ALBERT

Description: Over the last 20 years, a large number of instruments have provided plasma density measurements in Earth's topside ionosphere. In order to utilize all of the collected observations for empirical modeling, it is necessary to ensure that they do not exhibit systematic differences and are adjusted to the same reference frame. In this study, we compare satellite plasma density observations from Gravity Recovery and Climate Experiment (GRACE), Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC), CHAllenging Minisatellite payload (CHAMP), Swarm, and Communications/Navigation Outage Forecasting System (C/NOFS) missions. Electron densities retrieved from GRACE K-Band Ranging (KBR) system, previously shown to be in excellent agreement with incoherent scatter radar (ISR) measurements, are used as a reference. We find that COSMIC radio occultation (RO) densities are highly consistent with GRACE-KBR observations showing a mean relative difference of urn:x-wiley:21699380:media:jgra56751:jgra56751-math-0001, and therefore no calibration factors between them are necessary. We utilize the outstanding three-dimensional coverage of the topside ionosphere by the COSMIC mission to perform conjunction analysis with in situ density observations from CHAMP, C/NOFS and Swarm missions. CHAMP measurements are lower than COSMIC by urn:x-wiley:21699380:media:jgra56751:jgra56751-math-0002. Swarm densities are generally lower at daytime and higher at nighttime compared to COSMIC. C/NOFS ion densities agree well with COSMIC, with a relative bias of urn:x-wiley:21699380:media:jgra56751:jgra56751-math-0003. The resulting cross-calibration factors, derived from the probability distribution functions, help to eliminate the systematic leveling differences between the data sets, and allow using these data jointly in a large number of ionospheric applications.

Description: This work presents a new extension to B-Splines that enables them to model functions on directed tree graphs such as non-braided river networks. The main challenge of the application of B-splines to graphs is their definition in the neighbourhood of nodes with more than two incident edges. Achieving that the B-splines are continuous at these points is non-trivial. For both, simplification reasons and in view of our application, we limit the graphs to directed tree graphs. To fulfil the requirement of continuity, the knots defining the B-Splines need to be located symmetrically along the edges with the same direction. With such defined B-Splines, we approximate the topography of the Mekong River system from scattered height data along the river. To this end, we first test and validate successfully the method with synthetic water level data, with and without added annual signal. The quality of the resulting heights is assessed besides others by means of root mean square errors (RMSE) and mean absolute differences (MAD). The RMSE values are 0.26 m and 1.05 m without and with added annual variation respectively and the MAD values are even lower with 0.11 m and 0.60 m. For the second test, we use real water level observations measured by satellite altimetry. Again, we successfully estimate the river topography, but also discuss the short comings and problems with unevenly distributed data. The unevenly distributed data leads to some very large outliers close to the upstream ends of the rivers tributaries and in regions with rapidly changing topography such as the Mekong Falls. Without the outlier removal the standard deviation of the resulting heights can be as large as 50 m with a mean value of 15.73 m. After the outlier removal the mean standard deviation drops to 8.34 m.

Description: Hydrogen (δ2H) and oxygen (δ18O) isotopes of water extracted from speleothem fluid inclusions are important proxies used for paleoclimate reconstruction. In our study we use a cavity ring-down laser spectroscopy system for analysis and modified the approach of Affolter et al. (2014) for sample extraction. The method is based on crushing of small sub-gram speleothem samples in a heated and continuously water-vapour purged extraction line. The following points were identified: Injection of reference water shows a precision (1σ) of 0.4–0.5 ‰ for δ18O values and 1.1–1.9 ‰ for δ2H values for water amounts of 0.1–0.5 μl, which improves with increasing water amount to 0.1–0.3 ‰ and 0.2–0.7 ‰, respectively, above 1 μl. The accuracy of measurements of water injections and water-filled glass capillaries crushed in the system is better than 0.08 ‰ for δ18O and 0.3 ‰ for δ2H values. The reproducibility (1σ) based on replicate analysis of speleothem fluid inclusion samples with water amounts 〉0.2 μl is 0.5 ‰ for δ18O and 1.2 ‰ for δ2H values, respectively. Isotopic differences between the water vapour background of the extraction system and the fluid inclusions have no significant impact on the measured fluid inclusion isotope values if they are within 10 ‰ for δ18O and 50 ‰ for δ2H values of the background. Tests of potential adsorption effects with inclusion free spar calcite confirm that the isotope values are unaffected by adsorption for water contents of about 1 μl (fluid inclusion) water per g of carbonate or above. Fluid inclusion analysis on three different modern to late Holocene speleothems from caves in northwest Germany resulted in δ18O and δ2H values that follow the relationship as defined by the meteoric water line and that correspond to the local drip water. Yet, due to potential isotope exchange reactions for oxygen atoms, hydrogen isotope measurements are preferentially to be used for temperature reconstructions. We demonstrate this in a case study with a Romanian stalagmite, for which we reconstruct the 20th century warming with an amplitude of approximately 1 °C, with a precision for each data point of better than ±0.5 °C.

Description: Brachiopods present a key fossil group for Phanerozoic palaeo-environmental and palaeo-oceanographical reconstructions, owing to their good preservation and abundance in the geological record. Yet to date, hardly any geochemical proxies have been calibrated in cultured brachiopods and only little is known on the mechanisms that control the incorporation of various key elements into brachiopod calcite. To evaluate the feasibility and robustness of multiple Element/Ca ratios as proxies in brachiopods, specifically Li/Ca, B/Ca, Na/Ca, Mg/Ca, Sr/Ca, Ba/Ca, as well as Li/Mg, we cultured Magellania venosa, Terebratella dorsata and Pajaudina atlantica under controlled experimental settings over a period of more than two years with closely monitored ambient conditions, carbonate system parameters and elemental composition of the culture medium. The experimental setup comprised of two control aquariums (pH0 = 8.0 and 8.15, T = 10 °C) and treatments where pCO2 − pH (pH1 = 7.6 and pH2 = 7.35), temperature (T = 16 °C) and chemical composition of the culture medium were manipulated. Our results indicate that the incorporation of Li and Mg is strongly influenced by temperature, growth effects as well as carbonate chemistry, complicating the use of Li/Ca, Mg/Ca and Li/Mg ratios as straightforward reliable proxies. Boron partitioning varied greatly between the treatments, however without a clear link to carbonate system parameters or other environmental factors. The partitioning of both Ba and Na varied between individuals, but was not systematically affected by changes in the ambient conditions. We highlight Sr as a potential proxy for DIC, based on a positive trend between Sr partitioning and carbonate chemistry in the culture medium. To explain the observed dependency and provide a quantitative framework for exploring elemental variations, we devise the first biomineralisation model for brachiopods, which results in a close agreement between modelled and measured Sr distribution coefficients. We propose that in order to sustain shell growth under increased DIC, a decreased influx of Ca2+ to the calcifying fluid is necessary, driving the preferential substitution of Sr2+ for Ca2+ in the crystal lattice. Finally, we conducted micro-computed tomography analyses of the shells grown in the different experimental treatments. We present pore space – punctae – content quantification that indicates that shells built under increased environmental stress, and in particular elevated temperature, contain relatively more pore space than calcite, suggesting this parameter as a potential novel proxy for physiological stress and even environmental conditions.

Description: The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.

Description: Drought and heat events, such as the 2018 European drought, interact with the exchange of energy between the land surface and the atmosphere, potentially affecting albedo, sensible and latent heat fluxes, as well as CO2 exchange. Each of these quantities may aggravate or mitigate the drought, heat, their side effects on productivity, water scarcity and global warming. We used measurements of 56 eddy covariance sites across Europe to examine the response of fluxes to extreme drought prevailing most of the year 2018 and how the response differed across various ecosystem types (forests, grasslands, croplands and peatlands). Each component of the surface radiation and energy balance observed in 2018 was compared to available data per site during a reference period 2004–2017. Based on anomalies in precipitation and reference evapotranspiration, we classified 46 sites as drought affected. These received on average 9% more solar radiation and released 32% more sensible heat to the atmosphere compared to the mean of the reference period. In general, drought decreased net CO2 uptake by 17.8%, but did not significantly change net evapotranspiration. The response of these fluxes differed characteristically between ecosystems; in particular, the general increase in the evaporative index was strongest in peatlands and weakest in croplands. This article is part of the theme issue ‘Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale’.

Description: Up to 50% of the signal delays of L-band signals used in the Global Navigation Satellite System comes from the topside ionosphere and plasmasphere. In this study, we apply an Ensemble Kalman Filter (EnKF) approach to estimate the 4D electron density of the topside ionosphere and plasmasphere based on space-based STEC data. NeQuick model is used as background. The STEC measurements of eleven LEO satellites are used for the reconstructions. The majority of the approaches, working with EnKF, uses physics-based models for the propagation step. In our work, we investigate the question how the propagation step can be realized, in the case that a physical propagation model is not available or discarded due to computational burden. We explore different propagation models and compare them with the iterative reconstruction technique SMART+ for two periods of the year 2015 covering quiet to perturbed ionospheric conditions. We check the capability of the estimations to reproduce assimilated STEC as well as to reconstruct independent STEC measurements. The comparison with the assimilated STEC shows that during both periods all methods reduce the statistics (Median, SD, RMS) of the STEC residuals in comparison to the background model by up to 86%. In summary, the results indicate that the methods EnKF with exponential decay as propagation model (EnKFexp) and SMART+ perform best, reducing the independent STEC residuals by up to 64%, compared to the NeQuick model.

Description: The global and regional ionospheric maps are often used for a wide range of applications in geosciences, in particular to support precise positioning, but also for geophysical and atmospheric studies. There are currently many analysis centers and research groups providing operational and test VTEC maps. However, IGS ACs and other groups use different mathematical models and estimation techniques resulting different resolutions, accuracies and time delays of their products. Therefore, there is a need to compare and validate existing VTEC models. In this presentation, we present the overview talk about the work within the last 4 years of the IAG Joint Working Group (JWG 4.3.4) on validation of VTEC models for high-precision and high resolution applications. Among others, we evaluated (1) the accuracy and consistency of the IAAC GIMs during high and low solar activity periods of the 24th solar cycle, (2) the accuracy the two most popular ionospheric mapping functions - SLM and MSLM, (3) deterministic and stochastic approaches to VTEC modelling, (4) GIMs performance in single point and precise point positioning GNSS applications, (5) the accuracy and consistency of GNSS-derived VTEC maps and empirical models, (6) GIMs performance using external data (JASON) and GNSS, (7) the accuracy of new global ionosphere models.

Description: The rising demand for potable water in densely populated coastal regions has recently promoted growing research interest in detecting offshore freshened groundwater (OFG) worldwide. Recent geophysical studies along the continental margins offshore Israel, New Zealand, Malta, and the United States of America provide some examples of integrating geophysical and borehole data to constrain the spatial extent of OFGs and estimate their pore-water salinity. However, occurrences of OFGs and the interaction between terrestrial, carbonate-hosted groundwater systems with seawater are understudied in many coastal regions by complicated seafloor morphology. In this study, we investigate whether OFG can exist offshore a semi-arid carbonate coastline along the Maltese Islands and explore the possibility of sustainably exploiting these reservoirs as an unconventional source of potable water to relieve freshwater scarcity. We present an integration of 2-D resistivity models derived from marine controlled source electromagnetic (CSEM) measurements with 2-D and 3-D seismic data, core samples, borehole data, and geochemical measurements. Electrical resistivity models identify localized resistive anomalies (〉 10 ohm-m) offshore the northeastern coast of Gozo (the second-largest island in the Maltese archipelago). Furthermore, a resistive body is observed at ~ 300 m below sea level close to the coast of Gozo which extends northeastwards and disappears at ~ 8 km offshore. If the anomalous resistive body is associated with pore-water salinity variations or, alternatively, caused by lithological changes, will be discussed through an integrative geological model developed along each profile.

Description: The high-quality dual-frequency phase measurements of Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) system provide valuable opportunities to examine the Earth’s ionosphere. DORIS data from the Jason-3 satellite has also been available in Near-Real-Time (NRT) with a delay of a few hours. Such data are perfectly suited for independent validation and combination of Real-Time Global Ionospheric Maps (RT-GIM) derived from GNSS measurements. In this work, we analyzed the feasibility of using DORIS data to estimate the accuracy of GNSS-generated ionospheric models. To this end, the concept of DORIS differential Slant Total Electron Content (dSTEC) assessment is proposed. The consistency between DORIS and GNSS dSTEC assessments in the quality analysis of RT-GIMs was checked, and the overall Pearson correlation coefficient reaches 0.81 during the one-year test period. The DORIS dSTEC assessment can be used not only to estimate the accuracy of individual GIMs, but also to determine their weighting within a combination strategy. The performance of DORIS-dSTEC and GNSS-dSTEC combined GIMs is assessed by comparison to Jason-3 VTEC from the mission altimeter. The standard deviations are 4.71 TECu and 4.80 TECu for DORIS-dSTEC and GNSS-dSTEC combined GIMs, indicating the slightly better performance of DORIS-dSTEC combined RT-GIM in Jason-3 VTEC assessment. Overall, NRT DORIS data can be used to independently validate and combine GNSS-derived ionospheric maps. In the future, it is also envisaged that DORIS data can be directly incorporated into ionosphere modeling. To this end, the provision of NRT data from other DORIS missions is planned (e.g., Sentinel-3).