Copernicus

Kinematic response of ice-rise divides to changes in ocean and atmosphere forcing

Kinematic response of ice-rise divides to changes in ocean and atmosphere forcing Schannwell, Clemens; Drews, Reinhard; Ehlers, Todd A.; Eisen, Olaf; Mayer, Christoph; Gillet-Chaulet, Fabien

The majority of Antarctic ice shelves are bounded by grounded ice rises. These ice rises exhibit local flow fields that partially oppose the flow of the surrounding ice shelves. Formation of ice rises is accompanied by a characteristic upward-arching internal stratigraphy (“Raymond arches”), whose geometry can be analysed to infer information about past ice-sheet changes in areas where other archives such as rock outcrops are missing. Here we present an improved modelling framework to study ice-rise evolution using a satellite-velocity calibrated, isothermal, and isotropic 3-D full-Stokes model including grounding-line dynamics at the required mesh resolution (<500 m). This overcomes limitations of previous studies where ice-rise modelling has been restricted to 2-D and excluded the coupling between the ice shelf and ice rise. We apply the model to the Ekström Ice Shelf, Antarctica, containing two ice rises. Our simulations investigate the effect of surface mass balance and ocean perturbations onto ice-rise divide position and interpret possible resulting unique Raymond arch geometries. Our results show that changes in the surface mass balance result in immediate and sustained divide migration (>2.0 m yr⁻¹) of up to 3.5 km. In contrast, instantaneous ice-shelf disintegration causes a short-lived and delayed (by 60–100 years) response of smaller magnitude (<0.75 m yr⁻¹). The model tracks migration of a triple junction and synchronous ice-divide migration in both ice rises with similar magnitude but differing rates. The model is suitable for glacial/interglacial simulations on the catchment scale, providing the next step forward to unravel the ice-dynamic history stored in ice rises all around Antarctica.

Global sensitivity analysis and adaptive stochastic sampling of a subsurface-flow model using active subspaces

Global sensitivity analysis and adaptive stochastic sampling of a subsurface-flow model using active subspaces Erdal, Daniel; Cirpka, Olaf A.

Integrated hydrological modeling of domains with complex subsurface features requires many highly uncertain parameters. Performing a global uncertainty analysis using an ensemble of model runs can help bring clarity as to which of these parameters really influence system behavior and for which high parameter uncertainty does not result in similarly high uncertainty of model predictions. However, already creating a sufficiently large ensemble of model simulation for the global sensitivity analysis can be challenging, as many combinations of model parameters can lead to unrealistic model behavior. In this work we use the method of active subspaces to perform a global sensitivity analysis. While building up the ensemble, we use the already-existing ensemble members to construct low-order meta-models based on the first two active-subspace dimensions. The meta-models are used to pre-determine whether a random parameter combination in the stochastic sampling is likely to result in unrealistic behavior so that such a parameter combination is excluded without running the computationally expensive full model. An important reason for choosing the active-subspace method is that both the activity score of the global sensitivity analysis and the meta-models can easily be understood and visualized. We test the approach on a subsurface-flow model including uncertain hydraulic parameters, uncertain boundary conditions and uncertain geological structure. We show that sufficiently detailed active subspaces exist for most observations of interest. The pre-selection by the meta-model significantly reduces the number of full-model runs that must be rejected due to unrealistic behavior. An essential but difficult part in active-subspace sampling using complex models is approximating the gradient of the simulated observation with respect to all parameters. We show that this can effectively and meaningfully be done with second-order polynomials.

Effects of extraction conditions on the redox properties of soil organic matter (SOM) and its ability to stimulate microbial iron(III) mineral reduction by electron shuttling

Effects of extraction conditions on the redox properties of soil organic matter (SOM) and its ability to stimulate microbial iron(III) mineral reduction by electron shuttling Bai, Yuge; Subdiaga, Edisson; Haderlein, Stefan B.; Knicker, Heike; Kappler, Andreas

Soil organic matter (SOM), including humic substances (HS), is redox-active, can be microbially reduced, and transfers electrons in an abiotic reaction to Fe(III) minerals thus serving as electron shuttle. The standard procedure to extract HS from soil and separate them into humic acids (HA) and fulvic acids (FA) involves alkaline and acidic solutions potentially leading to unwanted changes in SOM chemical and redox properties. To determine the effects of extraction conditions on the redox and electron shuttling properties of SOM extracts, we prepared HS and SOM extracts from a forest soil applying either a combination of 0.1 M NaOH and 6 M HCl, or water (pH 7). Both chemical extractions (NaOH / HCl) and water extractions were done in separate setups under either oxic or anoxic conditions. Furthermore, we applied the NaOH / HCl treatment to a subsample of the water-extracted-SOM. We found that soil extraction with NaOH lead to ca. 100 times more extracted C and the extracted HS had 2–3 times higher electron exchange capacities (EEC) than SOM extracted by water. For water-extracted SOM, anoxic extraction conditions lead to about 7 times more extracted C and 1.5 times higher EEC than under oxic extraction conditions. This difference was probably due to the occurrence of microbial reduction and dissolution of Fe(III) minerals in the soil during the water extraction at neutral pH and the concomitant release of Fe(III) mineral-bound organic matter. NaOH / HCl treatment of the water-extracted SOM lead to 2 times higher EEC values in the HA isolated from the SOM compared to the water-extracted SOM itself, suggesting the chemical treatment with NaOH and HCl caused changes of redox-active functional groups of the extracted organic compounds. Higher EEC of extracts in turn resulted in a higher stimulation of microbial Fe(III) mineral reduction by electron shuttling, i.e. faster initial Fe(III) reduction rates, and in most cases also in higher reduction extents. Our findings suggest that SOM extracted with water at neutral pH should be used to better reflect environmental SOM redox processes in lab experiments and that potential artefacts of the chemical extraction method and anoxic extraction condition need to be considered when evaluating and comparing abiotic and microbial SOM redox processes.

First identification and quantification of detached-tip vortices behind a wind energy converter using fixed-wing unmanned aircraft system

First identification and quantification of detached-tip vortices behind a wind energy converter using fixed-wing unmanned aircraft system Mauz, Moritz; Rautenberg, Alexander; Platis, Andreas; Cormier, Marion; Bange, Jens

In the present study, blade-tip vortices have been experimentally identified in the wake of a commercial wind turbine using the Multi-purpose Airborne Sensor Carrier Mark 3 (MASC Mk 3) unmanned aircraft system (UAS) of the University of Tübingen. By evaluation of the wind components, detached blade-tip vortices were identified in the time series. From these measurements, the circulation and core radius of a pair of detached blade-tip vortices is calculated using the Burnham–Hallock (BH) wake vortex model. The presented data were captured under a dominating marine stratification about 2km from the North Sea coastline with northern wind direction. The measured vortices are also compared to the analytical solution of the BH model for two vortices spinning in opposite directions. The model has its origin in aviation, where it describes two aircraft wake vortices spinning in opposite directions.

An evaluation method is presented to measure detached-tip vortices with a fixed-wing UAS. The BH model will be used to describe wake vortex properties behind a wind energy converter (WEC). The circulation and core radius of detached blade-tip vortices will be calculated. Also a proposition of the model for WEC wake evaluations will be made to describe two independent co-rotating vortices. Quantifying blade-tip vortices helps to understand the process of vortices detaching from a rotor blade of a wind turbine, their development in the wake until finally dissipating in the far wake and contributing to overall atmospheric turbulence. This is especially interesting for set-ups of numerical simulations when setting the spatial resolution of the simulation grid.