bluuebunny/crossref_metadata_2025 huggingface dataset:
Future increases in lightning-ignited boreal fires from conjunct increases in dry fuels and lightning
Authors: Hessilt Thomas Duchnik , van der Werf Guido , Abatzoglou John T. , Scholten Rebecca C. , Veraverbeke Sander
Published: March 2021

Fire is the most important landscape disturbance in the boreal forest of North America. The boreal region is the largest terrestrial biome and stores approximately 35 % of the global soil carbon (C). Burned area has increased over the last decades and is projected to increase further in the future, potentially altering boreal forest ecosystems from a net sink of C to a source of C. Approximately 90 % of the burned area in the region originates from wildfires ignited by lightning strikes. It is therefore important to understand the drivers of lightning-induced wildfires to evaluate the consequences of possible future changes in lightning activity and ignition efficiency. Here, we evaluated lightning ignition efficiency, i.e. the probability that a lightning strike starts a fire, for Alaska and Northwest Territories between 2001 and 2018 in function of three sets of drivers: lightning characteristics, topography, and fire weather. Further, we projected the lightning ignition efficiency under the RCP8.5 scenario and combined it with predictions of future lightning activity to assess future lightning ignition.
            The logistic model demonstrated an overwhelming influence of fire weather on lightning ignition efficiency (area under the curve > 0.83), whereas lightning characteristics and topography contributed relatively little to the model performance. We found that short-term drying of the organic soils is the most important requirement for a lightning strike to start a fire. The average lightning ignition efficiency for Alaska and Northwest Territories increased with 54 ± 32 % and 44 ± 44 by 2100. Combined with future projections of lightning activity, we predicted a total increase in lightning ignition of up to 230 ± 20 % and 92 ± 56 % for Alaska and Northwest Territories by 2100. Future increases in lightning ignitions in the boreal forest will likely induce additional burned areas in regions with C-rich peatland and permafrost soils. Our research showed that the increased availability of dry fuels and increases in lightning will reinforce each other leading to more boreal fires and consequent C emissions.
Implementation of the barotropic tide in oceanic circulation models. Current tests with the NEMO model 
Authors: Morel Yves , Benshila Rachid , Tranchant Benoit , Chanut Jerome , Arbic Brian , Allain Damien , Lyard Florent , Carrere Loren , Koch-Larrouy Ariane
Published: March 2021

This study proposes a new methodology for implementing the barotropic tide in an ocean general circulation model (OGCM). The assumptions underlying this methodology are that the best barotropic tide solutions are computed by specialized models and that the fields that should be accurately reproduced by the OGCM are the transport fields from the specialized tide model. The target/reference solution for the OGCM is thus the projection of the tide model on the OGCM grid, for each tidal harmonic.

The proposed methodology involves little change of the OGCM modeland yields almost exactly the reference solution, with a cost that is belowmost of the current methodologies. It relies on the modification of the tidepotential, or more accurately, on the replacement of all terms associatedwith the tide (tide potential, self attraction and loading, tide dissipation,  ...) by a general tide forcing term in the barotropic momentum equationwhich is calculated from the –known- reference solution.

The tide forcing terms can be tricky to calculate as they depend on details of the OGCM numerical schemes (for both temporal and spatial operators). A general procedure, automatically adapting the chosen schemes, is proposed for their calculation, so that the procedure is independent of the model.  

Tests with academic configurations are first proposed to validate the methodology and its implementation, and the OGCM is chosen to be the NEMO (Nucleus for European Modelling of the Ocean) model.

A global ¼° configuration with realistic bathymetry and with FES tide solutions (Finite Element Solution) are then performed. Current tests show that when FES solutions are crudely interpolated on the NEMO grid, the methodology exactly reproduces the FES fluxes, but the associates NEMO SSH is very noisy in regions where FES has high resolution. This problem is currently addressed. To get rid of this problem, fluxes must be carefully integrated along each grid cell, so that the reproduced SSH is exactly an average of the FES SSH within the NEMO grid cell. Hopefully, we will be able to present final –clean- solutions at the conference.
Influence of the representation of convection on the mid-Holocene West African Monsoon
Authors: Jungandreas Leonore , Hohenegger Cathy , Claussen Martin
Published: March 2021

Global climate models have difficulties to simulate the northward extension of the monsoonal precipitation over north Africa during the mid-Holocene as revealed by proxy data. A common feature of these models is that they usually operate on too coarse grids to explicitly resolve convection, but convection is the most essential mechanism leading to precipitation in the west African monsoon region. Here, we investigate how the representation of tropical deep convection in the ICON climate model affects the meridional distribution of monsoonal precipitation during the mid-Holocene, by comparing regional simulations of the summer monsoon season (July to September, JAS) with parameterized (40km-P) and explicitly resolved convection (5km-E). 
The spatial distribution and intensity of precipitation, are more realistic in the explicitly resolved convection simulations than in the simulations with parameterized convection.
However, in the JAS-mean the 40km-P simulation produces more precipitation and extents further north than the 5km-E simulation, especially between 12° N and 17° N. The higher precipitation rates in the 40km-P simulation are consistent with a stronger monsoonal circulation over land. 
Furthermore, the atmosphere in the 40km-P simulation is less stably stratified and notably moister. The differences in atmospheric water vapor are the result of substantial differences in the probability distribution function of precipitation and its resulting interactions with the land surface. The parametrization of convection produces light and large-scale precipitation, keeping the soils moist and supporting the development of convection. 
In contrast, less frequent but locally intense precipitation events lead to high amounts of runoff in explicitly resolved convection simulations. The stronger runoff inhibits the moistening of the soil during the monsoon season and limits the amount of water available to evaporation. 
The activities of the IAPG - International Association for Promoting Geoethics: status and future perspectives
Authors: Di Capua Giuseppe , Peppoloni Silvia
Published: March 2021

The IAPG (https://www.geoethics.org) was founded in August 2012 with the aim to increase the awareness of the geoscientific community on ethical, social and cultural implications of geoscience knowledge, research, practice, education, and communication.
In this perspective, geoethics has been initially developed in the context of geosciences, as a rediscovery by geoscientists, and in some cases as a real process of consciousness-raising, of the social role that they can and should play in support of society to face global anthropogenic changes.
Currently the IAPG can count on more than 2600 geoscientists (belonging to 130 countries) and its IAPG network includes also 32 national sections, working to develop geoethics by focusing on local specific issues of each country, and 3 task groups. Many international organizations recognize, appreciate and support results achieved by the association, through affiliations, agreements of cooperation and partnerships.
The IAPG has coordinated numerous publications, both books and articles, supports a book series on geoethics and a new scientific, open-access, not-for-profit, peer-reviewed journal on geoethics and social geosciences, and promotes a school on geoethics.
This presentation provides an update on the status of IAPG activities, and on future perspectives.
RPAS-SfM 4D mapping of shallow landslides activated in a steep terraced vineyard
Authors: Mauri Luca , Straffelini Eugenio , Cucchiaro Sara , Tarolli Paolo
Published: March 2021

The presence of roads is closely linked with the activation of land degradative phenomena such as landslides. Factors such as ineffective road management and design, local rainfall regimes and specific geomorphological elements actively influence landslides occurrence. In this context, recent developments in digital photogrammetry (e.g. Structure from Motion; SfM) paired with Remotely Piloted Aircraft Systems (RPAS) increase our possibilities to realize low-cost and recurrent topographic surveys. This allows the realization of multi-temporal (hereafter 4D) and high-resolution Digital Elevation Models (DEMs), fundamental to analyse geomorphological features and quantify processes at the fine spatial and temporal resolutions at which they occur. In this research is presented a 4D comparison of geomorphological indicators describing a landslide-prone agricultural system, so as to detect the noticed high-steep slope failures. The possibility to analyse the evolution of landslide geomorphic features in steep agricultural systems through high-resolution and 4D comparison of such indicators is still a challenge to be investigated. In this research, we considered a case study located in the central Italian Alps, where two shallow landslides (L1, L2) were activated below a rural road within a terraced vineyard. The dynamics of the landslides were monitored through the comparison of repeated DEMs (DEM of Difference, i.e. DoD), that reported erosion values of above 20 m3 and 10 m3 for the two landslides zones and deposition values of more than 15 m3 and 9 m3 respectively. The elaboration of Relative Path Impact Index (RPII) highlighted the role played by the road in the alteration of surface water flow directions. Altered water flows were expressed by values between 2σ and 4σ of RPII close to the collapsed surfaces. The increasing of profile curvature and roughness index described landslides evolution over time. Finally, the multi-temporal comparison of features extraction underlined the geomorphological changes affecting the study area. The computation of the quality index underlined the accuracy of features extraction. This index is expressed in a range between 0 (low accuracy) and 1 (high accuracy) and resulted equal to 0.22 m, regarding the landslide observed during the first RPAS survey (L1-pre); 0.63 m, concerning the same landslide detected during the second RPAS survey (L1-post); 0.69 m for L2. Results prove the usefulness of high-resolution and 4D RPAS-based SfM surveys for the investigation of landslides triggering due to the presence of roads at hillslope scale in agricultural systems. This work could be a useful starting point for further studies of landslide-susceptible zones at a wider scale, to preserve the quality and the productivity of affected agricultural areas.
G-RUN ENSEMBLE: A multi-forcing observation-based global runoff reanalysis
Authors: Ghiggi Gionata , Humphrey Vincent , Seneviratne Sonia , Gudmundsson Lukas
Published: March 2021

Although river flow is the best-monitored variable of the terrestrial water cycle, the scarcity of available in situ observations in large portions of the world has until now hindered the development of consistent observational estimates with global coverage. Recently, fusing sparse in-situ river discharge observations with gridded precipitation and temperature using machine learning has shown great potential for developing global monthly runoff estimates (Ghiggi et al., 2019). However, the accuracy of the utilised gridded precipitation and temperature products is variable and the corresponding uncertainty in the resulting runoff and river flow estimates was not yet quantified.

Global-RUNoff ENSEMBLE (G-RUN ENSEMBLE) (Ghiggi et al., in review) provides a multi-forcing global reanalysis of monthly runoff rates at a 0.5° resolution, composed of up to 525 runoff simulations. The G-RUN ENSEMBLE is based on 21 different atmospheric forcing datasets, overall spanning the period 1901-2019. The reconstructions are benchmarked against a comprehensive set of global-scale hydrological models (GHMs) simulations, using a large database of river discharge observations that were not used for model training as a reference.

Overall, the G-RUN ENSEMBLE shows good accuracy compared to the set of GHMs evaluated, especially with respect to the reproduction of the dynamics and seasonality of monthly runoff rates. We found that the spread imposed by the atmospheric forcing data in the G-RUN ENSEMBLE is small compared to the spread observed within the ensemble of GHMs simulations driven with a subset of such forcings. This might occur because GHMs are more impacted by biases in the input meteorological forcing and are more susceptible to accumulate errors over the simulation time than the adopted machine learning approach.

In summary, the multi-forcing nature of the G-RUN ENSEMBLE allows to quantify the uncertainty associated with the currently available atmospheric forcings, thereby paving the way for more robust and reliable water resources assessments, climate change attribution studies, hydro-climatic process studies as well as evaluation, calibration and refinement of GHMs.

References

Ghiggi, G., Humphrey, V., Seneviratne, S. I., and Gudmundsson, L. 2019: GRUN: an observation-based global gridded runoff dataset from 1902 to 2014, Earth Syst. Sci. Data, 11, 1655–1674, https://doi.org/10.5194/essd-11-1655-2019.

Ghiggi, G., Humphrey, V., Seneviratne, S. I., and Gudmundsson, L.: G-RUN ENSEMBLE: A multi-forcing observation-based global runoff reanalysis, Water Res. Res., in review.
A priori and a posteriori analysis in Large eddy simulation of the two-dimensional decaying turbulence using Explicit filtering approach
Authors: Perezhogin Pavel , Glazunov Andrey
Published: March 2021

In this work, we consider turbulence closures of LES (Large Eddy Simulation) type for classical decaying 2D turbulence in a priori and a posteriori experiments using explicit filtering approach. According to Germano 1986 decomposition, full subfilter stress for Gaussian filter is decomposed into three Galilean-invariant parts: Leonard, Cross and Reynolds stresses. By analysing spectral transfer of energy and enstrophy, we show that Leonard stress redistributes resolved energy toward large scales and dissipates substantial part of enstrophy, while Cross stress provides an additional enstrophy dissipation at subfilter scales and Reynolds stress predominantly injects energy into middle scales (i.e., Kinetic Energy Backscatter). Substantial part of enstrophy dissipation is located on subfilter scales, and it should be accounted for by choosing base filter wide enough compared to mesh step of LES model. Otherwise, significant fraction of enstrophy dissipation will correspond to subgrid scale stress, which is less universal and harder to approximate. As a result of a priori analysis, we propose LES closure consisting of three parts: SSM (Scale Similarity Model), which is equivalent to Leonard stress, biharmonic Smagorinsky damping as a Cross stress counterpart and ADM (Approximate Deconvolution Model) approximation for Reynolds stress ("backscatter"). The proposed model have two free parameters: Smagorinsky constant and amplitude of the backscatter. These parameters are estimated in a posteriori experiments utilizing dynamic approach and energy-enstrophy balance equation, correspondingly. The proposed model have the following distinctive features: it reproduces energy and enstrophy transfer spectra in accordance to the individual components of the subfilter forces, "reproduces" base filter and reproduces energy growth in accordance to the filtered DNS (Direct Numerical Simulation) solution.

The work was supported by the Russian Foundation for Basic Research (projects 19-35-90023, 18-05-60184) and Moscow Center for Fundamental and Applied Mathematics (agreement with the Ministry of Education and Science of the Russian Federation No. 075-15-2019-1624).
A high resolution three-dimensional model of ocean tides for the pan-arctic region
Authors: Ali Alfatih , Muller Malte , Bertino Laurent , Melson Arne
Published: March 2021

As grid resolutions of operational ocean models are becoming finer and approach closer to the coast, the importance of inclusion of tidal forcing in high resolution operational ocean forecasting systems has increasingly been recognized. In the current work,  we present a 3D general ocean circulation model of ocean tides in the pan-Arctic region at ~3km horizontal grid resolution and 50 hybrid layers in the vertical, thus representing both barotropic and internal tides. The model system is based on the Hybrid Coordinate Ocean Model (HYCOM)  coupled with the Los Alamos Sea Ice Model (CICE). The results showed good agreement when compared with observations from tide gauges and a data-assimilative global barotropic tidal model. Among other results, the evaluation includes results for tidal amplitude and phase of the most energetic constituents (M2, S2, K1 and Q1).   The model system is currently operational and its development is supported by the Copernicus Marine Environment Monitoring Service (CMEMS) where its forecasts are disseminated.
Geomorphic variability and sediment dynamics in small catchments of dryland environments: Application into sand mining
Authors: Singh Abhishek kumar , Bhatt Nishith
Published: March 2021

The understanding of the sediment routing system and source-to-sink dynamics in a catchment is vital as it helps to assess areas undergoing erosion and deposition. This is significant in catchments which undergo active mining activities especially natural sand materials. The role of climate and natural erosional processes is vital in this as mining of sand is also affected by natural replenishment. In present study, we take a case study of a small catchment of 30km length ~ Chharri, situated in arid landscape of Kachchh of western India. Using geomorphic assemblage mapped using remote sensing and field investigation, we identified natural sub-sinks (depocenters) in the Chharri river valley. The investigation was validated by studying sediment profiles of the depocentral landforms in seasonal time series (pre-monsoon and post monsoon sessions). The changes in morphology, sediments accumulations were integrated to assess the natural sand replenishment in areas which had been undergoing mining activity. Based on time series data it was deduced that the small catchments in dry-land environments, the sand production and dynamics is modulated by type of vegetation, pattern in precipitation and human intervention. The results of such source-to-sink study have long-term implications on sand replenishment, mining activity and landscape evolution of such river basins.
Changes in Northwest Greenland Ice Sheet Elevation and Mass
Authors: Otosaka Inès , Shepherd Andrew , Groh Andreas
Published: March 2021

About a third of Greenland’s total ice losses come from the Northwest sector, a sector that includes a large number of marine-terminating outlet glaciers, which have all experienced widespread retreat triggered by ocean-induced melting. Here, we derive changes in surface elevation, volume and mass in the Northwest sector of the Greenland Ice Sheet using a decade of CryoSat-2 observations. We find an average elevation change rate of 18.7 ± 0.4 cm/yr, with rapid thinning at the ice sheet margins at a rate of 42.7 ± 0.9 cm/yr. We compare our CryoSat-2 rates of elevation change to airborne laser altimetry data from Operation IceBridge. Overall, there is a good agreement between the two datasets with a mean difference of 6.5 ± 0.5 cm/yr and standard deviation of 31.1 cm/yr. We further compute volume change, which we convert to mass change by testing three alternate density models and we find that the northwest sector has lost 386 ± 3.7 Gt of ice between July 2010 and July 2019. We compare our mass balance estimate to independent estimates from gravimetry and the mass budget method across different spatial scales. First, we compare the different estimates by splitting the sector into two and four regions. While our altimetry estimate is the least negative across all regions, the gravimetry and mass budget estimates alternate in recording the largest ice losses. We further compare mass changes derived from altimetry and the mass budget method in each of the 74 individual glacier basins of the Northwest sector. We find a high correlation of 0.81 between rates of mass change from altimetry and the mass budget method, with the highest differences recorded in Steenstrup-Dietrichson and Kjer Gletscher basins. Our comparisons show that the spatial pattern of the differences between mass balance estimates is complex, suggesting that discrepancies between techniques do not solely originate from one single region or technique. Finally, we explore several factors that could potentially bias our altimetry mass balance estimation, by investigating differences between satellite radar and airborne laser altimetry, the dependency on grid spatial resolution and the impact of using different density models.
Distinguishing shock-related microstructures in gneisses from the Vredefort impact structure, South Africa
Authors: Dellefant Fabian , Trepmann Claudia , Gilder Stuart , Sleptsova Iuliia , Kaliwoda Melanie , Weiss Benjamin
Published: March 2021

Shocked gneiss (~8 GPa) from the Vredefort impact structure (South Africa) contain planar fractures in quartz decorated by magnetite and ilmenite, which are commonly attributed to the impact event. However, the surface at Vredefort is riddled by lightning strikes, which also produce rapid pressure-temperature pulses that can modify the microstructure and the magnetic properties of the rocks. To understand the differences between lightning and impact-related shock effects, we investigated samples from two, 10 m-deep drill cores by Raman spectroscopy, polarized light microscopy/U-stage and electron microscopy/electron backscatter diffraction techniques. Magnetite and ilmenite within planar fractures in quartz occur at all depths, and are therefore intrinsic to the impact event, independent of lightning. Primary iron-bearing minerals were locally heated by the generation of shear fractures in neighboring quartz, leading to small volumes (micrometer scales) of melt intruding into nearby fractures. Frictional heating and rapid quenching of feldspar and quartz is indicated by localized, fine-grained aggregates along intragranular planar fractures as well as transgranular pseudotachylytic veins. On the other hand, altered ilmenite grains with exsolved magnetite occur only in gneisses from the uppermost 80 cm of both drill cores. When in contact with biotite, the ilmenite-magnetite boundaries are altered to chlorite, and the ilmenite is partly transformed to anatase. These alteration products contain fine-grained magnetite. It appears that lightning strikes altered the existing ilmenite-magnetite in the Vredefort samples to produce smaller, more single-domain like magnetite grains, consistent with the observed magnetic properties of the samples
An evaluation of generated source signals from machinery in conventional tunnelling and their possible application in a tunnel seismic while drilling (TSWD) system
Authors: Hartl Irene , Schlögel Ingrid , Wenighofer Robert , Gallistl Jakob
Published: March 2021

Geological conditions and their uncertainties are a major risk factor in underground construction projects. To ensure a fast, smooth and save completion of the excavation, a prediction of the geological conditions in front of the working face during tunnelling is a topic of great importance.

Various geophysical methods for a prediction of the conditions ahead of the tunnel face have been developed over the past years, yet, most of them being seismic techniques, which require a short interruption of the excavation to minimise noise interference. However, there is also the approach with TSWD which uses the working TBM (Tunnel Boring Machine) as a source signal and can thus work simultaneously with the excavation. Up to now, this concept has been applied primarily in mechanised tunnelling and there are hardly any applications in conventional tunnelling.

In the course of several practical experiments at the “Zentrum am Berg” in Eisenerz (Austria), different concepts for a transfer of TSWD from mechanised to conventional tunnelling were developed and tested at scale in an underground research facility. Three machines were used for these tests, an excavator with a hydraulic hammer attached as well as two different drilling jumbos. The devices were equipped with an accelerometer to pick up the source signal at its origin (pilot signal). Different sensor positions were tested using a sledge hammer as a source and evaluated in detail. Moreover, omnidirectional geophones of different sensitivities (4.5 Hz and 27 Hz) were tested and compared as transducers in the adjacent rock mass.

An essential part of the experiment analysis consisted of the evaluation of the source characteristics as well as the generated spectral bandwidth of the source signal from typical construction machines in conventional tunnelling. Consequently, the outcomes will be another step forward in the development of a TSWD exploration system also applicable to conventional tunnelling projects.
Towards DOAS measurements with a picometre spectral resolution
Authors: Kuhn Jonas , Bobrowski Nicole , Wagner Thomas , Platt Ulrich
Published: March 2021

Differential Optical Absorption Spectroscopy (DOAS) has proven to be very useful to study the composition and dynamics of Earth’s atmosphere. Compact grating spectrographs (GSs) with moderate spectral resolution (ca. 1nm) allow to quantify the absorption of many trace gases along atmospheric light paths from ground to space borne platforms.

Since the width of a rovibronic absorption line of a small molecule in the UV to near IR spectral range is in the picometre range, increasing the spectral resolution of DOAS measurements largely increases their selectivity and in many cases also their sensitivity. In addition, further trace gases (e.g. OH radicals) or isotopes of trace gases could be detected, while common problems due to Fraunhofer line undersampling were reduced. However, since high resolution GSs are bulky (immobile) instruments with a strongly reduced light throughput, hardly any high resolution DOAS measurements have been performed.

Since more than a century, Fabry Pérot Interferometers (FPIs) have been successfully used for high resolution spectroscopy in many scientific fields, where their light throughput advantage over grating spectrographs for higher resolving powers is well known. However, except for a few studies, FPIs
received hardly any attention in atmospheric trace gas remote sensing. We examine the light throughput of GSs and FPI spectrographs regarding spectral resolution and spectrograph size (i.e. mobility). We find that robust and mobile high resolution FPI spectrograph implementations can be by orders of magnitude smaller than GSs with the same spectral resolution. A compact high resolution FPI spectrograph prototype was already successfully tested in the field. Further, the light throughput can be optimised to allow for passive scattered sunlight measurements with similar SNR as moderate resolution DOAS measurements while, at the same time, attaining spectral resolutions in the picometre range.

High resolution FPI spectrographs might allow for a multitude of applications in atmospheric remote sensing. A few examples include scattered sunlight absorption measurements of many atmospheric trace gases and their isotopes, the quantification of tropospheric and volcanic OH radicals, high resolution O2 measurements for radiative transfer investigation and aerosol studies, and solar induced chlorophyll fluorescence quantification using Fraunhofer lines.
The analysis of streamflow variability and flood wave characteristics on the two lowland rivers in Croatia
Authors: Kovačević Martina , Potočki Kristina , Gilja Gordon
Published: March 2021

Climate changes affect streamflow variability on the long-term and short-term temporal scale. Consequently, analysis of changes in hydrological regime, but also in intensity and frequency of short-time flood events, enables better understanding of hydraulic and geomorphological processes in rivers. Changes in streamflow variability and flood wave characteristics may lead to intensifying riverbed erosion and lowering infrastructure safety, such as bridges over rivers. The aim of the study is to analyse hydrological regime for historical data from the selected gauging stations on the two large lowland rivers in Croatia: the Sava River and the Drava River. Analysis of the magnitude, frequency, variability, and timing of streamflow is conducted. Additionally, deterministic and probabilistic approach to determination of metrics that describe hydrograph shape is performed for the different threshold levels. Results obtained from this study will help in exploring riverbed erosion processes which may entail the increased scouring around bridge piers and consecutively impair the infrastructure reliability in the changing climate.

Acknowledgments

This work has been supported in part by Croatian Science Foundation under the project R3PEAT (UIP-2019-04-4046) and DOK-2020-01.
Extended-range predictability of sudden stratospheric warming events suggested by mode decomposition
Authors: Wu Zheng , Jiménez-Esteve Bernat , de Fondeville Raphael , Székely Eniko , Obozinski Guillaume , Domeisen Daniela
Published: March 2021

Major sudden stratospheric warmings (SSWs) are extreme wintertime circulation events of the Arctic stratosphere that are accompanied by a breakdown of the polar vortex. The stratospheric anomalies can propagate downward to the lower stratosphere and influence the weather of the troposphere and the surface for up to two months after the onset of SSW events. Therefore, SSWs can be an important source of predictability on subseasonal to seasonal (S2S) time scales over the Northern Hemisphere (NH) mid- and high- latitudes. However, SSWs themselves are difficult to forecast, with a predictability limit of around one to two weeks. Therefore, understanding the dynamical process that leads to the vortex breakdown is crucial to improve the predictability of SSWs, and ultimately, the weather at the Earth’s surface. To this end, we employ a mode decomposition diagnosis to analyze Ertel's potential vorticity (PV) equation by decomposing each term using empirical orthogonal functions (EOFs) of PV to study the vortex weakening process. With this method, a principal component (PC) tendency equation can be derived, which includes the evolution of the linear and nonlinear PV advection terms and indicates how they contribute to the vortex weakening. The results show that the linear advection term is the main contributor to the increase of PC tendency in the early stage of the warming and contains distinct signals that indicate the weakening of the vortex as early as 25 days before the onset of SSWs using ERA-interim daily data. Our results indicate that both the lead times of the onset of SSW events as well as the type of the event may be extended beyond the current predictability limit, promising to provide longer lead times for the prediction of surface weather. 
Global flood monitoring with GRACE/GRACE-FO
Authors: Latinovic Milena , Güntner Andreas , Flechtner Frank , Murböck Michael , Kwas Andreas
Published: March 2021

The German Aerospace Center and NASA's joint mission, the Gravity Recovery and Climate Experiment (GRACE) operational from 2002 until October 2017, provided measurements of Earth's gravity field anomalies. Its follow-on mission GRACE-FO, implemented by NASA and GFZ, was launched in May 2018 and continued to give us large-scale measurements of the Earth's gravity variations. These variations in gravity are used to determine anomalies of total water storage (TWSA) which can provide us with insights into global water redistribution on a monthly up to a daily basis.

Most common natural disasters that still require efficient early warning systems are floods. Floods are causing significant economic and humanitarian losses on a global scale and are triggered by the interaction of different hydro-meteorological processes (e.g. precipitation, sub-surface water storage, snow cover).    

We aim to explore GRACE and GRACE-FO products' possibilities to detect the water storage dynamics associated with floods in large river catchments. We include analysis of the basins' wetness states before the flood events, which eventually can give us early indicators of flood development. During the GRACE data period, we investigate around 2500 historical floods from the Dartmouth Flood Observatory (DFO). We acquire GRACE data with daily resolution from the latest releases of ITSG and GFZ for the spatial extent of DFO floods and reduce TWSA values by long-term trends and by average seasonal variability. Furthermore, we assess the available river discharge time series, during the GRACE period, obtained from the Global Runoff Data Centre (GRDC) for the flood event separation. We compare GRACE-based water storage anomalies to flood events' characteristics, like peak, volume, and duration. Results show the potential of GRACE-based TWSA to detect large-scale flood events.
Improving solar wind forecasting using Data Assimilation
Authors: Lang Matthew , Witherington Jake , Turner Harriet , Owens Mathew , Riley Pete
Published: March 2021

In terrestrial weather prediction, Data Assimilation (DA) has enabled huge improvements in operational forecasting capabilities. It does this by producing more accurate initial conditions and/or model parameters for forecasting; reducing the impacts of the “butterfly effect”. However, data assimilation is still in its infancy in space weather applications and it is not quantitatively understood how DA can improve space weather forecasts.

To this effect, we have used a solar wind DA scheme to assimilate observations from STEREO A, STEREO B and ACE over the operational lifetime of STEREO-B (2007-2014). This scheme allows observational information at 1AU to update and improve the inner boundary of the solar wind model (at 30 solar radii). These improved inner boundary conditions are then input into the efficient solar wind model, HUXt, to produce forecasts of the solar wind over the next solar rotation.

In this talk, I will be showing that data assimilation is capable of improving solar wind predictions not only in near-Earth space, but in the whole model domain, and compare these forecasts to corotation of observations from STEREO-B at Earth. I will also show that the DA forecasts are capable of reducing systematic errors that occur to latitudinal offset in STEREO-B’s corotation forecast.
Arctic Ocean tidal regime change across the Bolling-Allerod onset
Authors: Velay-Vitow Jesse , Peltier William Richard
Published: March 2021

Although currently microtidal, the Arctic Ocean is known to have been megatidal at Last Glacial Maximum (LGM) due to the Arctic Ocean basin being nearly entirely enclosed, with only Fram Strait connecting it to the global ocean. This allowed for the propagation of a gravest mode coastal Kelvin wave traveling anti-clockwise around the Arctic ocean. The transition from the megatidal regime at LGM to the mircotidal regime observed today is not well understood, and the factors which control the amplitude of the semidiurnal tidal constituents in the Arctic Ocean have not been fully determined in the literature. We investigate the Arctic tidal regime across the Bolling-Allerod (B-A) onset, 14.6-14.1 ka, finding that the Arctic Ocean is megatidal prior to B-A onset and weakens considerably thereafter. The period of time during which the Arctic tidal regime is enhanced is precisely the time at which high Arctic ice streams begin to deglaciate, indicating that the tides may play a causal role in forcing the rapid deglaciation of the sector of the Laurentide abutting the Arctic Ocean. We further show that the deglaciation of the Laurentide ice sheet, through the mechanisms of Glacial Isostatic Adjustment (GIA) and gravitationally self-consistent local reduction in sea level, causes an increase in the amplitude of the principal lunar and solar semidiurnal tidal constituents in the Arctic Ocean. Additionally, it is the collapse of the Barents sea ice sheet which significantly weakens the Arctic Ocean tidal regime. We report the contribution of each major terrestrial ice sheet to the relative sea-level rise at each of Barbados, Tahiti, and Sunda Shelf, finding that the gravitationally self-consistent GIA model employed accurately predicts the RSL change at each of these sites and determines that the contribution at Barbados from the Laurentide is smaller than the contribution at Tahiti or Sunda Shelf due to the flow of ocean water away from the deglaciating Laurentide and into the "far field."  We further show that the contribution to RSL at Barbados due to the collapse of the Barents Sea ice sheet is significant. 
New insights into structure and seismicity of the Central Alps from 3D Pg and Sg tomography and improved hypocenter relocations
Authors: Diehl Tobias , Kissling Edi , Herwegh Marco , Schmid Stefan
Published: March 2021

Accuracy of hypocenter location, in particular focal depth, is a precondition for high-resolution seismotectonic analysis of natural and induced seismicity. For instance, linking seismicity with mapped fault segments requires hypocenter accuracy at the sub-kilometer scale. In this study, we demonstrate that inaccurate velocity models and improper phase selection can bias absolute hypocenter locations and location uncertainties, resulting in errors larger than the targeted accuracy. To avoid such bias in densely instrumented seismic networks, we propose a coupled hypocenter-velocity inversion restricted to direct, upper-crustal Pg and Sg phases. The derived three-dimensional velocity models, combined with dynamic phase selection and non-linear location algorithms result in a highly accurate earthquake catalog, including consistent hypocenter uncertainties. We apply this procedure to about 60’000 Pg and 30’000 Sg quality-checked phases of local earthquakes in the Central Alps region. The derived tomographic models image the Vp and Vs velocity structure of the Central Alps’ upper crust at unprecedented resolution, including small-scale anomalies such as those caused by a Permo-Carboniferous trough in the northern foreland, Subalpine Molasse below the Alpine front or crystalline basement units within the Penninic nappes. The external Aar Massif is characterized by low Vp/Vs ratios of about 1.625-1.675 in the depth range of 2-6.5 km, which we relate to a felsic composition of the uplifted crustal block, possibly with increased quartz content. Finally, we discuss along-strike variations imaged by relocated seismicity in the Central Alps and demonstrate how joint interpretation of velocity structure and hypocenters provides additional constraints on lithologies of upper-crustal seismicity.
Burst of Unusual Quasi-10 day Wave During the 2019 Southern Sudden Stratospheric Warming
Authors: Wang Jack , Palo Scott , Forbes Jeffrey , Marino John , Moffat-Griffin Tracy
Published: March 2021

An unusual sudden stratospheric warming (SSW) occurred in the Southern hemisphere in September 2019. Ground-based and satellite observations show the presence of a transient westward-propagating quasi-10 day planetary wave with zonal wavenumber one during the SSW. The planetary wave activity maximizes in the MLT region approximately 10 days after the SSW onset. Analysis indicates the quasi-10 day planetary wave is symmetric about the equator which is contrary to theory for such planetary waves. 

Observations from MLS and SABER provide a unique opportunity to study the global structure and evolution of the symmetric quasi-10 day wave with observations of both geopotential height and temperature during these unusual atmospheric conditions. The space-based measurements are combined with meteor radar wind measurements from Antarctica, providing a comprehensive view of the quasi-10 day wave activity in the southern hemisphere during this SSW. We will also present the results of our mesospheric and lower thermospheric analysis along with a preliminary analysis of the ionospheric response to these wave perturbations.
What is the optimum solar proxy for long-term ionospheric studies?
Authors: Laštovička Jan
Published: March 2021

For long-term studies as ionospheric trends investigations we have to use proxies of solar activity, because homogenous and sufficiently long data series of solar ionizing radiation are not available. Here I deal with selection of the optimum solar proxy for yearly average and monthly median values near noon (11-13 LT). Six solar proxies are used, F10.7, F30, Mg II, He II, Fα (solar H Lyman alpha flux) and R (sunspot number), foF2 from European ionosondes Juliusruh, Pruhonice and Rome, and foE from Chilton and Juliusruh over the period 1976-2019. For yearly values Mg II is the optimum proxy (but it is available only since late 1978) for foF2, with F30 being the second best. For foE the optimum proxy appears to be F10.7. For monthly medians of January, April, July and October the general pattern is the same as for yearly values.
The response of Evapotranspiration to osmotic potential in small-scale lab lysimeters
Authors: Salman Adil , Durner Wolfgang , Joshi Deep C. , Naseri Mahyar
Published: March 2021

Drought and climatic change are among the main environmental stressors for the water and soil qualities. Soil water potential is the major soil-related factor controlling water availability to plants and their evapotranspiration. It consists of two main components: matric and osmotic potential. Although the effect of matric potential on plant evapotranspiration has been extensively studied under various conditions, there is still a lack of quantitative studies on the effects of osmotic potential on evapotranspiration.

In our study, we investigated the influence of soil osmotic potential on the evapotranspiration rate and cumulative evapotranspiration of grass planted in small laboratory lysimeters. A sandy loam soil material was packed in four lysimeters with a volume of 6000 cm3 and equal bulk density. The soil material was air dried, freed from roots and passed through a 2 mm sieve. Each lysimeter was equipped with soil sensors at two different depths to monitor soil moisture, bulk electrical conductivity, temperature, and matric potential. To obtain continuous mass balance measurements, each lysimeter was placed on a balance connected to the computer. Grass seeds were planted in each lysimeter at the same density and irrigated with distilled water until plant height was 12 cm. Irrigation water of two different qualities (EC= 0 and 4.79 dS/m) was then applied to produce different levels (0 and -0.17 MPa) of osmotic potential. The volumetric water content was adjusted to a value between 15 and 20 % in each lysimeter during the grass growth period. When the volumetric water content reached 15 %, irrigation water was added to the lysimeters to increase it to 20 %. Data were collected to calculate changes in osmotic potential relative to changes in total soil water potential. In addition, the relationship between osmotic potential and evapotranspiration rate during the growing season was determined.

Our results indicate a controlling role of soil osmotic potential on total soil water potential. This role results a significant reductions in evapotranspiration in response to increases in osmotic potential, in addition to effects on plant health. Osmotic potential has a significant function on total soil water potential when the soil becomes dry and poor water qualities are used in irrigation.
Two-Element Keyboard-Block Model Of Megathrust Earthquakes Generation For Central Kurils
Authors: Gabsatarov Yurii , Vladimirova Irina , Alexeev Dmitry , Lobkovsky Leopold
Published: March 2021

The strongest subduction earthquakes (M≥8) lead to the release of the huge amount of elastic stresses accumulated over hundreds or even thousands of years. Prediction of such earthquakes, causing significant socio-economic and environmental damage, is one of the most important and urgent tasks of geophysics.

To date, significant advances have been made in the field of earthquake prediction using models based on the concept of a continuous geophysical medium that ruptured coseismically along the main fault. As an alternative, models are proposed that take into account the fault-block structure of the continental margin, confirmed by seismological and oceanographic studies. In our study, we consider one of such models - a keyboard-block model (single-element) which combines the ideas of possible synchronous destruction of several adjacent asperities, mutual slip along a plane with variable friction depending on velocity, and subsequent healing of destructed portions of the medium under high-pressure conditions. This concept made it possible to simulate the displacement of surface points of frontal seismogenic blocks at all stages of the seismic cycle.

GNSS observations in subduction regions are carried out mostly on islands situated on the rear massif far from the seismogenic blocks. Strong multidirectional motion registered on GNSS stations during the seismic cycle, as well as seismological and geological data, clearly indicate that the rear part of the arc also has a complex structure and is divided into separate segments by large faults rooted into the contact zone of interacting lithospheric plates. We made a generalization (double-element) of the original model to consider the discontinuity of not only the frontal but also the rear part of the island arc.

We compared the earth's surface displacements during the seismic cycle in the Central Kurils, obtained within the framework of the continuous model, as well as the single-element and two-element keyboard models, to establish the influence of various configurations of the fault-block structure of the continental margin on the seismic cycle. We constructed the continuous model on the basis of our slip distribution model for the 2006 Simushir earthquake which indicates the interplate coupling patches prior to this earthquake.

This study was supported by the Russian Science Foundation (project 20–17-00140).