bluuebunny/crossref_metadata_2025 huggingface dataset:
LOFAR4SW – Space Weather Science and Operations with LOFAR
Authors: Rothkaehl Hanna , Matyjasiak Barbara , Baldovin Carla , Bisi Mario , Barnes David , Carley Eoin , Carozzi Tobia , Fallows Richard A. , Gallagher Peter T. , Mevius Maaijke , Robertson Stuart C. , Ruiter Mark , Verbiest Joris , Vermeulen Renne , Vilmer Nicole
Published: March 2021

Space Weather (SW) research is a very important topic from the scientific, operational and civic society point of view. Knowledge of interactions in the Sun-Earth system, the physics behind observed SW phenomena, and its direct impact on modern technologies were and will be key areas of interest.  The LOFAR for Space Weather (LOFAR4SW) project aim is to prepare a novel tool which can bring new capabilities into this domain. The project is realised in the frame of a Horizon 2020 INFRADEV call.  The base for the project is the Low Frequency Array (LOFAR) - the worlds largest low frequency radio telescope, with a dense core near Exloo in The Netherlands and many stations distributed both in the Netherlands and Europe wide with baselines up to 2000 km.  The final design of LOFAR4SW will provide a full conceptual and technical description of the LOFAR upgrade, to enable simultaneous operation as a radio telescope for astronomical research as well as an infrastructure working for Space Weather studies.  In this work we present the current status of the project, including examples of the capabilities of LOFAR4SW and the project timeline as we plan for the Critical Design Review later in 2021.
Idealized Numerical Modelling of Internal Waves Through Density Staircases
Authors: Schee Mikhail , Grisouard Nicolas
Published: March 2021

The Arctic Ocean contains a warm layer originating from the Atlantic Ocean below the pycnocline which has a thermohaline staircase structure that inhibits vertical mixing. If this heat were to rise to the surface, the rate of sea ice loss would increase dramatically. Wind stress and ice floes generate internal waves which can cause vertical mixing. As the ice cover in the Arctic continues to decline, it will be important to predict how these changing internal waves propagate through such stratification profiles. Here, we investigate how density staircases enhance or limit downward near-inertial wave propagation. We use direct numerical simulations to solve the Boussinesq equations of motion using spectral methods. We simulate the propagation of internal waves through a vertically stratified fluid which includes one or more steps (i.e., mixed layers). We find that we reproduce the results of laboratory experiments showing transmission and reflection of internal waves from one or two mixed layers. We then extend our parameter regime to simulate the propagation of internal waves through a more realistic stratification profile tending toward that of the Arctic pycnocline.
Drought severity and impact relationships: a multidimensional case study of recent droughts in the south-central semiarid prairies of the United States
Authors: Ochsner Tyson , Hagerman Amy , Krueger Erik , Lambert Dayton , Lambert Lixia , Weckler Paul
Published: March 2021

As earth’s climate is changing, there is evidence for abrupt shifts to hotter and drier climate conditions in some locations. These changes may be particularly harmful to coupled human and natural systems in semiarid regions because they exist on marginal water availability and can exhibit strong coupling between land surface and atmospheric conditions which can make droughts more persistent. In the south-central semiarid prairie ecoregion of the US, there has been a significant warming trend over the past 120 years. This ecoregion has also experienced some of the most severe drought conditions in the US during recent decades, particularly in southwest Oklahoma and adjacent portions of Texas. These drought conditions have complex, multi-dimensional impacts on coupled human and natural systems, which are often not adequately quantified or understood. Furthermore, even the severity of drought is often poorly or inconsistently measured, in part due to an overreliance on meteorological or remotely-sensed data as opposed to measurements of stored soil water, surface water, and groundwater. The objectives of this research are to 1) accurately measure the severity and multi-dimensional impacts of recent droughts in the south-central semi-arid prairies of the US and 2) clarify the relationships between drought severity and impacts. We utilize a case-study approach focused on southwest Oklahoma where in situ observational datasets allows for quantification of stored soil water, surface water, and groundwater. Drought impact types were recorded by the US Drought Impact Reporter and quantified using appropriate supplemental datasets and models.
Numerical Modelling of PFAS Transport and Groundwater Flow in the Vreta-Bålsta Esker
Authors: Pettersson Kevin , Mossadeghi Björklund Mona , Tatomir Alexandru
Published: March 2021

Per- and poly-fluoroalkyl substances represent a new group of anthropogenic environmental pollutants.
For instance, one of the most commonly used firefighting methods today especially within airports is the use of aqueous film forming foams (AFFFs). AFFFs contain PFAS, which give the foam the ability to easily spread over a large area. Some PFAS species have demonstrated adverse health effects already at low concentrations such as liver damage and reproductive harm. The strong chemical bonding between carbon and fluorine also results in PFAS to being resistant to thermal- and chemical degradation. Thus, they degrade very slowly in nature. In Sweden, the limit for PFAS in drinking water is 90 ng L-1. This means that water needs to be treated for PFAS in the water treatment plants in cases of contamination. where water with a concentration over the limit would be used.

Bålsta is a growing city, with is proximity to Sweden’s largest city and capital Stockholm and Sweden’s fourth largest city Uppsala. Both located about 50 kilometres away, makes Bålsta an attractive city for people to live in and commute to their job in in the larger cities. With this growth new drinking water sources are required in the future Vreta-Bålsta area is an esker with good aquifer properties. However, it is an industrial area, and in the area lies a landfill which has been used as a training ground for firefighting exercises. During these exercises AFFFs have been used and the area is polluted with an unknown amount of PFAS. As the area is planned to be used for artificial infiltration and extraction of groundwater, it needs to be tested for potential PFAS contamination. 

This study aims to investigate by means of numerical modelling different scenarios of PFAS transport from the contaminated. Using data about the topography, soil depth of the area together, hydraulic conductivity, porosity, precipitation and runoff a model of the area was constructed within GMS. The package MODFLOW was used to simulate the groundwater flow. Using the groundwater flow solution, with the PFAS transport is simulated with the package MT3DMS to produce a result of a possible spread of PFAS within the studied area to see which parts that could be contaminated or become contaminated in the future.

The results given from the model was that PFAS from the landfill would transport northeast bound. With the southern part of the area would stay clear from all contamination from the landfill.
Impact of subjective modeling decisions on hydrological modeling
Authors: Ann Alexander Ashlin , Nagesh Kumar Dasika
Published: March 2021

Modelers often make different decisions in building hydrologic models based on their experience and modeling philosophy. Consequently, a wide range of models is developed, which differ in many aspects of conceptualization and implementation. This diversity of models has been useful to explore a myriad of scientiﬁc and applied questions, but it has also led to great confusion on choosing the appropriate model configurations in compliance with the dominant processes in the study area. Also, modeling decisions during model configuration introduce subjectivity from the modeler. To provide guidance to select the best-suited model configuration for a catchment it is required to examine and evaluate the different model representations of hydrological processes and their impact on model simulations. In this study, we show that modeling decisions during the model configuration, beyond the model choice, also impact the model results. The framework, Structure for Unifying Multiple Modeling Alternatives (SUMMA; Clark et al., 2015a, b) is used in this study to disentangle the model components which helps to have a controlled and systematic evaluation of multiple models representations. The area chosen for the study is the Malaprabha catchment in the Karnataka state of India. The impact of the choice of parameterizations and parameter values on the model simulations are shown. To improve upon the traditional model evaluation methods, hydrological signatures are made use to have a hydrologically meaningful evaluation of model simulations. This study helped to identify the suitable model configuration for the Malaprabha catchment. Multiple working hypotheses during model configuration which is possible with the help of such flexible framework like SUMMA can provide insights on the impact of subjective modeling decisions.
Use of Coupled Human-Water Model for Evaluating the Impacts of the WEF Nexus on the Energy Potential of Crop Residues in Pakistan
Authors: Anderson Emma C. , Alizadeh Mohammad Reza , Adamowski Jan , Malard Julien , Inam Ahzar
Published: March 2021

Failure to consider interactions in the Water-Energy-Food (WEF) nexus can lead to unintended outcomes. In Pakistan, research has suggested that agricultural residues are a viable alternative renewable energy source to address the persistent energy shortfalls and reliance on imported diesel and heavy fuel oil. However, these studies assess the viability from a broad scale and do not adequately account for nexus interactions. For example, a quarter of irrigated land in Pakistan is salt-affected, adversely impacting crop (and residue) yields. Failure to consider climate change impacts on water availability and agricultural productivity also increases uncertainty. Finally, the effects of socioeconomic feedbacks and water management policies are not understood.  To address these challenges, this research applies a coupled physically-based (SAYSMOD), and group (stakeholder) built system dynamics model (P-GBSDM) of the agricultural system in the lower Rechna Doab, Pakistan, to assess the sub-regional viability of residue-based energy production in salt-affected and non-salt-affected lands. The modelled area (750 km2) is within a district found highly suitable for residue-based energy. The P-GBSDM, developed by Inam et al. (2017), captures the socioeconomic and spatially-distributed environmental feedbacks related to agricultural productivity, hydrological parameters and farmer's livelihood indicators. The P-GBSDM is amended for this research to estimate crop residue yield and potential energy production and feedbacks related to farmer income (from selling residues) and crop residue removal. The model is simulated for the years 2000-2030 under different climate change scenarios and stakeholder-suggested salinity management practices. Crop (and residue) yield, equivalent collection radius, farmer income, and soil salinity are used to evaluate the residue-based energy production in this area. Results are compared to literature values. Preliminary results suggest that estimates that do not consider the WEF nexus overestimate residue-based energy generation's potential. 
Meter-scale retrieval of industrial methane emissions using GHGSat’s satellite constellation
Authors: Strupler Mathias , Deglint Hanford , Gains David , Jervis Dylan , MacLean Jean-Philippe , McKeever Jason , Ramier Antoine , Shaw Warren , Tarrant Ewan , Varon Daniel , Young David
Published: March 2021

Actionable feedback to industrial operators is extremely valuable to help them reduce their greenhouse gas emissions. With this goal in mind, GHGSat launched in 2016 a demonstration satellite called GHGSat-D (“Claire”). It was the first satellite built specifically to detect and quantify methane emissions from individual sites.

With the launches of GHGSat-C1 (“Iris”) in September 2020 and of GHGSat-C2 (“Hugo”) planned in January 2021, GHGSat will have three methane-sensing meter-scale resolution satellites in orbit. In addition to those satellites, GHGSat has also deployed an aircraft version of the instrument to survey specific areas with even lower detection threshold thanks to its higher spatial resolution.

This presentation will show the improvements done since GHGSat-D that allow our instruments to reach column precision of  1% of background. With this enhanced sensitivity, sources such as oil and gas facilities, mines, landfills and dams can be measured from space. Emission quantification of various sources will be presented and will demonstrate that GHGSat-C1 is approaching its target detection threshold of 100 kg/h. We will also illustrate the complementarity of GHGSat’s instruments with Sentinel-5P, the first ones able to detect individual sources with low emission rates, the second able to measure daily and with high accuracy global methane concentrations. We will also discuss the data calibration and validation plan of our instruments. Finally, an update on the future expansion of GHGSat’s constellation will be given.
Line of Sight Refractivity from a standalone GNSS Receiver and Collocated Radiosonde data
Authors: Peng Feng , Fei Li , Jianguo Yan , Barriot Jean-Pierre
Published: March 2021

With its high temporal resolution, unique mesoscale sampling scale and full weather capability, GNSS is now contributing as an important tool for monitoring the global atmospheric environment. The GNSS tropospheric zenith delay and the corresponding precipitable water vapor data (PW) are already widely applied in many weather models. High precision GNSS processing also estimates tropospheric delay gradients, which contain azimuthal isotropic information about the state of the atmosphere. However, the application of GNSS tropospheric delay gradients is not yet fully explored because of several obstacles. First, it suffers from the satellite constellations geometry and multipaths, and the gradients estimations are noisier than the zenithal delays. Second, the delay gradients were first developed for positioning purposes. GNSS tomography takes advantage of the delay gradients but requires a dense GNSS network. Here we introduce a new method to obtain the line-of-sight wet refractivity from a stand-alone GNSS receiver. We assume that the wet refractivity is mainly governed by a scale height (exponential law) and that the departures from the decaying exponential can be mapped as a set of low degree 3D Zernike functions and Chebyshev polynomials. We show up examples of inversion with data acquired at the IGS station in Tahiti, French Polynesia. We will also discuss the possibility of joint inversions with other measurements, using radiosonde data as an example.
Establishment of groundwater baseline using end-member mixing analysis in the groundwater flow system approach
Authors: Rodriguez Padilla Susana , Olea Olea Selene , Escolero Fuentes Oscar
Published: March 2021

The aim of this research is to establish the groundwater baseline in a sub-basin located in the southwest of Mexico City, an area affected by anthropogenic activities.

The methodology consists of groundwater sampling in 40 sites to measure major ions and physicochemical parameters as temperature, pH, Eh, and total dissolved solids. The end-member mixing analysis was applied using the groundwater flow system approach. The groundwater baseline was established using flow components that were defined.

The main results are: to found four groundwater flow components: 1) local, 2) intermediate, 3) cold regional, and 4) hot regional; to established a groundwater baselines; to relate the anomalous concentrations of nitrate and sulfate due to anthropogenic activities in the area; to associate the fertilizer use, wastewater, and the canal leaching black waters as the principal sources of these concentrations.

The conclusions show the importance to use the groundwater flow system approach to differentiate natural processes as hydrochemical evolution due to water-rock interaction of the anthropogenic influence. In the context where groundwater is extracted without knowing its baseline and the anthropological implications, the groundwater flow system approach to permit generated best management and administration strategies.
The Pescadero Basin Complex, southern Gulf of California: structure, tectono-stratigraphic evolution and magmatism
Authors: Spelz Ronald M. , Ramírez-Zerpa Néstor , Contreras Juan , Yarbuh Ismael , González-Fernández Antonio , Caress David , Clague David , Zierenberg Robert , Paduan Jennifer B. , Negrete-Aranda Raquel , Fletcher John M. , Neumann Florian , Cousens Brian
Published: March 2021

The Pescadero Basin Complex (PBC) in the southern Gulf of California comprises three distinctive stretched rhomboid pull-apart basins separated by several short transforms. Multibeam and Autonomous underwater vehicle (AUV) bathymetry data collected at 40-m and 1-m resolution, respectively, combined with the processing and interpretation of three 2-D high-resolution multichannel seismic reflection profiles, were used to characterize the architecture of the entire PBC, as well as the internal structure of the northern Pescadero basin. Detailed mapping and cross-sectional kinematic modeling based on multichannel seismic images of the northern Pescadero basin reveals a highly evolved pull-part geometry, characterized by a well-defined ~1.8 km wide axial graben stretching ~32 km in an NNE-SSW direction. Both finite and incremental strain analyses carried out in this study point out that the PBC developed under sustained transtensional deformation, where the relative motion of the crustal blocks is oblique and divergent to the transforms or principal displacement zones (PDZ's), and subsidence is likely being accommodated by one of more décollement layers located at the bottom of a broad negative flower structure. We also present new geochemical data of lava flows with a N-MORB composition outcropping on the NE segment of the northern Pescadero axial graben, and lava-flow samples of E-MORB composition from an uplifted sediment hill on the western margin of the southern Pescadero basin. MORB samples from the PBC represent the northernmost surface flows known in the Gulf of California, highlighting that the PBC has evolved beyond being a pull-apart complex to having initiated seafloor spreading with new oceanic crust formation in response to the opening of the Gulf of California.
Improving Observations of Aggregate Snow Cover Properties on MOSAiC by Integrating Repeat Terrestrial Laser Scanning and In-Situ Data
Authors: Clemens-Sewall David , Macfarlane Amy , Polashenski Chris , Perovich Don , Jaggi Matthias , Raphael Ian , Schneebeli Martin , Wagner David
Published: March 2021

The spatial heterogeneity of the snow cover on Arctic sea ice impacts the coupled Ice-Ocean-Atmosphere system. This spatial heterogeneity manifests in both the spatial distribution of snow thickness and the material properties of that snow (e.g. density, specific surface area [SSA], thermal conductivity, salinity, etc). This presents a challenge for observing the aggregate snow cover properties. Most material properties can only be measured in-situ and it is logistically difficult to measure material properties at a large number of sites. Here, we address this challenge by integrating repeat Terrestrial Laser Scan (TLS) data and in-situ observations of snow properties on an area several hundred meters across. We used TLS to map the topography of this area at cm-scale vertical resolution on approximately a biweekly basis throughout the winter during MOSAiC. By comparing successive scans, we map the spatial extent of snow layers as they build up the snow cover. Concurrently, we made in-situ penetration resistance force measurements using a SnowMicroPen (SMP) to quantify the snow properties at sites within the measurement area. These weekly point measurements, with 3mm vertical resolution, provide details of the grain type, snow density and SSA stratigraphy. Combining the TLS and SMP observations enables us to extrapolate the layer-wise properties of the snow cover throughout the measurement area. We examine how consistent snow properties are within layers and use this information to quantify aggregate snow cover properties for the entire region. For example, by integrating SMP-derived density with TLS-derived layers we estimate aggregate change in snow mass for this region for selected periods of the winter.
Validation of the greenhouse gases observing satellite GOSAT using an ensemble of COCCON spectrometers
Authors: Frey Matthias M. , Dubravica Darko , Morino Isamu , Ohyama Hirofumi , Hori Akihiro , Blumenstock Thomas , Hase Frank , Gross Jochen , Tu Qiansi , Jacobs Nicole , Simpson William R. , Balis Dimitrios , Mermigkas Marios , Franklin Jonathan E. , Gottlieb Elaine
Published: March 2021

Greenhouse gases (GHGs) play a crucial role with respect to global warming. Therefore, precise and accurate observations of anthropogenic GHGs, especially carbon dioxide (CO2) and methane (CH4), are of utmost importance for the estimation of their emission strengths, flux changes and long-term monitoring. Satellite observations are well suited for this task as they provide global coverage. However, like all measurements these need to be validated. The Total Carbon Column Observing Network (TCCON) performs ground-based observations of GHGs with reference precision using high-resolution Fourier Transform infrared (FTIR) spectrometers. TCCON data are of high accuracy as TCCON uses species dependent scaling factors derived from in-situ reference measurements to be calibrated to the World Meteorological Organization (WMO) reference scale. For several satellites measuring GHGs TCCON data are the main validation source.

To further improve the global coverage of FTIR spectrometers and complement TCCON especially in remote areas, the COllaborative Carbon Column Observing Network (COCCON) was established. Until now the focus of COCCON was on the quality control of EM27/SUN spectrometers and dedicated campaigns to estimate emission strengths of CO2 and CH4 from local and regional sources, e.g. from cities, fracking areas or mining sites.

Here we present a global validation of the Greenhouse Gases Observation Satellite GOSAT using multiple spectrometers from the COCCON network. The COCCON instruments are stationed in Finland, Germany, Greece, Japan, Namibia, Sweden and the USA. The sites span a range of different atmospheric and observing conditions, from subtropical to subpolar regions, including boreal forests and deserts, low and high albedo surfaces, polluted and clean areas. Overall, we find a good agreement between GOSAT and COCCON measurements.
Comparing Seasonal Diffences of Black Carbon in Tibetan Plateau transported from South Asia using WRF-Chem
Authors: Wu Shuoqiu , Ma Xiaoyan
Published: March 2021

The melting of glaciers and snow on the Qinghai-Tibet Plateau, known as the Earth’s “Third Pole” and “World Water Tower”, is source of fresh water for hundreds of millions of people in South Asia, Southeast Asia, and East Asia, but it is now suffering from an unprecedented crisis. The black carbon deposited on the surface of the glacier will reduce the snow albedo and absorb more solar radiation, leading to accelerated melting of ice and snow.Previous studies have shown that black carbon from South Asia is one of the main sources of the Qinghai-Tibet Plateau, and the transportation of black carbon to the Qinghai-Tibet Plateau presents obviously seasonal differences.However, the transport of black carbon from South Asia to the Qinghai-Tibet Plateau in different seasons shows a completely opposite trend to wind field conditions.This study uses the WRF-Chem model to study the transmission mechanism of South Asian black carbon to the Tibetan Plateau in April (pre-monsoon), July (summer monsoon) and December (winter monsoon).MIX emission inventory and Peking University's global black carbon emission inventory (PKU-BC) were involved to analyze the seasonal distribution of black carbon concentration, dry and wet deposition in the Qinghai-Tibet Plateau and South Asia, and the distribution of BC concentration and wind field at different altitudes.Combined with the vertical distribution of BC concentration across the Himalayas, the transport mechanism of black carbon in South Asia to Qinghai-Tibet Plateau in different seasons is studied.In the selected three months, December had the highest surface black carbon concentration in South Asia and the Qinghai-Tibet Plateau, while July had the lowest black carbon concentration; Mainly because of the large amount of wet deposition of black carbon brought about by the heavy precipitation in South Asia in July;According to the vertical distribution of black carbon,black carbon can climb up the hillside and eventually reach the southern slope of the Qinghai-Tibet Plateau in April. In July, black carbon is mainly distributed below 3km. In December, black carbon can be uplifted to 4-5km, and finally transported into Qinghai-Tibet Plateau.
Evaluation of a new Japanese reanalysis (JRA-3Q) in a pre-satellite era
Authors: Naoe Hiroaki , Kobayashi Shinya , Kosaka Yuki , Chiba Jotaro , Tokuhiro Takayuki , Harada Yayoi
Published: March 2021

This study evaluates the latest Japanese Reanalysis for Three Quarters of a Century (JRA-3Q) conducted by the Japan Meteorological Agency (JMA), focusing on a semi-period of pre-satellite era (1960s and 1970s). The reanalysis is the third Japanese global atmospheric reanalysis covering the period from late 1940s onward, which is produced with the JMA's operational system as of December 2018. The atmospheric model has a TL479 horizontal resolution and 100 vertical layers up to 0.01 hPa, and the core component of the JRA-3Q data assimilation system is the 6-hourly 4D-Var of the atmospheric state with a T319-resolution inner model. Because there are only few global-covered observational datasets during the pre-satellite era, evaluation of the JRA-3Q is mainly to conduct an intercomparison of other reanalysis datasets such as representation Japanese 55-year Reanalysis (JRA-55), a JRA-55's subset of atmospheric reanalysis assimilating conventional observations only (JRA-55C), and version 3 of the Twentieth Century Reanalysis (20CRv3), and also an intercomparison of JRA-3Q between the pre-satellite and satellite eras. Emphasis of this evaluation during the non-satellite era is placed on the representation of tropical circulation, the consistency in time of the reanalysed fields, detection of tropical cyclones, and the quality of the stratospheric water vapor and ozone. For example, the surface circulation over the tropical Africa is improved by means of reducing spurious anticyclonic circulation anomalies that were found in JRA-55. Although the atmospheric model can produce self-generated quasi-biennial oscillation (QBO) by introducing non-orographic gravity wave drag, the evaluation reveals that JRA-3Q has a shorter period of around one year in the middle stratosphere and diminished QBO amplitude in the lower stratosphere, indicating that representation of the QBO in JRA-3Q is not as good as that in JRA-55.
Comparative Analysis of the Measured and Modeled Equatorial Thermospheric Wind Climatology 
Authors: Khadka Sovit , Gerrard Andrew , Fedrizzi Mariangel , Dandenault Patrick , Meriwether John
Published: March 2021

The thermospheric winds play an important role in the vertical and horizontal couplings of the upper atmosphere by modulating neutral and plasma dynamics. A large variety of observation techniques and numerical as well as empirical models have been developed to understand the behavior of thermospheric winds. The Fabry-Perot interferometer (FPI) is a widely used ground- and satellite-based optical instrument for the thermospheric winds observations in the upper atmosphere. Due to solar contamination of the fainter airglow emission during the daytime, most of the ground-based interferometric wind measurements are limited to the nighttime period only. Despite these constraints, the Second‐generation, Optimized, Fabry‐Perot Doppler Imager (SOFDI) is designed for both daytime and nighttime measurements of thermospheric winds from OI 630‐nm emission and is currently operating at the Huancayo, Peru, near the geomagnetic equator. In this study, we present a comparative analysis of the observed SOFDI wind climatological data and several other modeled results including, but not limited to, Horizontal Wind Model 2014 (HWM-14), Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPe) model with and without implementing Prompt Penetration Electric Field (PPEF), Whole Atmosphere Model (WAM), SAMI3 model, and Magnetic mEridional NeuTrAl Thermospheric (MENTAT) model. We examine the relative performances of these models in the context of the direct-measured thermospheric winds. The day and nighttime modeled winds show an excellent agreement with the SOFDI wind data at the equatorial latitude, except for the daytime zonal winds. Further, this analysis gives a comprehensive picture of how well the measured winds provided by the SOFDI instrument and various models represent the features of the equatorial thermosphere. We also investigate and give an overview of the sources, drivers, effects, and possible mechanisms of the wind variability in the low-latitude thermosphere.
Numerical and Theoretical Investigation on the Origin of the Multiple Peaks of the H/V Ratio Curve
Authors: Xiao Wanbo , Lu Siqi , Wang Yanbin
Published: March 2021

Despite the popularity of the horizontal to vertical spectral ratio (HVSR) method in site effect studies, the origin of the H/V peaks has been controversial since this method was proposed. Many previous studies mainly focused on the explanation of the first or single peak of the H/V ratio, trying to distinguish between the two hypotheses — the S-wave resonance and ellipticity of Rayleigh wave. However, it is common both in numerical simulations and practical experiments that the H/V ratio exhibits multiple peaks, which is essential to explore the origin of the H/V peaks.

The cause for the multiple H/V peaks has not been clearly figured out, and once was simply explained as the result of multi subsurface layers. Therefore, we adopted numerical method to simulate the ambient noise in various layered half-space models and calculated the H/V ratio curves for further comparisons. The peak frequencies of the H/V curves accord well with the theoretical frequencies of S-wave resonance in two-layer models, whose frequencies only depend on the S wave velocity and the thickness of the subsurface layer. The same is true for models with varying model parameters. Besides, the theoretical formula of the S-wave resonance in multiple-layer models is proposed and then supported by numerical investigations as in the cases of two-layer models. We also extended the S-wave resonance to P-wave resonance and found that its theoretical frequencies fit well with the V/H peaks, which could be an evidence to support the S-wave resonance theory from a new perspective. By contrast, there are obvious differences between the higher orders of the H/V ratio peaks and the higher orders of Rayleigh wave ellipticity curves both in two-layer and multiple-layer models. The Rayleigh wave ellipticity curves are found to be sensitive to the Poisson’s ratio and the thickness of the subsurface layer, so the variation of the P wave velocity can affect the peak frequencies of the Rayleigh wave ellipticity curves while the H/V peaks show slight change. The Rayleigh wave ellipticity theory is thus proved to be inappropriate for the explanation of the multiple H/V peaks, while the possible effects of the Rayleigh wave on the fundamental H/V peak still cannot be excluded.

Based on the analyses above, we proposed a new evidence to support the claim that the peak frequencies of the H/V ratio curve, except the fundamental peaks, are caused by S-wave resonance. The relationship between the P-wave resonance and the V/H peaks may also find further application.
GRACE-FO accelerometer data: An alternative approach using Least Squares Spectral Analysis
Authors: Tzamali Myrto , Pagiatakis Spiros
Published: March 2021

Technological advances in satellite geodesy have been demanding more and more accurate gravity field models but also precise measurements of the movement of water along the Earth system. GRACE-FO (GFO) mission is dedicated to monitor the Earth with a purpose of estimating the gravity field and the hydrological cycles. For the extraction of monthly gravity field models the non-gravitational accelerations are essential. The performance of GFO accelerometers (ACC) is not the optimal.  The ACC measurements present immense spikes, spurious signals and bias jumps on all three axes affecting the validity of the measurements. The bias jumps are similar to those presented at GRACE measurements and they have been related to the satellites’ entrance to and exit from the Earth’s shadow. The dominant spikes, mainly appearing in the equatorial region, have been connected to the thermal sensitivity of the instrument or the orientation of the magnetic field lines. We propose an alternative dataset generated from Level 1A of GFO C with corresponding Gaussian weights and an optimal correction of the bias jumps, along with the estimation of linear and quadratic trends using the Least Squares methodology in the frequency domain and in all three axes. The method does not remove spikes, nor does it interpolate missing values. The new 1B dataset with estimated variances shows no spike effects in the frequency domain contrastingly to the existing ACT Level 1B data. Also, a preliminary analysis of the daily amplitudes of the orbital period and semi-period components of the ACT Level 1B data set spanning one year, reveals a strong periodic signal of ~ 153 days. This signal vanishes when the proposed weighted data set is used. This signal could be related to calibration deficiencies or a systematic error in the ACC data that requires further study. The same weighted filtering approach is proposed for the ACC measurements of Swarm C satellite, a LEO constellation that measures the magnetic field of the Earth. The ACC measurements of Swarm display low signal to noise ratio due to an increased thermal sensitivity of the instrument. A weighted Gaussian filter applied on the Swarm ACC measurements reduces the contribution of the dominant spikes in the frequency domain and displays the non-gravitational signals more clearly leading to a more extended use of Swarm non-gravitational accelerations measurements.
A numerical study of the mechanism of injection-induced and triggered seismicity at the Pohang Enhanced Geothermal Systems project, South Korea
Authors: Kim Kwang-Il , Yoo Hwajung , Park Seheok , Yim Juhyi , Xie Linmao , Min Ki-Bok , Rutqvist Jonny
Published: March 2021

Hydraulic stimulation for the creation of an Enhanced Geothermal System (EGS) reservoir could potentially reactivate a nearby fault and result in man-made earthquakes. In November 15, 2017, an Mw 5.5 earthquake, the second largest after the initiation of the South Korean national instrumental monitoring system, occurred near an EGS project in Pohang, South Korea. The earthquake occurred on a previously unmapped fault, that is here denoted the Mw 5.5 Fault. A number of previous studies to model the hydraulic stimulation in the Pohang EGS project have been carried out to identify the mechanism of seismic events. Those previous studies focused on coupled hydro-mechanical processes without the consideration of pre-existing fractures and thermal effects. This study presents an investigation of the mechanisms of induced and triggered seismicity in the Pohang EGS project through three-dimensional coupled thermo-hydro-mechanical numerical simulations. Fractures intersecting the open-hole sections of two deep boreholes, PX-1 and PX-2, clearly indicated by field observations are modeled along with the Mw 5.5 Fault. Models of stress-dependent permeability models are calibrated based on the numerical reproduction of the pressure-time evolution during the field hydraulic stimulations. The Coulomb failure stress change at the Mw 5.5 Fault is calculated to quantify the impact of five hydraulic stimulations. In the case of PX-2 stimulations, the pore pressure buildup results in a volumetric expansion of the reservoir and thereby the perturbation of stresses is transferred to the Mw 5.5 Fault. The volumetric contraction of the reservoir by the temperature reduction could slightly perturb the stress distribution at the Mw 5.5 Fault. In the case of PX-1 stimulations, shear slip of the PX-1 fracture is explicitly modeled. The modeling shows that transfer of the shear stress drop by the shear slip stabilizes the Mw 5.5 Fault, which is consistent with the field observation that the seismicity was not induced at the Mw 5.5 Fault by the PX-1 stimulations. The cooling-induced thermal stress additionally reduces the effective normal stress of PX-1 fracture. Thus, some additional shear slip of the PX-1 fracture is induced by the thermal effect. However, the modeling shows that for both PX-1 and PX-2 stimulations, thermally-induced stress perturbations are very small compared to pressure-induced stress perturbations.
Isotopes of molecular nitrogen, oxygen, and argon in the South Pole ice core document local and global climate change through the last deglaciation.
Authors: Morgan Jacob , Buizert Christo , Severinghaus Jeff
Published: March 2021

Ice core gas records are an invaluable paleoclimatic archive. The three most abundant gases in air, nitrogen (N2), oxygen (O2), and argon (Ar), provide paleoclimatic information about both global and regional processes including tropical rainfall patterns and local surface temperature changes. We present a large dataset of elemental and isotopic ratios of N2, O2, and Ar (O2/N2, Ar/N2, δ15N, δ18O, & δ40Ar) from the South Pole Ice Core between 0 – 52,000 yr BP, with a focus on high precision δ15N and δ40Ar measurements between 5,000 – 32,000 yr BP. The unprecedented precision of our measurements allows us to use δ15Nexcess (= δ15N - δ40Ar/4) to reconstruct past temperature change at the South Pole. Although this proxy has been widely applied in Greenland, this is the first time it has been successfully applied to Antarctic ice and provides a valuable independent check on the more traditional water isotopes temperature proxy. We find good agreement between the two during the relatively stable climate of the glacial period and the Holocene. However the temperature reconstructions diverge during the deglaciation. We present several hypotheses that could explain the discrepancy and look to other emerging ice core temperature proxies to support our interpretation.
Numerical modeling of the consequences of "marine heatwaves" in the North Pacific for the Arctic Ocean
Authors: Golubeva Elena , Platov Gennady , Kraineva Marina
Published: March 2021

As a result of the analysis of the NOAA surface temperature observational data (Huang et al., 2020), the periods corresponding to "marine heatwaves" in the northeastern Pacific Ocean (2013-2019) were identified. Marine heatwaves were defined as exceeding the 90th percentile threshold. The same analysis of the temperature in the Bering Strait's immediate vicinity showed anomalously warm waters in the same years. Analysis of the pressure field, which forms the atmosphere's dynamic state and affects the water circulation system of the Bering Sea, allowed us to assume the inflow of anomalously warm Pacific waters into the Chukchi Sea. To analyze the North Pacific heatwaves' consequences for the Arctic Ocean, we carried out two numerical experiments using the regional ocean and sea ice model SibCIOM (Golubeva et al., 2018) and NCEP/NCAR atmospheric reanalysis data (Kalnay et al., 1996). The first numerical experiment was carried out to calculate hydrodynamic and ice fields from January 2000 to November 2020 (Experiment 1). On the Arctic and the Pacific Ocean boundary in the Bering Strait, we used the monthly average climatic values ​​of the transport, temperature, and salinity of waters coming from the Pacific Ocean. Experiment 2 was carried out from 2014 to November 2020. The calculated values ​​of hydrological and ice characteristics obtained in Experiment 1 were used as the initial state for this experiment. In contrast to Experiment 1,  a heat flux exceeding the average climatic values ​​was set at the Bering Strait in Experiment 2. Its assignment was provided by using temperature values ​​from observational data in the Bering Strait vicinity (Huang et al., 2020). Comparison of monthly average hydrological and ice fields obtained in two numerical experiments and analysis of numerical results showed that an increase in the temperature of the Pacific waters entering the Arctic shelf through the Bering Strait leads to an increase in the heat content of the Chukchi Sea waters, heat transfer by currents in the surface and subsurface layers, a gradual increase in the heat content of the Beaufort Sea, and the reduction of Arctic ice cover. The increase in heat content in Experiment 2 for the Beaufort Sea was obtained in both the upper 50-meter and 250-meter layers.

The research is supported by the Russian Science Foundation, grant №. 19-17-00154.
Ozone Photochemistry in New York City and Baltimore based on Aircraft Observations
Authors: Ren Xinrong , Stratton Phillip , Daley Hannah , Dickerson Russell
Published: March 2021

Aircraft observations of ozone, ozone precursors, and meteorological parameters were made over the New York City (NYC) and Baltimore areas during ozone exceedance events in summer 2018-2020.  Despite the continued reduction in anthropogenic emissions, ozone exceedance events still frequently occurred in the NYC area.  Ozone production efficiency, defined as the ratio of the ozone production rate to the NOx oxidation rate, calculated using these observations,  was about 14 ppb ozone produced per ppb NOx oxidized. This high ozone production efficiency likely contributes to the persistent ozone exceedance problem over the Long Island Sound and Connecticut coastal area, downwind of NYC under prevailing southwesterly winds.  There is some evidence for a decreasing trend although COVID-19 restrictions had an impact on 2020 emissions.  A box model, constrained by observations, was used to examine atmospheric photochemical oxidation processes.  Ozone production rates and their sensitivity to nitrogen oxides (NOx) and volatile organic compounds (VOCs) were calculated based on the model results. In general ozone production is VOC sensitive near emission sources and NOx sensitive away from source regions. While the Baltimore area is predominantly in the NOx sensitive region, the NYC area is transitioning from VOC sensitive to NOx sensitive.  Preliminary results show that controlling both NOx and VOCs reduces ozone production in the NYC area. Reducing VOCs can reduce ozone production in emission source regions and reducing NOx can reduce ozone production farther away from the source regions. The results from this work strengthen our understanding of ozone production and provide scientific information for emission control strategies to reduce air pollution in ozone non-attainment areas.
Global structure and properties of ULF waves in the ion foreshock observed in a Hybrid-Vlasov simulation.
Authors: Zhang Kun , Dorfman Seth , Ganse Urs , Turc Lucile , Shi Chen
Published: March 2021

Energetic ions reflected and accelerated by the Earth’s bow shock travel back into the solar wind, forming the ion foreshock, and generate ultralow frequency (ULF) waves. Such ULF waves have been extensively studied over the past few decades using satellite measurements. However, the spatial variations of the wave properties cannot be well resolved by satellite observations due to the limited number of available spacecraft simultaneously inside the ion foreshock. Therefore, we conduct a global survey of the ULF wave properties in the ion foreshock through analysis of a Vlasiator (a hybrid-Vlasov code) simulation. Previous studies validated that this simulation well reproduced Earth’s foreshock and the ULF waves in it [e.g., Palmroth et al., 2015; Turc et al., 2018]. Here we focus on the wave properties, including frequency, ellipticity, polarization, wave normal angle and growth rate, of the well-known 30-sec wave and its multiple harmonics. We report that the ULF waves near the edge of the foreshock are very different from the waves in the center of the foreshock. We also show the related ion distribution and discuss the connection between the observed ion beams and ULF waves, aiming at understanding the cause of the observed differences in wave properties.

 

This study is supported by NASA grant 80NSSC20K0801. Vlasiator is developed by the European Research Council Starting grant 200141-QuESpace, and Consolidator grant GA682068-PRESTISSIMO received by the Vlasiator PI. Vlasiator has also received funding from the Academy of Finland. See www.helsinki.fi/vlasiator

 

Palmroth, M., et al. (2015), ULF foreshock under radial IMF: THEMIS observations and global kinetic simulation Vlasiator results compared, J. Geophys. Res. Space Physics, 120, 8782–8798, doi:10.1002/2015JA021526.

Turc, L., Ganse, U., Pfau-Kempf, Y., Hoilijoki, S., Battarbee, M., Juusola, L., et al. (2018). Foreshock properties at typical and enhanced interplanetary magnetic field strengths: results from hybrid-Vlasov simulations. Journal of Geophysical Research: Space Physics, 123, 5476–5493. doi:10.1029/2018JA025466.