bluuebunny/crossref_metadata_2025 huggingface dataset:
Atmospheric gravity waves in a gas centrifuge
Authors: Schlutow Mark
Published: March 2021

Field observations of nonlinear atmospheric gravity waves are sparse and involved due to many challenges for the instrumentation. Due to these complications of field measurements, laboratory experiments are an indispensable tool.

As of today, all laboratory experiments on gravity waves have in common that they were performed with water as the working fluid. Due to flow similarities, most of the features observed in the water tanks are equally valid for the atmosphere. However, one particular property of air cannot be emulated by water: compressibility. Especially for the dynamics of nonlinear waves, compressibility plays a significant role.

We propose a laboratory experiment by means of a rapidly rotating gas centrifuge. The centrifugal forces act on the gas like the gravitational pull causing a stratified compressible working fluid. In this device, atmosphere-like gravity waves would be observable under controlled and replicable conditions for the first time.

We show that the waves in a centrifuge would theoretically behave like their atmospheric counterparts; they exhibit the same dispersion and polarization relations. Futhermore, spinning the centrifuge with the right frequency, there is a clear scale separation between acoustic and gravity waves. In addition to the centrifugal force, the Coriolis force acts in the same plane potentially spoiling the similarities. However, the influence of the Coriolis force on the wave is negligibly small.
Temporal variations of atmospheric ammonia (NH3) derived from over a decade of IASI satellite measurements
Authors: Van Damme Martin , Clarisse Lieven , Franco Bruno , Sutton Mark A , Erisman Jan Willem , Wichink Kruit Roy , van Zanten Margreet , Hadji-Lazaro Juliette , Hurtmans Daniel , Clerbaux Cathy , Coheur Pierre-François
Published: March 2021

The Infrared Atmospheric Sounding Interferometer (IASI) mission consists of a suite of three infrared sounders providing today over 13 years of consistent global measurements (from end of 2007 up to now). In this work we use the recently developed version 3 of the IASI NH3 dataset to derive global, regional and national trends from 2008 to 2018. Reported national trends are analysed in the light of changing anthropogenic and pyrogenic NH3 emissions, meteorological conditions and the impact of sulphur and nitrogen oxides emissions. A case study is dedicated to the Netherlands. Temporal variation on shorter timescales will also be investigated.
Complex rotated CCA: a method to correlate lagged geophysical variables
Authors: Rieger Niclas , Corral Alvaro , Turiel Antonio , Olmedo Estrella
Published: March 2021

The nature of the climate system is very complex: a network of mutual interactions between ocean and atmosphere lead to a multitude of overlapping geophysical processes. As a consequence, the same process has often a signature on different climate variables but with spatial and temporal shifts. Orthogonal decompositions, such as Canonical Correlation Analysis (CCA), of geophysical data fields allow to filter out common dominant patterns between two different variables by maximizing cross-correlation. In general, however, CCA suffers from (i) the orthogonality constraint, which tends to produce unphysical patterns, and (ii) the use of direct correlations, which leads to signals that are merely shifted in time being considered as distinct patterns.

In this work, we propose an extension of CCA, complex rotated CCA (crCCA), to address both limitations. First, we generate complex signals by using the Hilbert transforms. To reduce the spatial leakage inherent in Hilbert transforms, we extend the time series using the Theta model, thus creating an anti-leakage buffer space. We then perform the orthogonal decomposition in complex space, allowing us to detect out-of-phase signals. Subsequent Varimax rotation removes the orthogonal constraints to allow more geophysically meaningful modes.

We applied crCCA to a pair of variables expected to be coupled: Pacific sea surface temperature and continental precipitation. We show that crCCA successfully captures the temporally and spatially complex modes of (i) seasonal cycle, (ii) canonical ENSO, and (iii) ENSO Modoki, in a compact manner that allows an easy geophysical interpretation. The proposed method has the potential to be useful especially, but not limited to, studies on the prediction of continental precipitation by other climate variables. An implementation of the method is readily available as a Python package.
Present distribution and variability of water flow, suspended sediment load, and geochemical runoff in the Volga River delta
Authors: Zavadskaya Mariya , Zavadskiy Alexandr , Lychagin Mikhail
Published: March 2021

Environmental changes in the Volga River delta attract attention of researchers due to increasing anthropogenic pressure on the river catchment, global climate changes, and natural fluctuation cycles of the Caspian Sea level. These changes significantly affect the hydrological regime and erosion-accumulative processes in the Volga delta.

In 2018-2020, the authors conducted series of field hydrological and geochemical studies within the Volga River delta, which covered all major systems of the deltaic water streams. We have determined water discharge, suspended sediment concentration, content of dissolved and suspended chemical elements (ICP-MS/ICP-AES). The results obtained in the study provide a comprehensive view of the current spatial and temporal distribution of the water flow, suspended sediment yield, and geochemical flows along the main branches, numerous channels and rivers during the both high and low water periods, and also allow us to compare them with long-term trends established by previous studies.

We have found that the present distribution of water flow and suspended sediment load within the Volga delta differs from the second half of the XX century. It determines the ongoing restructuring of the water balance, and transformation of the rate of erosion and accumulative processes within the main systems of the deltaic branches. The study allowed delineating the Volga Delta by zones of erosion, transition, and accumulation of suspended matter in different phases of the water regime in relation to the modern basis of erosion, which is the current level of the Caspian Sea.

The distribution of geochemical runoff within the Volga Delta is determined by water and sediment runoff, as well as concentrations of dissolved and suspended forms of chemical elements. The largest geochemical runoff passes through the Bakhtemir and Buzan systems. During the low-water period, the volumes of geochemical runoff of these branches are similar. During the flood period, despite the increasing share of Buzan water runoff, the prevailing fluxes of matter pass through the Bakhtemir system due to higher concentrations of elements and larger solid discharge.

The ratio of dissolved and suspended elements in flows is determined mainly by the properties of chemical elements. According to the percentage ratio of dissolved and suspended forms, we divided elements into 3 groups: 1) migrating mainly in dissolved forms (Na, Ca, Sr, Mg, Mo, U, K, Li, Ba, As, Sb), 2) migrating mainly in suspended forms (Pb, Y, Zr, Ti, Mn, Fe, Al, REE), 3) elements with changeable behavior depending on geochemical and hydrological conditions (Ni, Cd, Zn, Co, Cu, et al.).

The research was supported by RFBR project No. 18-05-80094 and №17-05-41174-RGS.
Measurement error-filtered machine learning in digital soil mapping
Authors: van der Westhuizen Stephan , Heuvelink Gerard , Hofmeyr David
Published: March 2021

Digital soil mapping (DSM) may be defined as the use of a statistical model to quantify the relationship between a certain observed soil property at various geographic locations, and a collection of environmental covariates, and then using this relationship to predict the soil property at locations where the property was not measured. It is also important to quantify the uncertainty with regards to prediction of these soil maps. An important source of uncertainty in DSM is measurement error which is considered as the difference between a measured and true value of a soil property.

The use of machine learning (ML) models such as random forests (RF) has become a popular trend in DSM. This is because ML models tend to be capable of accommodating highly non-linear relationships between the soil property and covariates. However, it is not clear how to incorporate measurement error into ML models. In this presentation we will discuss how to incorporate measurement error into some popular ML models, starting with incorporating weights into the objective function of ML models that implicitly assume a Gaussian error. We will discuss the effect that these modifications have on prediction accuracy, with reference to simulation studies.
Laboratory and field studies of preferential flow dynamics in unsaturated fractured porous media
Authors: Rüdiger Florian , Bartsch Kim , Nimmo John R. , Kordilla Jannes
Published: March 2021

Recharge dynamics within the vadose zone (variable saturation conditions) of consolidated fractured rock formations are an ongoing challenge when it comes to process understanding and predictive modeling. The proper delineation of fast (macropores, fractures, conduits) and slow (matrix) flow components in these systems and their interaction with each other remains a complex puzzle and holds a key to enhance process-based infiltration models.

We conducted laboratory and field experiments to study infiltration dynamics through porous-fractured systems. Laboratory experiments were carried out with analogue fracture networks on meter scale. Orthogonal networks were created by placing equally sized blocks with a constant gap between to glass plates, which were mount by metal clamps. Vertical flow through different network configurations (apertures, intersection types, topology, flow rates) was studied for (1) porous media (sandstone) and (2) non-porous media (glass) to delineate the control of network features on flow dynamics, as well as the effect of fracture-matrix interaction. Matrix imbibition was found to strongly control the preferential flow velocity during flow path evolution. Higher infiltration rates lead to more by-pass at fracture intersections, whereas low infiltration rates favor flow partitioning into horizontal fractures. Vertical flow progression within the non-porous network is significantly faster due to the lack of imbibition. Semi-analytical tools, such as transfer functions, and source-responsive dual-domain models are tested to reproduce the experimental data and to incorporate key features of fracture networks in future modeling approaches. We additionally obtained experimental data from infiltration dynamics at porous-fractured field sites on meter scale to compare them to the well-controlled laboratory experiments and to evaluate the applicability of the results to actual field processes.
Global chlorophyll a concentrations of phytoplankton functional types with detailed uncertainty assessment using multi-sensor ocean color and sea surface temperature products
Authors: Xi Hongyan , Losa Svetlana N. , Mangin Antoine , Garnesson Philippe , Bretagnon Marine , Demaria Julien , A. Soppa Mariana , Hembise Fanton d'Andon Odile , Bracher Astrid
Published: March 2021

With the extensive use of ocean color (OC) satellite products, diverse algorithms have been developed in the past decades to observe the phytoplankton community structure in terms of functional types, taxonomic groups and size classes. There is a need to combine satellite observations and biogeochemical modelling to enable comprehensive phytoplankton groups time series data and predictions under the changing climate. A prerequisite for this is continuous long-term satellite observations from past and current OC sensors with quantified uncertainties are essential to ensure their application. Previously we have configured an approach, namely OLCI-PFT (v1), to globally retrieve total chlorophyll a concentration (TChl-a), and chlorophyll a concentration (Chl-a) of multiple phytoplankton functional types (PFTs). This algorithm is developed based on empirical orthogonal functions (EOF) using satellite remote sensing reflectance (Rrs) products from the GlobColour archive (https://www.globcolour.info/). The algorithm can be applied to both, merged OC products and Sentinel 3A OLCI data. Global PFT Chl-a products of OLCI-PFT v1 are available on CMEMS under Ocean Products since July 2020. Lately we have updated the approach and established the OLCI-PFT v2 by including sea surface temperature (SST) as input data. The updated version delivers improved global products for the aforementioned PFT quantities. The per-pixel uncertainty of the retrieved TChl-a and PFT Chl-a products is estimated and validated by taking into account the uncertainties from both input data (satellite Rrs and SST) and model parameters through Monte Carlo simulations and analytical error propagation. The uncertainty of the OLCI-PFT products v2 was assessed on a global scale. For PFT Chl-a products this has been done for the first. The uncertainty of OLCI-PFT v2 TChl-a product is in general much lower than that of the TChl-a product generated in the frame of the ESA Ocean Colour Climate Change Initiative project (OC-CCI). The OLCI-PFT algorithm v1 and v2 have also been further adapted to use a merged MODIS-VIRRS input. Good consistency has been found between the OLCI-PFT products derived from using input data from the different OC sensors. This sets the ground to realize long-term continuous satellite global PFT products from OLCI-PFT. Satellite PFT uncertainty, as provided for our products, is essential to evaluate and improve coupled ecosystem-ocean models which simulate PFTs, and furthermore can be used to improve these models directly via data assimilation.
A Micromechanics-based Multiscale Approach toward Continental Deformation and Tectonic Processes
Authors: Lu Lucy (Xi) , Jiang Dazhi , Beall Adam , Fagereng Ake
Published: March 2021

The Earth’s lithosphere has abundant structures and fabrics generated by various tectonic processes. These geological features span a wide range of characteristic lengths, from crystal lattice spacing to the dimensions of lithospheric plates. Using field observations of exhumed geological features, we aim to understand the rheological behaviour of Earth’s lithosphere. However, our direct field and laboratory observations are limited to the most accessible scales, typically from outcrops to microscopes. There is therefore a significant intrinsic scale gap between direct observations and the tectonic processes operating along plate boundaries. A micromechanics-based Multi-order Power-Law Approach (MOPLA) has been developed to bridge this scale gap. MOPLA treats the heterogeneous rock mass as a continuum of rheologically distinct elements. The rheological properties and the strain rate and stress fields of the constituent elements and the composite material are computed by solving partitioning and homogenization equations self-consistently. The partitioned ‘local’ fields in individual elements are related to small-scale geological features. The ‘bulk’ fields and the homogenized rheological properties are associated with tectonic processes and the macroscopic behaviour of the heterogeneous rock mass. The algorithm of MOPLA is implemented in a MATLAB package and has been successfully applied to various studies on multiscale deformation in the lithosphere. In this work, we will introduce this multiscale approach and also briefly introduce our ongoing work on characterising the rheological behaviour of a heterogeneous subduction shear zone using MOPLA.
Cluster analysis in the studying of stress relation in the Vrancea-zone
Authors: Czirok Lili , Kuslits Lukács , Gribovszki Katalin
Published: March 2021

The SE-Carpathians produce significant geodynamic activity due to the current subduction process. The strong seismicity in the Vrancea-zone is its most important indicator. The focus area of these seismic events is relatively small, around 80*100 km and the distribution of their locations is quiet dense.

The authors have carried out cluster analyses of the focal mechanism solutions estimated from local and tele-seismic measurements and stress inversions to support the recent and previously published studies in this region. They have applied different pre-existing clustering methods – e.g. HDBSCAN (hierarchical density-based clustering for applications with noise) and agglomerative hierarchical analysis – considering to the geographical coordinates, focal depths and parameters of the focal mechanism solutions of the used seismic events, as well. Moreover, they have attempted to improve a fully-automated algorithm for the classification of the earthquakes for the estimations. This algorithm does not call for the setting of hyper-parameters, thus the affection of the subjectivity can be reduced significantly and the running time can be also decreased. In all cases, the resulted stress tensors are in close agreement with the earlier presented results.
Climatic and Biotic Controls on Topographic Asymmetry at the Global Scale
Authors: Smith Taylor , Bookhagen Bodo
Published: March 2021

Insolation differences play a primary role in controlling microclimate and vegetation cover, which together influence the development of topography. Topographic asymmetry (TA), or slope differences between terrain aspects, has been well documented in small-scale, field-based, and modeling studies. Here we combine a suite of environmental (e.g., vegetation, temperature, solar insolation) and topographic (e.g., elevation, drainage network) data to explore the driving mechanisms and markers of TA on a global scale. Using a novel empirical TA analysis method, we find that (1) steeper terrain has higher TA magnitudes, (2) globally, pole-facing terrain is on average steeper than equator-facing terrain, especially in mid-latitude, tectonically quiescent, and vegetated landscapes, and (3) high-elevation and low-temperature regions tend to have terrain steepened towards the equator. We further show that there are distinct differences in climate and vegetation cover across terrain aspects, and that TA is reflected in the size and form of fluvial drainage networks. Our work supports the argument that insolation asymmetries engender differences in local microclimates and vegetation on opposing terrain aspects, which broadly encourage the development of asymmetric topography across a range of lithologic, tectonic, geomorphic, and climatic settings.
CRAMPON: a model- and observation-based near real-time platform for glacier mass balance monitoring in Switzerland
Authors: Landmann Johannes Marian , Huss Matthias , Künsch Hans Rudolf , Ogier Christophe , Geibel Lea , Sold Leo , Farinotti Daniel
Published: March 2021

As glaciers shrink, high interest in their near real-time mass balance arises. This is mainly for two reasons: first, there are concerns about water availability and short-term water resource planning, and second, glaciers are one of the most prominent indicators of climate change, resulting in a high interest of the broader public.

To satisfy both interests regarding information on near real-time mass balance, we are running the project CRAMPON – “Cryospheric Monitoring and Prediction Online”. Within this project, we set up an operational assimilation platform where it is possible to query daily mass balance estimates in near real-time, i.e. updated with a lag of max. 24 hours. During the operational alpha phase, we increase the amount of modelled glaciers and assimilated observations steadily. We start with about 15 glaciers from the Glacier Monitoring Switzerland (GLAMOS) program, for which time series of seasonal mass balances from the glaciological method are available. After that, we expand our set of modelled glaciers to about 50 glaciers that have frequent geodetic mass balances in the past, and finally to all glaciers in Switzerland. The assimilated observations reach from the operational GLAMOS seasonal mass balance observations via daily point mass balances from nine in situ cameras providing instantaneous ablation rates to satellite-derived albedo and snow distribution on the glacier.
As basis for the platform, we run an ensemble of three temperature index and one simplified energy balance melt models. This ensemble takes gridded temperature, precipitation and radiation as input and aims at quantifying uncertainties of the produced daily mass balances. To determine uncertainties in the model prediction of a current mass budget year correctly, we run the models with parameter distributions we have fitted on individual parameter sets calibrated in the past. Since a purely model-based prediction can reveal high uncertainties though, we choose a sequential data assimilation approach in the form of a Particle Filter to constrain this uncertainty with observations, whenever available. We have customized the standard Particle Filter to (1) use a resampling method that is able to keep models in the ensemble despite a temporary bad performance, and (2) allow parameter and model probability evolution over time.

In this contribution, we focus on giving a holistic overview over the already existing platform features and discuss the future developments. We plan to make the calculated mass balances publicly available in summer 2021, and to extend this platform to the global scale at a later stage.
Fluid Flow Modeling using Geochemistry to Characterize the Songwe Medium Temperature Geothermal System - Tanzania
Authors: Asnin Sitti Nur , Nnko Martha , Josephat Sadock , Mahecha Albano , Mshiu Elisante , Bertotti Giovanni , Brehme Maren
Published: March 2021

A geothermal area with only bicarbonate thermal water discharges at medium temperature requires a more integrated analysis than used in classical geochemical exploration. This signature is typical for steam-heated water, which commonly occurs at the margins of a geothermal system. However, these waters can also rise from carbonate rich layers in the central part of the field. Our study shows that fluid flow modeling can identify the exact source, flow pathways and temperatures of reservoir fluids based on water-rock interaction. For the first time, we present a conceptual geothermal fluid flow model based on geochemical data for the Songwe geothermal system in Tanzania.

Thermal springs discharge along NW-SE fracture zones in two separate areas: the central Songwe graben (Iyola, Main springs, Rambo and Kaguri) and eastern Songwe graben (Ikumbi). The discharge temperatures of springs range between 37 and 85 oC with Na-HCO3 type, and carbonate deposits surrounding most of the springs. We estimated fluid temperatures for a depth of 2.5km by applying K-Mg and Na-K-Ca (Mg correction) geothermometers, suggesting that reservoir fluids reach temperatures between 125 and 148 oC. We reconstructed reservoir fluid characteristics for that temperatures and propose oversaturated minerals (volcanics, clays, carbonates, apatites, weathered metamophics and hydrothermal minerals) as a model result of interaction between the deep fluids and certain lithologies. Comparison between the modeled oversaturated minerals with minerals in the springs (calcite, aragonite, analcime, muscovite, and smectite) suggests that Kaguri spring water is a result of interaction between deep reservoir fluids with all lithologies, passed on the way to the surface (Metamorphics, Karoo group and Red Sandstone). The fluid signature of Kaguri springs suggest an upflow zone of the geothermal system. Further, our model with oversaturated minerals shows that the thermal water from the reservoir flows laterally along the Red Sandstone layer to the eastern part of study area. It appears as Rambo springs, south of Kaguri springs, and as Main springs and Iyola to the west. The outflow zone might be continuing towards Ikumbi springs, where the fluids also interact with volcanic units. The proposed model shows that carbonate dissolution from the Red sandstone layer is the most common water-rock interaction. The carbonate is embedded in pores and fractures and occurs as matrix in the sandstone. The water-rock interaction is dominated by HCO3- and Na and seen in carbonate depositions at all springs.
Surface circulation in the Ligurian Sea: an assessment of satellite radar altimeter-derived geostrophic currents.
Authors: Picco Paola , Vignudelli Stefano , Repetti Luca , Demarte Maurizio
Published: March 2021

Recent improvements of satellite altimeter observations allow to approach investigations on the surface ocean circulation even in those regions where the slope associated to dynamic structures is reduced. The capability to detect the main dynamic features and their variability from satellite radar altimetry in the Ligurian Sea (Western Mediterranean) is here assessed.

Altimeter data from X-TRACK products recently released are used for this study: the time series of satellite-based- currents along the track n.044, which crosses the Ligurian Sea from the Corsica Channel to the Ligurian coast, is analysed. The temporal sampling is about 10 days and the along-track resolution is 7 km. Geostrophic currents computed from satellite radar altimetry are checked for consistency against the dynamic topography obtained from concurrent CTD casts collected during recent oceanographic campaigns carried out by the Italian Hydrographic Institute along the track. A more detailed assessment of the computed current velocities is based on the analysis of long-term ADCP measurements from a fixed mooring deployed from 2004 to 2006 in the Central Ligurian Sea (43°47.77’ N; 9°02.85’ E) 40 nm from the coast, quite close to the altimeter track. An RD&I 300 kHz upward-looking ADCP sampled the upper layer at 8 m vertical resolution. Currents in the upper layer (0-100 m) are almost barotropic with the variability due to the wind confined to the upper few meters. In order to define an appropriate metrics to compare currents from different measuring systems, EOF analysis of ADCP profiles have proved to be a good tool to filter out the high frequency and wind driven currents, thus enhancing the contribution of the geostrophic component.
Bound Coherent Structures Propagating on the Free Surface of Deep Water
Authors: Dremov Sergey , Kachulin Dmitry , Dyachenko Alexander
Published: March 2021

               The work presents the results of studying the bound coherent structures propagating on the free surface of ideal incompressible fluid of infinite depth. Examples of such structures are bi-solitons which are exact solutions of the known approximate model for deep water waves — the nonlinear Schrödinger equation (NLSE). Recently, when studying multiple breathers collisions, the occurrence of such objects was found in a more accurate model of the supercompact equation for unidirectional water waves [1]. The aim of this work is obtaining and further studying such structures with different parameters in the supercompact equation and in the full system of nonlinear equations for potential flows of an ideal incompressible fluid written in conformal variables. The algorithm used for finding the bound coherent objects was similar to the one described in [2]. As the initial conditions for obtaining such structures in the framework of the above models, the NLSE bi-soliton solutions were used, as well as two single breathers numerically found by the Petviashvili method and placed in a same point of the computational domain. During the evolution calculation the initial structures emitted incoherent waves which were filtered at the boundaries of the domain using the damping procedure. It is shown that after switching off the filtering of radiation, periodically oscillating coherent objects remain on the surface of the liquid, propagate stably during one hundred thousand characteristic wave periods and do not lose energy. The profiles of such structures at different parameters are compared.

This work was supported by RSF grant 19-72-30028 and RFBR grant 20-31-90093.

[1] Kachulin D., Dyachenko A., Dremov S. Multiple Soliton Interactions on the Surface of Deep Water //Fluids. – 2020. – Т. 5. – №. 2. – С. 65.

[2] Dyachenko A. I., Zakharov V. E. On the formation of freak waves on the surface of deep water //JETP letters. – 2008. – Т. 88. – №. 5. – С. 307.
Connecting gravity field, moment of inertia, and core properties in Jupiter through empirical structure models
Authors: Neuenschwander Benno A. , Helled Ravit , Movshovitz Naor , Fortney Jonathan J.
Published: March 2021

Constraining Jupiter's internal structure is crucial for understanding its formation and evolution history. Recent interior models of Jupiter that fit Juno's measured gravitational field suggest an inhomogeneous interior and potentially the existence of a diluted core. These models, however, strongly depend on the model assumptions and the equations of state used. A complementary modelling approach is to use empirical structure models. 
These can later be used to reveal new insights on the planetary interior and be compared to standard models. 
Here we present empirical structure models of Jupiter where the density profile is constructed by piecewise-polytropic equations. With these models we investigate the relation between the normalized moment of inertia (MoI) and the gravitational moments J2 and J4. 
Given that only the first few gravitational moments of Jupiter are measured with high precision, we show that an accurate and independent measurement of the MoI value could be used to further constrain Jupiter's interior. An independent measurement of the MoI with an accuracy better than ~0.1% could constrain Jupiter's core region and density discontinuities in its envelope. 
We find that models with a density discontinuity at ~1 Mbar, as would produce a presumed hydrogen-helium separation, correspond to a fuzzy core in Jupiter. 
We next test the appropriateness of using polytropes, by comparing them with empirical models based on polynomials. 
We conclude that both representations result in similar density profiles and ranges of values for quantities like core mass and MoI.
Geodetic point surface mass balances: A new approach to determine point surface mass balances on glaciers from remote sensing measurements
Authors: Vincent Christian , Cusicanqui Diego , Jourdain Bruno , Laarman Olivier , Six Delphine , Gilbert Adrien , Walpersdorf Andrea , Rabatel Antoine , Piard Luc , Gimbert Florent , Gagliardini Olivier , Peyaud Vincent , Arnaud Laurent , Thibert Emmanuel , Brun Fanny , Nanni Ugo
Published: March 2021

Mass balance observations are very useful to assess climate change in different regions of the world. As opposed to glacier-wide mass balances, which are influenced by the dynamic response of each glacier, point mass-balances provide a direct climatic signal that depends on surface accumulation and ablation only. Unfortunately, major efforts are required to conduct in situ measurements on glaciers. Here, we propose a new approach that determines point surface mass balances from remote sensing observations. We call this balance the geodetic point surface mass balance. From observations and modelling performed on Argentière and Mer de Glace glaciers over the last decade, we show that the vertical ice flow velocity changes are small in areas of low bedrock slope. Therefore, assuming constant vertical velocities in time for such areas and provided that the vertical velocities have been measured for at least one year in the past, our method can be used to reconstruct annual point surface mass balances from surface elevations and horizontal velocities alone. We demonstrate that the annual point surface mass balances can be reconstructed with an accuracy of about 0.3 m w.e. a-1 using the vertical velocities observed over the previous years and data from Unmanned Aerial Vehicle images. Given the recent improvements of satellite sensors, it should be possible to apply this method to high spatial resolution satellite images as well.
Exploring the circular polarisation of low-frequency solar radio bursts with LOFAR and estimating the coronal magnetic field
Authors: Morosan Diana , Kumari Anshu , Räsänen Juska , Kilpua Emilia , Zucca Pietro , Bisi Mario , Dabrowski Bartosz , Krankowski Andrzej , Magdalenić Jasmina , Mann Gottfried , Rothkaehl Hanna , Vocks Christian
Published: March 2021

The Sun is an active star that often produces numerous bursts of electromagnetic radiation at radio wavelengths. In particular, low frequency (< 150 MHz)  radio bursts have recently been brought back to light with the advancement of novel radio interferometric arrays. However, the polarisation properties of solar radio bursts have not yet been explored in detail, especially with the Low Frequency Array (LOFAR). Here, we explore the circular polarisation of type III radio bursts and a type I noise storm and present the first Stokes V low frequency radio images of the Sun with LOFAR in tied array mode observations. We find that the degree of circular polarisation for each of the selected bursts increases with frequency for fundamental plasma emission, while this trend is either not clear or absent for harmonic plasma emission. In the case of type III bursts, we also find that the sense of circular polarisation varies with each burst, most likely due to their different propagation directions, despite all of these bursts being part of a long-lasting type III storm. Furthermore, we use the degree of circular polarisation of the harmonic emission of type III bursts to estimate the coronal magnetic field at distances of 1.4 to 4 solar radii from the centre of the Sun. We found that the magnetic field has a power law variation with a power index in the range 2.4-3.6, depending on the individual type III burst observed.
Predicting geo-effectiveness of CMEs with EUHFORIA coupled to OpenGGCM
Authors: Maharana Anwesha , Scolini Camilla , Raeder Joachim , Poedts Stefaan
Published: March 2021

The EUropean Heliospheric FORecasting Information Asset (EUHFORIA, Pomoell and Poedts, 2018) is a physics-based heliospheric and CME propagation model designed for space weather forecasting. Although EUHFORIA can predict the solar wind plasma and magnetic field parameters at Earth, it is not designed to evaluate indices like Disturbance-storm-time (Dst) or Auroral Electrojet (AE) that quantify the impact of the magnetized plasma encounters on Earth’s magnetosphere. To overcome this limitation, we coupled EUHFORIA with Open Geospace General Circulation Model (OpenGGCM, Raeder et al, 1996) which is a magnetohydrodynamic model of Earth’s magnetosphere. In this coupling, OpenGGCM takes the solar wind and interplanetary magnetic field obtained from EUHFORIA simulation as input to produce the magnetospheric and ionospheric parameters of Earth. We perform test runs to validate the coupling with real CME events modelled using flux rope models like Spheromak and FRi3D. We compare these simulation results with the indices obtained from OpenGGCM simulations driven by the measured solar wind data from spacecrafts like WIND. We further discuss how the choice of CME model and observationally constrained parameters influences the input parameters, and hence the geomagnetic disturbance indices estimated by OpenGGCM. We highlight limitations of the coupling and suggest improvements for future work. 
The potential of Self-Organising maps clustering to characterise a harvested peatland using airborne radiometric data and OS digital elevation model.
Authors: daly Eve , O Leary David
Published: March 2021

Peatlands are becoming recognized as important carbon sequestration centres. Through restoration projects of peatlands in which the water table is raised, they may become carbon neutral or possibly carbon negative. Restoration projects require a knowledge of intra-peat variation across potentially large spatial areas. This is often difficult with traditional in-situ point measurements. The integration of multidimensional geophysical datasets and digital elevation models, combined with modern data analytical techniques, may provide a rapid means of accessing intra-peat variation. In this study, an airborne radiometric survey, being flown nationally over the Republic of Ireland, combined with a digital elevation model, is used to delineate areas within an industrial peatland where peat thickness is less than 1m. Radiometric data are particularly suited to peat studies as they are sensitive to water content and peat thickness and require relatively little expert knowledge to utilise. Peat, as a mostly organic material, acts as a low signal environment where variations in the signal are linked to intra-peat variation of thickness, density and/or water content. This study uses an unsupervised machine learning, self-organizing map clustering methodology to group the study site into three zones interpreted as 1) the edge of the bog where peat layer is thinning or there is influence on the radiometric signal from non-peat soils outside of the bog, 2) the normal peat conditions where thickness and saturation appear as a relative constant in the radiometric response, and 3) areas where the peat is either thinner or drier. A ground geophysical survey was conducted to verify this interpretation. The delineation of such spatial variations in the radiometric response could aid any restoration project in the initial stages or act as a baseline study to monitor changes to the peatland during and after a restoration project is complete. Future work will see this methodology extended to other peatland types such as blanket bogs and natural raised bogs, as well as the integration of concurrent airborne electromagnetic data to link the near-surface radiometric response to the deeper vadose zone and define a more comprehensive classification scheme for these peatland sites.
Experimental study on dynamic mechanism of sheet erosion processes on steep grassland in the loess region of China
Authors: Guo Qi , Wang Zhanli
Published: March 2021

Sheet erosion has been the major erosion process on steep grassland since the Grain-for-Green project was implemented in 1999 in the Loess Plateau with serious soil erosion, in China. Quantifying sheet erosion rate on steep grassland could improve soil erosion estimation on loess hillslopes and provide scientific support for effectively controlling soil erosion and rationally managing grassland. Simulated rainfall experiments were conducted on grassland plot with vegetation coverage of 40% under complete combination of rainfall intensities of 0.7, 1.0, 1.5, 2.0 and 2.5 mm min-1 and slope gradients of 7°, 10°, 15°, 20° and 25°. Results showed that sheet erosion rate (SE), varying from 0.0048 to 0.0578 kg m-2 min-1, was well described by binary power function equation (SE = 0.0026 I1.306S0.662) containing rainfall intensity and slope gradient with R2 = 0.940. The logarithmic equation of shear stress (SE = 0.084 + Ln (τ)) and the power function equation of stream power (SE = 1.141 ɷ1.073) could be used to predict sheet erosion rate. Stream power (R2 = 0.903) was a better predictor of sheet erosion than shear stress (R2 = 0.882). However, predictions based on flow velocity, unit stream power, and unit energy were unsatisfactory. The stream power was an excellent hydrodynamic parameter for predicting sheet erosion rate. The sheet erosion process of grassland slope was also affected by the raindrop impact except the dynamic action of sheet flow. The combination of stream power and rainfall kinetic energy (KE) among different rainfall physical parameters had the most closely relationship with the sheet erosion rates, which is also better than the stream power only, and a binary power function equation (SE = 0.221 ω0.831KE0.416) could be used to predict sheet erosion rate on grassland slope with R2 = 0.930. The study results revealed the dynamic mechanism of the sheet erosion process on steep grassland in the loess region of China.
Rescue of groundwater level time series: how to identify and treat errors
Authors: Bikše Jānis , Retike Inga , Kalvāns Andis , Dēliņa Aija , Babre Alise , Popovs Konrāds , Jemeļjanova Marta , Zelenkevičs Artjoms , Baikovs Artūrs , Avotniece Zanita
Published: March 2021

Groundwater level time series are the basis for various groundwater-related studies. The most valuable are long term, gapless and evenly spatially distributed datasets. However, most historical datasets have been acquired during a long-term period by various operators and database maintainers, using different data collection methods (manual measurements or automatic data loggers) and usually contain gaps and errors, that can originate both from measurement process and data processing. The easiest way is to eliminate the time series with obvious errors from further analysis, but then most of the valuable dataset may be lost, decreasing spatial and time coverage. Some gaps can be easily replaced by traditional methods (e.g. by mean values), but filling longer observation gaps (missing months, years) is complicated and often leads to false results. Thus, an effort should be made to retain as much as possible actual observation data.

In this study we present (1) most typical data errors found in long-term groundwater level monitoring datasets, (2) provide techniques to visually identify such errors and finally, (3) propose best ways of how to treat such errors. The approach also includes confidence levels for identification and decision-making process. The aim of the study was to pre-treat groundwater level time series obtained from the national monitoring network in Latvia for further use in groundwater drought modelling studies.

This research is funded by the Latvian Council of Science, project “Spatial and temporal prediction of groundwater drought with mixed models for multilayer sedimentary basin under climate change”, project No. lzp-2019/1-0165.
Quantifying the mechanical response of the Izaña area (Tenerife) to sustained groundwater withdrawal 
Authors: Lamur Anthony , De Angelis Silvio , Marrero Rayco , Lavallée Yan , Gonzalez Pablo J.
Published: March 2021

Surface water resources on volcanic islands with moderate rainfall and relatively high permeability are usually scarce or non-existent. As such, life and local economies of these islands mostly relies on groundwater exploitation. It is therefore important to characterise the sustainability of volcanic aquifer systems. In short, an aquifer is deemed in equilibrium when the recharge rate equals or exceeds the exploitation rate. The Izaña area in Tenerife Island (Canary Islands, Spain) has been exploited since the 1900s via a series of ~30 horizontal drilling or water galleries coming from both flanks of the NE-Ridge. Since exploitation began, the water table has dropped continuously, in some area even more than 200 m. Since the 2000s, aquifer dynamics (compaction) have been observed using InSAR indicating a subsidence rate of up to 2 cm per year.

Here, we investigate a suite of rock samples collected. The samples were collected at several water galleries aiming to be representative of the aquifer materials from the Izaña area. We first characterise the basic physical properties of each samples (porosity, permeability, solid density) before quantifying the elastic parameters (Young’s modulus, Poisson ratio) and uniaxial strength of the lithologies collected. We also measure Vp under dry and wet conditions (i.e. different saturation levels) to assess whether water saturation can alter the velocity of P-waves passing through those rocks.

Preliminary results show that connected porosities range from 0.16 to 45%, conferring a wide range of mechanical response to increasing effective pressure, with strength ranging from 18 – 315 MPa and Young’s moduli ranging from 3 – 57 GPa. In a similar fashion, results for Vp measurements also exhibit a range of values (~1.5 – 4.5 km/s). These data show that materials present in the aquifer are extremely varied, suggesting that both fluid flow and observed deformation are likely to be controlled by the weakest, most porous lithologies.

These results will further be integrated with the lithostratigraphic record of the aquifer in order to model the mechanical response of the aquifer to changes in effective pressures, and specifically pore pressure reduction with water extraction. Additionally, chemical and textural analysis will provide insights on the evolution of the porous network at different alteration levels, here serving as a proxy for time at saturation in the aquifer. Finally, we aim to compare the experimental results from laboratory measurements to those of hydro-geophysical measurements that will be collected in the field starting in mid-2021.
Spatiotemporal dynamics of CO2 flux in Basel city centre
Authors: Stagakis Stavros , Feigenwinter Christian , Zurbriggen Etienne , Pitacco Andrea , Vogt Roland
Published: March 2021

Independent, timely and accurate monitoring of urban CO2 emissions is important to assess the progress towards the Paris Agreement goals, evaluate the mitigation potential of the implemented actions and support urban planning, policy- and decision-making processes. However, there are several challenges towards achieving comprehensive urban emission monitoring at the required scales, which are mainly related to the complexities in the urban form, the urban function and their interactions with the atmosphere. Cities are highly heterogeneous mosaics of CO2 sources and sinks. Typically, the main emission sources in an urban neighbourhood are vehicles and buildings, while the contribution of human, plant and soil respiration can be also significant depending on population density and green area fraction. At the same time, urban vegetation acts as carbon sink, mitigating urban emissions locally. This study attempts to unravel the complex urban CO2 flux dynamics by modelling each component separately (i.e. building emissions, traffic emissions, human metabolism, photosynthetic uptake, plant respiration, soil respiration) based on high resolution geospatial, meteorological and population activity datasets. The case study is the city centre of Basel, Switzerland. The models are calibrated and evaluated using Eddy Covariance measurements of CO2 flux from two permanent tower sites in the city centre, covering a significant part of the study area. Moreover, an extended field campaign for the measurement of the biogenic components (i.e. photosynthetic uptake, plant respiration, soil respiration) has been active since the summer of 2020, involving regular chamber flux measurements and soil stations across the study area. The study reveals the spatial and temporal complexity of the urban CO2 flux dynamics both diurnally and seasonally. The relative contribution of each flux component to the seasonal cycle is presented, while the mitigation potential of urban vegetation is evaluated. Cross-comparison between model outputs and Eddy Covariance measurements are discussed in respect to source area variability, airflow complexity in the urban canopy layer and irregular unrecognized emission sources.