bluuebunny/crossref_metadata_2025 huggingface dataset:
Particle Acceleration in Earth’s Global Magnetosphere: a Multiple Step Process
Authors: Richard Robert L. , Schriver David , Berchem Jean , El-Alaoui Mostafa , Lapenta Giovanni , Walker Raymond J.
Published: March 2020

Particle velocity distribution functions measured by spacecraft show that suprathermal ion and electron populations are a common feature of Earth’s magnetosphere.  An outstanding question has been to determine the acceleration processes that lead to the formation of these suprathermal particle populations. Very often, it has been challenging to explain the high levels of energy reached by these particles by simply invoking local processes such as magnetic reconnection. In this presentation, we investigate the hypothesis that suprathermal particle populations increase if the acceleration occurs over multiple steps through different acceleration mechanisms at different spatial locations in Earth’s magnetosphere.  For example, particles transported to the magnetotail which have been accelerated first in the dayside reconnection region could be further accelerated in the tail reconnection regions and then gain additional energy through Fermi and/or betatron acceleration as they convected back to the dayside magnetopause. Since local kinetic processes dominate the acceleration of ions and electrons in the magnetosphere, it has been difficult to validate that hypothesis. Multiple reconnection sites and different possible acceleration regions are too distant to be included in a single kinetic simulation and global hybrid simulations cannot describe electron acceleration.  To address this research problem we leverage our simulation capabilities by combining three different simulation techniques: global magnetohydrodynamic (MHD) simulations, large-scale kinetic (LSK) particle tracing simulations, and large-scale particle in cell (PIC) simulations.  First, we carry out an MHD simulation driven by upstream solar wind and interplanetary magnetic field conditions for a specific time interval featuring active magnetospheric reconnection.  Then we use an implicit PIC simulation of dayside reconnection with initial and boundary conditions from the MHD simulation.  Next, we follow suprathermal particles from the PIC simulation globally through the MHD fields using LSK to assess their transport into the magnetotail. A final step is to perform a PIC simulation embedded in the MHD simulation of magnetotail process including the suprathermal particles arriving from the dayside as determined from the LSK simulation.  Preliminary results indicate that particles energized by dayside reconnection are more likely to reach the magnetotail reconnection region. In addition, the development of enhanced high-energy tails in the particle distributions is promoted by previous energization steps during particle transport to the magnetotail reconnection region.
New trends in Multihazards Probabilistic Safety Assessment for nuclear installations: the H2020-NARSIS Project
Authors: Foerster Evelyne , Bazargan-Sabet Behrooz , Daniell James , Gehl Pierre , Vardon Philip J. , Duvvuru Mohan Varenya K. , Rastiello Giuseppe , Štrubelj Luka , Ragon Florence
Published: March 2020

The methodology for Probabilistic Safety Assessment (PSA) of Nuclear Power Plants (NPPs) has been used for decades by practitioners to better understand the most probable initiators of nuclear accidents by identifying potential accident scenarios, their consequences, and their probabilities. However, despite the remarkable reliability of the methodology, the Fukushima Dai-ichi nuclear accident in Japan, which occurred in March 2011, highlighted a number of challenging issues (e.g. cascading event - cliff edge - scenarios) with respect to the application of PSA questioning the relevance of PSA practice, for such low-probability but high-consequences external events. Following the Fukushima Dai-ichi accident, several initiatives at the international level, have been launched in order to review current practices and identify shortcomings in scientific and technical approaches for the characterization of external natural extreme events and the evaluation of their consequences on the safety of nuclear facilities.

The H2020 project “New Approach to Reactor Safety ImprovementS” (NARSIS, 2017-2021) aims at proposing some improvements to be integrated in existing PSA procedures for NPPs, considering single, cascade and combined external natural hazards (earthquakes, flooding, extreme weather, tsunamis). It coordinates the research efforts of eighteen partners encompassing leading universities, research institutes, technical support organizations (TSO), nuclear power producers and suppliers, reactor designers and operators from ten countries.

The project will lead to the release of various tools together with recommendations and guidelines for use in nuclear safety assessment, including a Bayesian-based multi-risk framework able to account for causes and consequences of technical, social/organizational and human aspects and as well as a supporting Severe Accident Management decision-making tool for demonstration purposes.

The NARSIS project has now been running for two years and a half, and the first set of deliverables and tools have been produced as part of the effort of the consortium. Datasets have been collected, methodologies tested, states of the art have been produced, and various criteria and plans developed. First results have started to emerge and will be presented here.
Characterization of Interannual and Seasonal Variability of Hydro-Climatic Trends in the Upper Indus Basin
Authors: Liaqat Muhammad Usman , Ranzi Roberto , Grossi Giovanna , Mahmood Talha
Published: March 2020

A major part of Pakistan’s economy is dependent upon agriculture which is irrigated from the water resources of the Upper Indus Basin (UIB). Therefore the human impact of hydroclimatic variability in this area is of paramount importance. The Upper Indus Basin is characterized by uncertain hydro-climatic behaviour with changing patterns in different sub-basins. Many studies have worked on hydro-climatic trends at basin scale but only few studies focused on the hydroclimate, precipitation dynamics and their magnitude at sub-basin level. Based upon this scenario, high resolution seasonal and annual climatology of UIB was developed. It is based on precipitation normals 1995-2017 obtained from four different gridded satellite datasets (Aphrodite, Chirps, PERSIANN-CDR and GPCC) as well as quality- controlled high and mid elevation ground observations (1250–4500 m a.s.l.). The quality-control of the gridded dataset is computed by the anomaly method. In order to, evaluate the data quality of the gridded rainfall, four statistics i.e., BIAS, Mean Absolute Error (MAE), Mean Absolute Percentage Error (MAPE) and Root Mean Square Error (RMSE) are used in this study. Using running trends and spectral analysis with multi-gauge based anomaly, the study analyses the precipitation and runoff   seasonal and annual temporal variability at sub-basin scale. For this, Mann–Kendall test was employed to detect the presence of any trend while their slope is calculated by Theil Sen’s slope method. The nonparametric Pettitt Test was also used in this study to eventually identify the change point in hydro-climatic time series. The results indicated that bias corrected CHIRPS precipitation datasets performed better in simulating precipitation with RMSE, MAE, MAPE [%] and BIAS followed by APHRODITE. The annual and seasonal precipitation climatology exhibited higher precipitation in the lower side of the basin. The comparison between short and long duration climatologies is being investigated as well. The annual running trend analysis of precipitation exhibited a very slight change whereas a more significant increase was found in the winter season (DJF) and most of sub-basins feature a significant decreasing rate in precipitation and constant change point within the monsoon period (JJA). Similarly, trend analysis for runoff in main rivers of Upper Indus Basin at Gilgat, Indus (Besham Qila, Bunji) exhibit nonsignificant increase except Hunza and Indus at Kharmong which are showed decrease annual trends and will be further investigated for seasonal patterns. Overall, these findings would assist to better understand precipitation, snow- and ice-melt runoff dynamics, addressing the hydroclimatic behaviour of the Karakoram region.
A significant acceleration of ice volume discharge preceded a major retreat of a West Antarctic paleo-ice stream
Authors: Bart Phil , Tulaczyk Slawek
Published: March 2020

For the period between 14.7 and 11.5 cal. (calibrated) kyr B.P, the sediment flux of Bindschadler Ice Stream (BIS; West Antarctica) averaged 1.7 × 108 m3 a−1. This implies that BIS velocity averaged 500 ± 120 m a-1. At a finer resolution, the data suggest two stages of ice stream flow. During the first 2400 ± 400 years of a grounding-zone stillstand, ice stream flow averaged 200 ± 90 m a-1. Following ice-shelf breakup at 12.3 ± 0.2 cal. kyr B.P., flow accelerated to 1350 ± 580 m a-1. The estimated ice volume discharge after breakup exceeds the balance velocity by a factor of two and implies ice mass imbalance of ~40 Gt a-1 just before the grounding zone retreated >200 km. We interpret that the paleo-BIS maintained sustainable discharge throughout the grounding-zone stillstand first due to the buttressing effect of its fringing ice shelf and then later (i.e., after ice-shelf breakup) due to the stabilizing effects of grounding-zone wedge aggradation. Major paleo–ice stream retreat, shortly after the ice-shelf breakup that triggered the inferred ice flow acceleration, substantiates the current concerns about rapid, near-future retreat of major glaciers in the Amundsen Sea sector where Pine Island and Thwaites Glaciers are already experiencing ice-shelf instability and grounding-zone retreat that have triggered upstream-propagating thinning and ice acceleration.
The effect of geometrical irregularities on damage zone width: Modeling and field observations
Authors: Tal Yuval , Faulkner Daniel
Published: March 2020

Geological and geophysical observations of fault zones reveal that fault cores are surrounded by regions of damaged rocks consist of fractures at a wide range of length scales with decaying intensity with distance from the fault core. The main mechanisms proposed for the development of off-fault damage include slip on faults with geometrical irregularities, migrating process zones, and dynamic damage from the passage of earthquake ruptures. Field observations of relatively deep exhumed fault zones have shown that fault damage zone width scales with the displacement on a fault. In this study, we combine such observations with numerical modeling to test what is the dominant mechanism producing off-fault damage at depth of several kilometres.

The field data [Faulkner et al., 2011] include measurements of micro-fracture damage zone width from small displacement fault zones within the Atacama fault zone in northern Chile that formed at ∼6 km depth within a dioritic protolith. An increase in damage zone width with displacement is clearly seen. We perform simulations of slip on synthetic faults, with roughness properties similar to that of natural faults, and examine how the total slip and roughness characteristics affect the extent and intensity of inelastic deformation to constrain the geometrical and frictional properties that could generate the observed damage. To accurately account for the effects of geometrical irregularities on the fault and allow slip that is large relative to the size the minimum roughness wavelength, we use the mortar finite element method, in which non-matching meshes are allowed across the fault and the contacts are continuously updated. Inelastic deformation of the bulk is modelled with Drucker–Prager viscoplasticity, which is a simple choice for describing cracked medium and is closely related to the Mohr–Coulomb model. Our results indicate that, for the depth and fault lengths in the field data, geometrical irregularities produce the scaling of damage zone width with displacement observed in the field and suggest that this, rather than the other mechanisms, produce most of the damage.
Contrasting sources of variability in subtropical and subpolar Atlantic overturning
Authors: Kostov Yavor , Johnson Helen L. , Marshall David P. , Forget Gael , Heimbach Patrick , Holliday N. Penny , Li Feili , Lozier M. Susan , Pillar Helen R. , Smith Timothy
Published: March 2020

The Atlantic meridional overturning circulation (AMOC) is pivotal for regional and global climate due to its key role in the uptake and redistribution of heat, carbon and other tracers. Establishing the causes of historical variability in the AMOC can tell us how the circulation responds to natural and anthropogenic changes at the ocean surface. However, attributing observed AMOC variability and inferring causal relationships is challenging because the circulation is influenced by multiple factors which co-vary and whose overlapping impacts can persist for years.  Here we reconstruct and unambiguously attribute variability in the AMOC at the latitudes of two observational arrays to the recent history of surface wind stress, temperature and salinity. We use a state-of-the-art technique that computes space- and time-varying sensitivity patterns of the AMOC strength with respect to multiple surface properties from a numerical ocean circulation model constrained by observations. While on inter-annual timescales, AMOC variability at 26°N is overwhelmingly dominated by a linear response to local wind stress, in contrast, AMOC variability at subpolar latitudes is generated by both wind stress and surface temperature and salinity anomalies. Our analysis allows us to obtain the first-ever reconstruction of subpolar AMOC from forcing anomalies at the ocean surface.
Oceanic density/pressure gradients and slope currents
Authors: Huthnance John M. , Inall Mark , Fraser Neil
Published: March 2020

Eastern boundary currents are some of the most energetic features of the global ocean, contributing significantly to meridional mass, heat and salt transports. We take a new look at the form of an oceanic slope current in equilibrium with oceanic density gradients. We depth-integrate the linearised x and y momentum and continuity equations, assume an equilibrium force balance in the along-slope direction (no along-slope variation in the along-slope flow), and zero cross-slope flow at a coastal boundary. We relate the bottom stress to a bottom velocity via a simple boundary friction law (the precise details are easily modified), and then derive an expression for the slope current velocity by integrating upwards using thermal wind shear. This provides an expression for the slope current as a function of depth and of cross-slope coordinate, dependent on the oceanic density field and surface and bottom stresses.

This new expression for the slope current allows for more general forms of oceanic density fields than have been treated previously. Wind stress is also now considered. The emphasis here is on understanding the simplified equilibrium force balance rather than the evolution towards that balance. There is a direct relationship between the slope current strength, friction and along-slope forcing; also between the total along-slope forcing and bottom Ekman transport, illustrating that “slippery” bottom boundaries in literature are a direct consequence of unrealistically assuming zero along-slope pressure gradient. We demonstrate the utility of the new expression by comparison with a high resolution hydrodynamic numerical model.
ITRS 2020 realization: the new situation for scale realization
Authors: Seitz Manuela , Blossfeld Mathis , Glomsda Matthias , Angermann Detlef
Published: March 2020

For the ITRS realization 2020 input data series of high quality from SLR, VLBI, GNSS and DORIS will be provided by the respective Technique Services ILRS, IVS, IGS and IDS. The observation data are currently reprocessed applying agreed state-of-the-art models. Between ITRF2014 and the ITRS realization 2020, many model changes had to be implemented for the individual techniques which nearly all have an impact on the realized scale. To mention the most important ones: (i) For SLR, estimated mean range biases will be introduced for each station and satellite. (ii) In VLBI analysis, the gravitational deformation of the telescopes will be considered for some stations. (iii) In GNSS analysis, highly-precise Galileo satellite phase center calibrations allow for the realization of the scale from Galileo observations. Calibrating GPS phase center corrections accordingly might even allow for a scale realization from the complete GNSS time series history. (iv) In case of DORIS, which was not involved in the ITRS scale realization up to now (as wasn’t GNSS), a new antenna type and changes in elevation cut-off and observation down-weighting will also have an impact on the scale.

It is a fact, that many of the model changes are not analyzed in detail with respect to their impact on the ITRF scale, and neither is the combination of model changes. The situation is even more complex since the ITRS realizations 2014, ITRF2014 on the one hand and DTRF2014 and JTRF2014 on the other hand, differ significantly with respect to the realized scale.

The poster summarizes and categorizes the various model changes and their impact on the scale. It critically discusses the new situation, which is a challenge for ITRS 2020 scale realization and will lead to significant differences between ITRF2020 and its predecessor ITRF2014.
Arctic Ocean Sea Ice Area Extent Cyclicity and Non-Stationarity
Authors: Muskett Reginald , Akasofu Syun-Ichi
Published: March 2020

Arctic sea ice is a key component of the Arctic hydrologic cycle. This cycle is connected to land and ocean temperature variations and Arctic snow cover variations, spatially and temporally. Arctic temperature variations from historical observations shows an early 20th century increase (i.e. warming), followed by a period of Arctic temperature decrease (i.e. cooling) since the 1940s, which was followed by another period of Arctic temperature increase since the 1970s that continues into the two decades of the 21st century. Evidence has been accumulating that Arctic sea ice extent can experience multi-decadal to centennial time scale variations as it is a component of the Arctic Geohydrological System. 

We investigate the multi-satellite and sensor daily values of area extent of Arctic sea ice since SMMR on Nimbus 7 (1978) to AMSR2 on GCOM-W1 (2019). From the daily time series we use the first year-cycle as a wave-pattern to compare to all subsequent years-cycles through April 2020 (in progress), and constitute a derivative time series. In this time series we find the emergence of a multi-decadal cycle, showing a relative minimum during the period of 2007 to 2014, and subsequently rising. This may be related to an 80-year cycle (hypothesis). The Earth’s weather system is principally driven the solar radiation and its variations. If the multi-decadal cycle in Arctic sea ice area extent that we interpret continues, it may be linked physically to the Wolf-Gleissberg cycle, a factor in the variations of terrestrial cosmogenic isotopes, ocean sediment layering and glacial varves, ENSO and Aurora.

Our hypothesis and results give more evidence that the multi-decadal variation of Arctic sea ice area extent is controlled by natural physical processes of the Sun-Earth system. 
Spread of denudational processes in seepage channels in a postglacial area (north-western Poland)
Authors: Mazurek Małgorzata
Published: March 2020

Erosion by emerging groundwater (i.e., seepage erosion or groundwater sapping) is the primary mechanism initiating stream channels (so-called seepage channels) and headward growth in lowland areas with a high infiltration capacity of sediment where the surface runoff is relatively rarely observed. Around groundwater outflows, as a result of the impact of denudational processes, develops an alcove, which is an amphitheatrical depression, often of steep slopes, separated from the slopes of the initial depression with clear edges. A spring-formed alcove is the upper boundary of a concentrated flow of water and sediment transport between well-marked channel margins. The influence of groundwater remains one of the least understood factors in the landform evolution in the postglacial zone of Western Pomerania (north-western Poland).

Morphometric and lithological surveying of about 80 spring-formed alcoves were studied in the southern part of the Parsęta catchment (NW Poland) made it possible to identify morphological effects of seepage erosion which are combined with surface wash and mass movement processes. The co-occurrence of various denudational processes in the headwater zones produces variations in the accumulation conditions, and as a result, a diversity of deposits. The mineral series includes erosional pavements, colluvium, and alluvial deposits. Changes in hydrodynamic conditions are favourable to organic accumulation (peats and organic-mineral muds) as well as  chemical and biochemical deposition (calcareous tufa and precipitation of Fe-oxides). Seepage channels grow when they attract enough groundwater to remove clastic material from the heads. Depending on the discharge volume of the outflow from the ten observed spring-formed alcoves (1-73 L/s), products of mechanical denudation (4-54 mg/L) are transported from the slope system to the fluvial system.

The morphometry of the spring-formed alcoves as well as deposits found in them reflect stages of their development. Changes in the development of the channel heads occur as a result of variations in the groundwater table that are due to changes in climatic conditions or land use. The determination of the place and formation of the beginning of a river channel initiated by groundwater outflows is of key importance for the modelling of the development of a stream network.
Observing the cryosphere with millimetre wave radar: The case study of Rhône Glacier
Authors: Harcourt William D. , Macfarlane David G. , Robertson Duncan A. , Rea Brice , Spagnolo Matteo
Published: March 2020

Improving our understanding of the processes governing mass loss from the cryosphere is inhibited by a lack of data at high spatial and temporal resolution. Satellite sensors can provide regional to global scale coverage of glacier processes but fail to resolve processes that occur rapidly, for example glacier calving. To observe these processes, the glaciology community must invest in new techniques that can monitor these processes adequately and fill this major research gap. Here, we will discuss the implementation of an exciting new radar system that is capable of imaging glacial terrain at a high angular resolution and during most weather conditions. The system, named AVTIS2, operates at 94 GHz (~3 mm) and offers a compromise between imaging resolution and penetration through atmospheric obscurants. AVTIS2 scans mechanically across a scene of interest in defined increments of azimuth and elevation angles and generates a 3D data cube of backscattered power. We use a point to maximum power criterion to generate point clouds and construct Digital Elevation Models (DEMs) of the terrain. Because AVTIS2 is a real aperture radar it does not require the phase stability of interferometric radars and can acquire DEMs irrespective of local environmental conditions. In this work, we have used the AVTIS2 radar to map Rhône Glacier in the Swiss Alps, representing the first ever time a millimetre wave radar has been used in this way. To improve our understanding of the performance of AVTIS2 for mapping glaciers, we have characterised the scattering properties of glacial ice at 94 GHz by calculating its Radar Cross Section (RCS). This is key to understanding the performance of AVTIS2 for mapping glaciers. This study represents the first investigation into the reflectivity of ice at millimetre wavelengths and the utility of millimetre wave radar as a surveying tool. We will report on the future application of this instrument in glaciological studies and the unique perspective it can offer.
Future Snow Water Equivalent and Snowmelt Extremes from NA-CORDEX Ensembles
Authors: Cho Eunsang , McCrary Rachel R. , Jacobs Jennifer M.
Published: March 2020

Snowpack and snowmelt driven extreme events can have large societal and economic consequences. Extreme snow can damage infrastructure and buildings. Snow meltwater is a dominant driver of severe spring flooding in the north-central and -eastern U.S. and southern Canada with impacts to the built and natural environments. However, the currently there is very limited guidance regarding the magnitude of “future” snow-driven extremes in a changing climate as needed to plan, design, and manage potentially vulnerable infrastructure and ecosystems. Regional climate models (RCMs) are commonly used to study and quantify regional climate changes, even though the ability of these models to accurately represent snow varies. In this study, trends and designs of extreme 25- and 100-year snowpack (snow water equivalent; SWE) and snowmelt events are estimated in the mid and late 21st century using the North America - Coordinated Regional Climate Downscaling Experiment (NA-CORDEX) ensemble of RCMs under Representative Con-centration Pathways 8.5 (RCP 8.5). This study aims to answer the following three research questions:

How much will snow-driven extreme events be changed in the mid and late 21st century?

Which regions have the largest differences among models?

Which RCM models are the source of these regional uncertainties?
Persistent high latitude amplification over the past 10 million years
Authors: Liu Xiaoqing , Huber Matthew , Foster Gavin L , Leckie R Mark , Zhang Yi Ge
Published: March 2020

When the Earth warms, the high latitudes often warm more than the low latitudes, a phenomenon commonly known as high latitude amplification. Although high latitude amplification has been observed by both climate data and models, the trajectory of high latitude amplification in our future changing climate is uncertain. Pacific-wide reconstructions of sea surface temperature variability from past climates are important for establishing the historical records of high latitude amplification. Multiple extratropical temperature records have been established for the past 10 million years (Myr). However, it is debated whether the warmest end member, the Western Pacific Warm Pool (WPWP), warmed during the late Miocene (~12 to 5 million years ago, Ma) and Pliocene (5 to 3 Ma). Here we present new multi-proxy, multi-site paleotemperature records from the WPWP. These results, based on lipid biomarkers and foraminiferal Mg/Ca, unequivocally show warmer temperatures in the past, and a secular cooling over the last 10 Myr. We combine these new data, along with the previously established paleotemperature records, to reveal a persistent pattern of change in the Pacific described by a high latitude amplification factor of ~1.7, which does not seem to be affected by the major climate changes over the past 10 Myr. The evolution of spatial temperature gradients in the Pacific is also evident in climate model output and instrumental observations covering the last 160 years, and thus appears to be a robust and predictable feature of the climate system. These results therefore confirm that climate models can capture the major features of past climate change, providing increased confidence in their predictions of future patterns that are likely to be similar to those reconstructed here.
Interaction of Interplanetary Shocks with the Moon
Authors: Zhou Xiaoyan , Omidi Nojan
Published: March 2020

In this presentation, we use data from THEMIS-ARTEMIS spacecraft and electromagnetic hybrid (kinetic ions, fluid electrons) simulations to describe the nature of the interaction between interplanetary shocks and the Moon. In the absence of a global magnetic field and an ionosphere at the Moon, solar wind interaction is controlled by (1) absorption of the core solar wind protons on the dayside; (2) access of supra-thermal and energetic ions in the solar wind to the lunar tail; (3) penetration and passage of the IMF through the lunar body. This results in a lunar tail populated by energetic ions and enhanced magnetic field in the central tail region. In general, ARTEMIS observations show a clear jump in the magnetic field strength associated with the passage of the interplanetary shock regardless of the position in the tail. Compared to the shock front observed in the solar wind, the magnetic field strength in the tail is stronger both upstream and downstream of the shock which is consistent with the expectations of larger field strengths in the tail. In addition, the transition from upstream to downstream magnetic field strength takes longer time as compared to the solar wind, indicating the broadening in space of the shock transition region. In contrast, plasma observations show that depending on the position of the spacecraft in the tail, a density enhancement in association with the shock front may or may not be observed. Using the observed solar wind conditions, we have used hybrid simulations to examine the interaction of interplanetary shocks with the Moon. The results indicate that by virtue of IMF passage through the lunar body, the magnetic field shock front also passes through the Moon and as such a jump in the magnetic field strength is observed throughout the lunar tail in association with the passage of the shock. As expected, the field strength in the upstream and downstream regions in the tail are larger than the corresponding values in the solar wind. In addition, the passage of the shock through the lunar tail is associated with the broadening of the shock front. The absorption of the core solar wind protons on the dayside introduces a density hole in the shock front as it passes through the Moon and the lunar tail and, as such, the shock front as a whole is disrupted. This hole is gradually filled with the ambient plasma while it travels further down the tail until eventually the shock front is fully restored a few lunar radii away from the Moon. The simulation results are found to be consistent with ARTEMIS observations. Here we also discuss the impacts of shock Mach number on the interaction. These results depict the lunar environment under transient solar wind conditions, which provide helpful information for the NASA’s plan to return humans to the Moon.
Concurrent Miocene Antarctic ice sheet growth and CO2 increase caused by disequilibrium
Authors: Stap Lennert , Knorr Gregor , Lohmann Gerrit
Published: March 2020

Geological evidence indicates considerable Antarctic ice volume variations during the early to mid-Miocene. Hitherto, ice modelling studies have mostly used equilibrium simulations to explain this variability. In these simulations, the gain in precipitation due to increased temperatures has to outweigh the loss caused by increased ice melt, to obtain simultaneous ice sheet growth and CO2 level rise. Here, conceptualising ice dynamical model results, we find that this is not a necessary condition for the transiently evolving Miocene Antarctic ice sheet. Instead, ice volume increase when CO2 levels are rising can also be explained as a consequence of disequilibrium between the transiently changing ice volume and forcing climate. This disequilibrium permits a continuation of ice sheet growth after a gradual CO2 decline. When the CO2 level is increased again, the ice sheet is still adapting to a relatively large equilibrium volume. Lowering the periodicity of the forcing leads to a larger disequilibrium, and consequently larger CO2-ice volume phase differences. Furthermore, amplified forcing variability increases ice volume variations, because the growth and decay rates depend on the forcing. It also leads to a reduced average ice volume, which is induced by the growth rate generally being smaller than the decay rate. We therefore submit that retrieval of high resolution proxy-CO2 records covering the Miocene would be very beneficial to constrain ice modelling studies.
A detailed look at the cumulus-valve mechanism and its potential implications for cloud-base cloudiness
Authors: Vogel Raphaela , Bony Sandrine
Published: March 2020

Most uncertainty in the warming response of trade-wind cumuli in climate models occurs near cloud base and is associated with model diversity in the strength of shallow convective mixing. In contrast to climate models, cloud-base cloudiness in large-eddy simulations (LES) and in observations is relatively insensitive to changes in the environment. The cumulus-valve mechanism provides a conceptual framework for understanding changes in cloud-base cloudiness in response to changes in the shallow-convective mass flux (M)—an important measure for convective mixing. The mechanism assumes that M keeps the mixed-layer top close to the lifting condensation level, which could explain a larger cloud-base cloudiness with larger M if the increase in M was mostly due to an increasing area fraction of cumuli. Here we use real-case LES over the tropical Atlantic to understand if cloud-base cloudiness increases with increasing M.

We find that M explains a lot of the variations in cloud-base cloudiness (correlation coefficient R=0.86), but the maximum relative humidity at the mixed-layer top (RHmax) needs to be considered additionally to explain the nighttime behavior of cloud-base cloudiness (R=0.95). The coupling of M and RHmax through adjustments in the sub-cloud layer depth is crucial for regulating cloud-base cloudiness. Inability of GCMs to adjust the sub-cloud layer depth in response to a change in M may likely contribute to their overestimated trade-cumulus cloud feedback. The simulated relationships will be compared to measurements from the EUREC4A field campaign.
Estimates of carbon budgets consistent with global warming of 1.5-2°C from ensembles of simulations with the MIT Earth System Model of intermediate complexity: the role of non-CO2 GHGs and SO2 emissions
Authors: Sokolov Andrei , Morris Jennifer , Paltsev Sergey
Published: March 2020

We present estimates of carbon budgets for different levels of surface air temperature (SAT) increase from multiple 400-member ensembles of simulations with the MIT Earth System Model of intermediate complexity (MESM). Ensembles were carried out using distributions of climate parameters affecting climate system response to external forcing obtained by comparison of historical simulations with available observations.

First, to evaluate MESM performance, we ran two ensembles: one with MESM forced by 1% per year increase in CO2 concentrations (and with non-CO2 greenhouse gases (GHG) at pre-industrial level) and the other with GHG concentrations from the RCP 8.5 scenario. Distributions of climate characteristics describing model response to increasing CO2 concentrations (e.g. TRC and TCRE) as well as values of carbon budgets of exceeding different SAT levels agree well with published estimates.

Then we ran a number of ensembles with MESM driven by emissions produced by the MIT Economic Projection and Policy Analysis (EPPA) Model. Our results show that under stringent mitigation policy concerning non-CO2 GHGs, the SAT increase can be kept below 2°C relative to pre-industrial with 66% probability through the end of the 21st century without negative CO2 emissions. The SAT increase can also be restricted to 1.5°C with 50% probability if such policy is implemented immediately. If GHG emissions follow the path implied by the Paris Agreement pledges through 2030, then it would require either an unrealistically sharp drop in non-CO2 GHG emissions or negative CO2 emissions to stay below 1.5°C. Keeping the temperature increase below a chosen value (1.5°C or 2°C) beyond 2100 will most likely require negative CO2 emissions in part due to difficulties in restricting agricultural methane emissions.

Further analysis shows that temperature change during the 21st century is also significantly affected by the assumption concerning decrease of SO2 emissions from energy-intensive industries. Implementation of technologies resulting in reduction of those emissions will decrease the probability of SAT staying below a given limit by about 7-12%. This will affect the time when negative CO2 emissions will become necessary to prevent temperature increase.
River temperature dynamics downstream of a shallow reservoir: process-based modelling to evaluate thermal mitigation strategies
Authors: Moore Dan , West Dave
Published: March 2020

This study focused on Alouette River, located in south coastal British Columbia. During summer, water is released from shallow reservoir at a near-constant rate from an outlet about 6-10 m below the water  surface. Outlet temperatures in summer 2013 were initially cool hypolimnetic water, followed by alternating cool and warm water associated with an internal seiche, and finally dominated by warm epilimnetic water during the period of highest water temperature. An energy-balance model was used to evaluate potential strategies to ameliorate thermal habitat conditions for Pacific salmon downstream of the dam. Restoration of deforested banks that represented 4% of the reach length reduced daily maximum temperatures by only about 0.5 °C , while releasing more flow exacerbated temperatures during the warmest week of the year. The only effective strategy for thermal amelioration would be to release water from deeper in the reservoir.
Overlooked volatile production from Arctic permafrost triggered by global warming
Authors: Li Haiyan , Mäki Mari , Kohl Lukas , Väliranta Minna , Bäck Jaana , Bianchi Federico
Published: March 2020

Permafrost thaw, as a consequence of climate warming, liberates large quantities of frozen organic carbon in the Arctic regions. The response of gaseous carbon release upon permafrost thaw might play a crucial role in the future evolution of atmosphere-land fluxes of biogenic gases such as volatile organic compounds (VOCs), a group of reactive gases and the dominant modulator of tropospheric oxidation capacities. Here, we examine the response of volatile release from Finnish Lapland permafrost soils to temperature increase in a series of laboratory incubation experiments. The experiments show that when the temperature rises from 0 °C to 15 °C, various VOC species are significantly emitted from the gradually thawing soils. The VOC fluxes from thawing permafrost are on average four times as high as those from active layer. Acetic acid and acetone dominate the total volatile emissions from both permafrost and active layer, with significant amounts of aromatics and terpenes detected as well. The emission rate and the composition of volatile release from thawing soils are highly responsive to temperature variations. As temperature increases, more less volatile compounds are released, i.e., sesquiterpenes and diterpenes. Collectively, these results demonstrate the highly overlooked volatile production from thawing permafrost, which will create a stronger permafrost carbon-climate feedback.
Global atmospheric carbon monoxide budget 2000–2017 inferred from multi-species atmospheric inversions
Authors: Zheng Bo , Chevallier Frederic , Yin Yi , Ciais Philippe , Fortems-Cheiney Audrey , Deeter Merritt , Parker Robert , Wang Yilong , Worden Helen , Zhao Yuanhong
Published: March 2020

Atmospheric carbon monoxide (CO) has been decreasing since 2000 as observed by both satellite- and ground-based instruments, but global bottom-up emission inventories surprisingly estimate increasing anthropogenic CO emissions concurrently. In this study, we use a multi-species atmospheric Bayesian inversion approach to attribute satellite-observed atmospheric CO variations to its sources and sinks in order to achieve full closure of the global CO budget during 2000–2017. Our observation constraints include satellite retrievals of the total column mole fraction of CO from MOPITT, formaldehyde (HCHO) from OMI, and methane (CH4) from GOSAT, which are all major components of the atmospheric CO cycle. Three inversions are performed to use the observation data to the maximum extent possible as they become available and assess the consistency of inversion results to the assimilation of more trace gas species. We identify a declining trend in the global CO budget since 2000 (three inversions are broadly consistent), driven by reduced anthropogenic emissions in the U.S. and Europe (both likely from the transport sector), and in China (likely from industry and residential sectors), as well as by reduced biomass burning emissions globally, especially in Equatorial Africa (associated with reduced burned areas). We show that the trends and drivers of the inversion-based CO budget are not affected by the inter-annual variation assumed for prior CO fluxes. All three inversions estimate that surface CO emissions contradict the global bottom-up inventories in the world’s top two emitters for the sign of anthropogenic emission trends in China (e.g., here −0.8 ± 0.5% yr−1 since 2000 while the prior gives 1.3 ± 0.4% yr−1) and for the rate of anthropogenic emission increase in South Asia (e.g., here 1.0 ± 0.6% yr−1 since 2000 smaller than 3.5 ± 0.4% yr−1 in the prior inventory). The comparison of the three inversions with different observation constraints further suggests that the most complete constrained inversion that assimilates MOPITT CO, OMI HCHO, and GOSAT CH4 has a good representation of the global CO budget, therefore matches best with independent observations, while the inversion only assimilating MOPITT CO tends to underestimate both the decrease in anthropogenic CO emissions and the increase in the CO chemical production.
Designing a multi-objective framework for forecast-based action of extreme rains in Peru
Authors: Lala Jonathan , Bazo Juan , Block Paul
Published: March 2020

The last few years have seen a major innovation within disaster management and financing through the emergence of standardized forecast-based action protocols. Given sufficient forecasting skill and lead time, financial resources can be shifted from disaster response to disaster preparedness, potentially saving both lives and property. Short-term (hours to days) early warning systems are common worldwide; however, longer-term (months to seasons) early actions are still relatively under-studied. Seeking to address both, the Peruvian Red Cross has developed an Early Action Protocol (EAP) for El Niño-related extreme precipitation and floods. The EAP has well-defined risk metrics, forecast triggers, and early actions ranging from 5 days to 3 months before a forecasted disaster. Changes in climate regimes, forecast technology, or institutional and financial constraints, however, may significantly alter expected impacts of these early actions. A robust sensitivity analysis of situational and technological constraints is thus conducted to identify benefits and tradeoffs of various actions given various future scenarios, ensuring an adaptive and effective protocol that can be used for a wide range of changing circumstances.
Anomalous Topography, Bathymetry, Crust and Mantle in the North Atlantic Region
Authors: Thybo Hans , Artemieva Irina
Published: March 2020

The whole North Atlantic region has highly anomalous topography and bathymetry. Observations show evidence for substantial topographic change with rapid onshore uplift close to the Atlantic coast and simultaneous subsidence of basins on the continental shelves, most likely throughout the Mesozoic.

We present a review of geophysical data and interpretation of the whole region with emphasis on data relevant for assessing topographic change. We review the available data on topography, bathymetry, density, seismic velocity, and heat flow and present interpretations of the structure and composition of the crust and lithospheric mantle.

We find that most of the northern North Atlantic Ocean has anomalously shallow bathymetry although it follows the “normal” square-root-of-age dependence, which however is elevated by up-to 2 km. The heat flow variation follows the square-root-of-age dependence, although heat flow is anomalously low on the spreading ridges around and on Iceland. In apparent contrast, exceptionally low seismic velocities are observed along the spreading ridges around and below Iceland. Near-zero free-air gravity anomalies indicate that the oceanic areas are mainly in isostatic equilibrium, whereas anomalously low Bouguer anomalies indicate low density in the uppermost mantle. Anomalously thick oceanic crust is observed along the Greenland-Iceland-Faro Ridge and extending into the Davis Strait. We propose that the anomalous bathymetry is caused by compositional variation in the lithosphere, which indicates that the lithosphere in the ocean may include remnants of continental lithosphere.

The onshore circum-Atlantic areas show rapid uplift close to the coast with rates up-to 3 cm/yr. This is surprisingly associated with strong positive free-air gravity anomalies which predicts isostatic subsidence. However, negative free-air gravity anomalies in onshore Canada and Bothnian Bay explain recent uplift in the shields as isostatic rebound after glaciation. Archaean lithosphere is everywhere thick in both Greenland and Fennoscandia, Proterozoic areas have thinner lithosphere and Palaeozoic-Mesozoic areas have very thin lithosphere. It is enigmatic that the presumed Archaean-Proterozoic Barents Sea region is submerged and includes deep sedimentary basins.
Towards scale independent hydrological forecasting in regulated semi-arid regions
Authors: Shrestha Pallav Kumar , Lorenz Christof , Najafi Husain , Thober Stephan , Rakovec Oldrich , Samaniego Luis
Published: March 2020

Semi-arid regions are characterized by low annual precipitation that exhibit large seasonal fluctuations. While semi-arid regions cover 3.6% of the globe, 13% of world’s documented reservoirs (GRanD database) are within 100 km of semi-arid regions to fulfill water demand year-round. Reservoirs are known to increase evaporation and significantly change hydrologic regime downstream. Accurate representation of reservoirs and scale independent modeling is indispensable for reliable hydrologic forecasting systems in semi-arid regions. To address this, the mesoscale hydrological model (mHM, git.ufz.de/mhm) is augmented with a new lake/reservoir module (multiscale lake module, mLM). The objective is to measure the performance of a scalable seasonal forecasting model chain with and without reservoirs.

The experimental setup constitutes the SaWaM (http://grow-sawam.org/) project study regions encompassing seven semi-arid basins and 15 reservoirs of high significance across three continents (Sao Francisco, Jaguaribe, Piranhas in Brazil, Blue Nile, Atbara in Sudan, Karun in Iran, Chira-Catamayo in Ecuador).The calibration of mHM parameters and its initial conditions for forecsating are obtained using the spatially disaggregated ERA5 (ERA-SD, ≈ 10 km, starting 1981) climate reanalysis data. The calibrated model is forced with an ensemble of 25 realisations of ECMWF-SEAS5 seasonal hindcasts which are bias corrected and spatially disaggregated (BCSD, ≈10 km) using ERA-SD. The 2010–2016 hindcasting experiment generates hydrological forecasts with lead time of upto six months. The performance of the model chain BCSD-mHM-mLM and BCSD-mHM are evaluated using the Brier Skill Score.

Preliminary results show that incorporating reservoirs in the model improves the performance of mHM (average NSE improvement ≈ +0.1 for the period 1990–2010) and the overarching forecasting model chain. Sub-grid level lake delineation and in-/outflow calculations of mLM result in scalable reservoir states and fluxes and thus overall scalable basin hydrology. Seamless forecasts for soil moisture, streamflow, reservoir inflow and reservoir water level are achieved across scales (≈10 km to ≈1 km) showing skills to up to two months lead time. This study is the first step towards an operational hydrological seasonal forecasting system which has potential to significantly improve water management, specially in semi-arid regions.