic anemometer, which requires a careful handling. In consequence, a sector-wise planar-fit is recommended, disregarding the sector influenced by the anemometer's mounting structure. This can reduce occurrences of invalid momentum flux data, whilst no effect on scalar fluxes can be seen.
As a main topic, this thesis investigates the application of process-based modelling to estimate turbulent flux exchange between the surface and the atmosphere for typical surface types on the Tibetan Plateau. Therefore a case study has been carried out at Nam Co, Tibetan Plateau. Turbulent flux measurements over dry and wet grassland as well as over a shallow lake have been conducted during the summer monsoon season of 2009, and modelled with the land surface scheme SEWAB and a hydrodynamic multilayer model for the lake. Adaptations were implemented to the land surface scheme with regard to the special conditions on the Tibetan Plateau, such as extreme diurnal variation of surface temperature and variation in soil moisture, further called TP version. The analysis includes a consequent model comparison with eddy-covariance data, using model parameters derived independently rather than applying optimisation strategies. Specific attention has been devoted to the impact of observed energy balance closure and its correction, establishing a new correction method according to the Buoyancy flux.
The land surface model reasonably represented the dry and the wet grassland site by only setting the site-specific model parameters, and the TP version performed overall better than the original version, while laboratory measurements of soil parameters failed to improve model performance in comparison to standard parameter values. Soil temperature and moisture measurements as well as field based knowledge of the soil type have been identified as minimum requirements for model parameter acquisition. Lake surface fluxes have been modelled reliably, the lake depth has been taken into account. These results can be transferred to any lake on the Tibetan Plateau given the required forcing data including a representative lake surface temperature.
The choice of the surface model and the selection of the energy balance closure correction method are inter-related problems. The correction partitions the balance residual to the sensible and latent heat flux. This can be typically done according to the Bowen ratio, or according to the presented new method which attributes a larger fraction to the sensible heat flux. Testing both methods leads to partly ambiguous model performance, especially with respect to the used parameter sets. It clearly leads to shifts in model bias, while the R² metric suggests higher model compatibility to the Bowen ratio correction. The latter agrees with previous findings with respect to SEWAB modelling, but is in contradiction with recent experimental findings, attributing the closure gap to secondary circulations, driven by buoyancy. Future research on model structure should account for such processes.
As expected, the flux measurements showed distinct differences between the investigated land use types in magnitude and dynamics. The used models were able to resolve these differences in general with contrasts between surface types exceeding model errors. This must be considered when validating regional flux estimates with eddy-covariance data from the dry Nam Co station. The findings from this thesis provide the basis to process eddy-covariance data on the required level as described above.