bluuebunny/crossref_metadata_2025_split huggingface dataset:
Modeling Kelvin‐Helmholtz Instability at the High‐Latitude Boundary Layer in a Global Magnetosphere Simulation
Authors: Michael A. T. , Sorathia K. A. , Merkin V. G. , Nykyri K. , Burkholder B. , Ma X. , Ukhorskiy A. Y. , Garretson J. 
Published: September 2021

AbstractThe Kelvin‐Helmholtz instability at the magnetospheric boundary plays a crucial role in solar wind‐magnetosphere‐ionosphere coupling, particle entry, and energization. The full extent of its impact has remained an open question due, in part, to global models without sufficient resolution to capture waves at higher latitudes. Using global magnetohydrodynamic simulations, we investigate an event when the Magnetospheric Multiscale (MMS) mission observed periodic low‐frequency waves at the dawn‐flank, high‐latitude boundary layer. We show the layer to be unstable, even though the slow solar wind with the draped interplanetary magnetic field is seemingly unfavorable for wave generation. The simulated velocity shear at the boundary is thin () and requires commensurately high spatial resolution. These results, together with MMS observations, confirm for the first time in fully three‐dimensional global geometry that KH waves can grow in this region and thus can be an important process for energetic particle acceleration, dynamics, and transport.
Recent Increases in Exposure to Extreme Humid‐Heat Events Disproportionately Affect Populated Regions
Authors: Rogers Cassandra D. W. , Ting Mingfang , Li Cuihua , Kornhuber Kai , Coffel Ethan D. , Horton Radley M. , Raymond Colin , Singh Deepti 
Published: September 2021

AbstractExtreme heat research has largely focused on dry‐heat, while humid‐heat that poses a substantial threat to human‐health remains relatively understudied. Using hourly high‐resolution ERA5 reanalysis and HadISD station data, we provide the first spatially comprehensive, global‐scale characterization of the magnitude, seasonal timing, and frequency of dry‐ and wet‐bulb temperature extremes and their trends. While the peak dry‐ and humid‐heat extreme occurrences often coincide, their timing differs in climatologically wet regions. Since 1979, dry‐ and humid‐heat extremes have become more frequent over most land regions, with the greatest increases in the tropics and Arctic. Humid‐heat extremes have increased disproportionately over populated regions (∼5.0 days per‐person per‐decade) relative to global land‐areas (∼3.6 days per‐unit‐land‐area per‐decade) and population exposure to humid‐heat has increased at a faster rate than to dry‐heat. Our study highlights the need for a multivariate approach to understand and mitigate future harm from heat stress in a warming world.
Slippery Bottom Boundary Layers: The Loss of Energy From the General Circulation by Bottom Drag
Authors: Ruan Xiaozhou , Wenegrat Jacob O. , Gula Jonathan 
Published: September 2021

AbstractBottom drag is believed to be one of the key mechanisms that remove kinetic energy from the ocean's general circulation. However, large uncertainty still remains in global estimates of bottom drag dissipation. One significant source of uncertainty comes from the velocity structures near the bottom where the combination of sloping topography and stratification can reduce the mean flow magnitude, and thus the bottom drag dissipation. Using high‐resolution numerical simulations, we demonstrate that previous estimates of bottom drag dissipation are biased high because they neglect velocity shear in the bottom boundary layer. The estimated bottom drag dissipation associated with geostrophic flows over the continental slopes is at least 56% smaller compared with prior estimates made using total velocities outside the near‐bottom layer. The diagnostics suggest the necessity of resolving the bottom boundary layer structures in coarse‐resolution ocean models and observations in order to close the global kinetic energy budget.
Seismic Reflection and Electrical Resistivity Imaging Support Pre‐Quaternary Glaciation in the Rocky Mountains (Unaweep Canyon, Colorado)
Authors: Patterson Anna , Behm Michael , Chwatal Werner , Flores‐Orozco Adrian , Wang Yichuan , Soreghan Gerilyn S. 
Published: September 2021

AbstractUnaweep Canyon (Uncompahgre Plateau, Colorado) represents an enigmatic landscape with a complex evolution. Interpretations for its origin have ranged from ancestral fluvial erosion in the late Cenozoic to glacial erosion in the Paleozoic, or some combination thereof, with significant implications for global climatic and large‐scale tectonic reconstructions. To address the conflicting interpretations, we acquired a high‐resolution seismic reflection profile to investigate the depth, structure, and sedimentary infill in the canyon. The data set is further complemented with an electrical resistivity survey. Integrated with other geophysical and geological data, the results show an overdeepened Precambrian basement with transverse U shape and support the hypothesis of a pre‐Quaternary glacial origin. Our data constitute the first detailed image of a buried pre‐Quaternary glacial valley in North America; if substantiated with core studies, these results have far‐reaching implications for our understanding of global ice houses as well as the tectonic conditions, enabling preservation of such systems.
Magnetospheric Multiscale Observations of an Expanding Oxygen Wave in Magnetic Reconnection
Authors: Kolstø Håkon Midthun , Norgren Cecilia , Hesse Michael , Chen Li‐Jen , Tenfjord Paul , Spinnangr Susanne Flø , Kwagala Norah 
Published: September 2021

AbstractHeavier plasma species such as oxygen ions can have a large impact on the magnetic reconnection process. It has been hypothesized that the acceleration of demagnetized oxygen ions by the Hall electric field will lead to the formation of an oxygen wave that expands into the exhaust. By comparing data from NASA's Magnetospheric Multiscale mission to a fully kinetic particle‐in‐cell simulation, we can for the first time provide observational evidence of such an expanding oxygen wave. The wave is characterized by an oxygen jet consisting of cold ions directed toward the neutral sheet associated with a density cavity. This density cavity forms as the are subject to collective acceleration by the Hall electric field leaving behind a region of low‐density oxygen ions. Our results are important for the understanding of the role and effect of oxygen ions in magnetic reconnection.
Mineral Luminescence Observed From Space
Authors: Köhler Philipp , Fischer Woodward W. , Rossman George R. , Grotzinger John P. , Doughty Russell , Wang Yujie , Yin Yi , Frankenberg Christian 
Published: September 2021

AbstractMethods developed to explore the luminescent properties of the moon facilitated the development of techniques to infer terrestrial solar‐induced chlorophyll fluorescence (SIF) from satellite instruments. While single SIF retrievals are inherently noisy, averaging many retrievals allows us to obtain highly accurate estimates. We analyzed several years of aggregated SIF data collected by the TROPOspheric Monitoring Instrument (TROPOMI) and the Orbiting Carbon Observatory‐2 (OCO‐2) over nonvegetated areas to explore the potential of SIF retrievals beyond the realm of photosynthesis. The fundamentally different retrievals at varying wavelengths in the near‐infrared reveal that about 10% of all barren surfaces are weakly luminescent, while a few areas luminesce strongly—amounts comparable to SIF from vegetation. By means of lithological maps, we attributed the strongest luminescence signals to exposed carbonate sedimentary rocks. Besides a detailed evaluation of the signal properties, we discussed implications for SIF data sets and other remote sensing products.
Enhanced Visibility of Subduction Slabs by the Formation of Dense Hydrous Phase A
Authors: Cai Nao , Qi Xintong , Chen Ting , Wang Siheng , Yu Tony , Wang Yanbin , Inoue Toru , Wang Duojun , Li Baosheng 
Published: September 2021

AbstractPhase A (Mg7Si2O8(OH)6) is one of the important dense hydrous magnesium silicates in subducting slab, because it forms after the breakdown of antigorite serpentine and could be the dominant hydrous phase in the upper‐mantle deep slab for the water transportation into deep Earth. In this study, the compressional (P) and shear (S) wave velocities of phase A were measured at simultaneous pressure and temperature conditions up to 11 GPa and 1073 K. Combined with elastic properties of olivine and pyroxenes, we calculate the hydration effect on the velocities of harzburgite lithology in cold subduction zones throughout the depth ranges where phase A is thermodynamically stable. Our calculations suggest that the hydration increases both P‐ and S‐wave velocities of harzburgite; at ∼5 wt% hydration, its seismic detectability is enhanced by 1%–1.5% in velocity contrasts relative to its anhydrous counterpart.
Shock Mach Number Estimates Using Incomplete Measurements
Authors: Gedalin Michael , Russell Christopher T. , Dimmock Andrew P. 
Published: September 2021

AbstractThe Mach number is one of the most important parameters of collisionless shocks. The accuracy of its observational determination is compromised by several complications. Incomplete measurements of plasma parameters significantly contribute to the uncertainty, along with the errors of the normal determination. A set of CLUSTER observed shocks is analyzed using several methods for finding the shock normal and to circumvent the shortcomings of the plasma data. A relation between the maximum magnetic compression and the Alfvénic Mach number is established. It is proposed as a proxy for the Mach number estimate when measurements are incomplete.
Repeating Nontectonic Seasonal Stress Changes and a Possible Triggering Mechanism of the 2019 Ridgecrest Earthquake Sequence in California
Authors: Kim Jeonghyeop , Holt William E. , Bahadori Alireza , Shen Weisen 
Published: September 2021

AbstractHere we characterize the 13‐year history of nontectonic horizontal strain anomalies across the regions surrounding Ridgecrest, CA, using cGPS data from January 2007. This time‐dependent model reveals a seasonality in the nontectonic strain anomalies and the associated Coulomb stress changes of ∼±0.5–2 kPa. In the area surrounding the epicenters of the 2019 Ridgecrest earthquake sequence of July, we find that the seasonal preseismic Coulomb stress changes peaked every early summer (May and June) during the last 13 years including during June 2019, a month prior to the large events. In addition, our statistical tests confirm that more strike‐slip earthquakes (Mw ≥ 2) occur during times when seasonal stress changes are increasing on right‐lateral faults in comparison with times when stresses are decreasing. These results suggest that the timing of the 2019 Ridgecrest earthquakes may have been modulated by nontectonic seasonal stress changes. The dynamic source of the seasonal nontectonic strain/stress anomalies, however, remains enigmatic. We discuss a possible combination of driving forces that may be attributable for the seasonal variations in nontectonic strain/stress anomalies, which captured in cGPS measurements.
Characterization of Irreversible Land Subsidence in the Yazd‐Ardakan Plain, Iran From 2003 to 2020 InSAR Time Series
Authors: Mirzadeh Sayyed Mohammad Javad , Jin Shuanggen , Parizi Esmaeel , Chaussard Estelle , Bürgmann Roland , Delgado Blasco Jose Manuel , Amani Meisam , Bao Han , Mirzadeh Seyyed Hossein 
Published: September 2021

AbstractGroundwater extraction rates exceeding recharge are occurring throughout Iran to sustain industrial and agricultural activities, resulting in land deformation in many areas, particularly in the Yazd‐Ardakan Plain (YAP) in central Iran's dry and desert regions. In this study, Interferometric Synthetic Aperture Radar (InSAR) time series analysis and statistical models are applied to characterize the controls on land subsidence in the YAP from 2003 to 2020. Our results reveal the existence of a northwest‐southeast elongated area of 234.45 km2 experiencing subsidence at rates up to 15 cm/yr. In the YAP, the international Airport, railway, transit road, and several industrial and historical sites are threatened by the differential subsidence. Well data confirm that groundwater levels have decreased by 18 meters between 1974 and 2018, driving the compaction of sediments within the underlying aquifer system. Our statistical analysis shows that the thickness of a shallow, clay‐rich aquitard layer controls the extent of the observed subsidence and the Independent Component Analysis of the InSAR time series shows that inelastic compaction is dominated. This work reveals that current groundwater extraction practices in central Iran are not sustainable and result in permanent subsidence, ground fractures with impact on infrastructures, and a permanent decrease in water storage capacity.
The Finicky Nature of Earthquake Shaking‐Triggered Submarine Sediment Slope Failures and Sediment Gravity Flows
Authors: Gomberg J. , Ariyoshi K. , Hautala S. , Johnson H. P. 
Published: September 2021

AbstractSince 2011, seafloor temperatures, pressures, and seismic ground motions have been measured by the seafloor cabled Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) on the Nankai margin. These measurements, high‐resolution bathymetry, and abundant contextual information make the DONET region seem ideally suited to provide constraints on seismic shaking‐triggered sediment slope failures and gravity flows, particularly since numerous published studies have linked paleo‐ to modern earthquakes to failures and flows within the DONET. The occurrences of the local 2016 M6.0 Mie‐ken and regional M7.0 Kumamoto earthquakes within and at regional distances, respectively, from the DONET data set provided an opportunity to explore this potential. We used DONET seismic recordings of the posited triggering shaking and to search for submarine slide signals and continuous temperature and pressure data to detect pulses of warm and densified water indicative of passing flows. We developed and applied a variety of analytical methods to eliminate signals generated by water column processes, while leaving slope failures and sediment gravity flow anomalies as residuals. Our explorations yielded no evidence that earthquake shaking initiated either phenomenon, which we suggest reflects the finicky nature both of the detection of and the physical processes that contribute to slope failures and flows (i.e., both require satisfying precise suites of conditions). Nonetheless, this negative result, our analyses, and the estimates of physical properties we derived for them, provide useful lessons and inputs for future studies.
Distinguishing Between Regression Model Fits to Global Mean Sea Level Reconstructions
Authors: Zhu Yingli , Mitchum Gary T. , Doran Kara S. , Chambers Don P. , Liang Xinfeng 
Published: September 2021

AbstractGlobal mean sea level (GMSL) has been rising since the last century, posing a serious challenge for the coastal areas. A variety of regression models have been utilized for determining GMSL rise over the past one hundred years, resulting in a large spread of sea level rise rates and multidecadal variations. In this study, we develop a new nonparametric noise model that is data‐dependent and considers overfitting due to regression. The noise model is used to determine whether one regression model has significantly better skill than others over the period 1900–2010. The choices of background noise and GMSL reconstruction influence whether two sea level models can be statistically distinguished. With our new nonparametric noise spectra, the differences of model skills in explaining sea level variance are significant only in 34% of model comparisons. However, stepwise trends with three inflection points are significantly more skillful than the linear, quadratic, or exponential trend for most GMSL reconstructions, suggesting the importance of multidecadal variability of sea level rise in the twentieth century. Nevertheless, stepwise trend models cannot be distinguished from models with a long‐term harmonic oscillation, indicating that the shape of multidecadal variability is not conclusive. The multidecadal variability is also significant in the steric and barystatic sea level contributions and is related to both natural and anthropogenic forcings. GMSL predictions based on regression fits in the twentieth century underestimate the sea level rise rate over the period 2011–2020 because the sea level acceleration in the recent decade (2011–2020) is not well represented.
A Deep Water Dispersion Experiment in the Gulf of Mexico
Authors: Meunier Thomas , Pérez Brunius Paula , Rodríguez Outerelo Javier , García Carrillo Paula , Ronquillo Argelia , Furey Heather , Ramsey Andrée , Bower Amy 
Published: September 2021

AbstractThe Deep Water Horizon oil spill dramatically impacted the Gulf of Mexico from the seafloor to the surface. While dispersion of contaminants at the surface has been extensively studied, little is known about deep water dispersion properties. This study describes the results of the Deep Water Dispersion Experiment (DWDE), which consisted of the release of surface drifters and acoustically tracked RAFOS floats drifting at 300 and 1,500 dbar in the Gulf of Mexico. We show that surface diffusivity is elevated and decreases with depth: on average, diffusivity at 1,500 dbar is 5 times smaller than at the surface, suggesting that the dispersion of contaminants at depth is a significantly slower process than at the surface. This study also examines the turbulent regimes driving the dispersion, although conflicting evidences and large uncertainties do not allow definitive conclusions. At all depths, while the growth of dispersion and kurtosis with time supports the possibility of an exponential regime at very short time scales, indicating that early dispersion is nonlocal, finite size Lyapunov exponents support the hypothesis of local dispersion, suggesting that eddies of size comparable to the initial separation (6 km), may dominate the early dispersion. At longer time scales, the quadratic growth of dispersion is indicative of a ballistic regime, where a mean shear flow would be the dominating process. Examination of the along‐ and across‐bathymetry components of float velocities supports the idea that boundary currents could be the source for this shear dispersion.
Geostrophy Assessment and Momentum Balance of the Global Oceans in a Tide‐ and Eddy‐Resolving Model
Authors: Yu Xiaolong , Ponte Aurélien L. , Lahaye Noé , Caspar‐Cohen Zoé , Menemenlis Dimitris 
Published: September 2021

AbstractThe future wide‐swath satellite altimeters, such as the upcoming Surface Water Ocean Topography (SWOT) mission, will provide instantaneous 2D measurements of sea level down to the spatial scale of O(10 km) for the first time. However, the validity of the geostrophic assumption for estimating surface currents from these instantaneous maps is not known a priori. In this study, we quantify the accuracy of geostrophy for the estimation of surface currents from a knowledge of instantaneous sea level using the hourly snapshots from a tide‐ and eddy‐resolving global numerical simulation. Geostrophic balance is found to be the leading‐order balance in frontal regions characterized by large kinetic energy, such as the western boundary currents and the Antarctic Circumpolar Current. Everywhere else, geostrophic approximation ceases to be a useful predictor of ocean velocity, which may result in significant high‐frequency contamination of geostrophically computed velocities by fast variability (e.g., inertial and higher). As expected, the validity of geostrophy is shown to improve at low frequencies (typically 0.5 cpd). Global estimates of the horizontal momentum budget reveal that the tropical and mid‐latitude regions where geostrophic balance fails are dominated by fast variability and turbulent stress divergence terms rather than higher‐order geostrophic terms. These findings indicate that the estimation of velocity from geostrophy applied on SWOT instantaneous sea level maps may be challenging away from energetic areas.
Observations Show Charge Density of Volcanic Plumes is Higher Than Thunderstorms
Authors: Haley Sean , Behnke Sonja , Edens Harald , Thomas Ronald 
Published: September 2021

AbstractWe analyze slow electric field change and lightning mapping measurements to provide insight into the characteristics of volcanic lightning and the associated implications on charging processes and the charge structure of a Vulcanian eruption plume. Data were obtained during a multi‐instrumental field campaign at Sakurajima volcano in 2015 when the Showa crater was active. We combine the electric field change and lightning mapping data from one explosive eruption on June 6, 2015 to identify individual flashes. From this, we interpret the flash type and polarity. In addition, the long‐time constant of the electric field change instrument allowed measurement of the quasi‐static field associated with charge separation in the eruption plume. We find that both intracloud and cloud‐to‐ground discharges occurred, and the polarity of cloud‐to‐ground discharges were all negative. The quasi‐static field measurement showed the plume carried a net negative charge. We calculate both the total charge transferred by cloud‐to‐ground discharges and the net charge density of the eruption plume. We find that cloud‐to‐ground discharges transfer an average of −0.41C per flash and the net charge density was −33C/. The percent error is at least 200%, due to uncertainty in the antenna gain. We show that these estimates are consistent with lightning that is 100 m in length. Further, the average flash rate during the first 8 s following the onset of eruption was five flashes per second. After that time, the flash rate abruptly decreased, which may be related to the end of gas‐thrust forcing.
Modulation of the Bifurcation in Radiative‐Convective Equilibrium by Gray‐Zone Cloud and Turbulence Parameterizations
Authors: Shi Xiaoming , Fan Yiyuan 
Published: September 2021

AbstractThis study investigates the mechanisms by which small‐scale turbulence and cloud physics determine the organization of large‐scale convection in radiative‐convective equilibrium (RCE), an idealization of the tropical atmosphere. Under uniform forcings similar to typical tropical conditions, the atmosphere in RCE might spontaneously separate into dry and moist regions on scales of 100–1,000 km, with convective clouds aggregating into a cluster in the latter. This phenomenon is known as convective self‐aggregation. Herein, we demonstrate that subtle changes in assumptions related to cloud physics and turbulence on scales of 1 km can dictate the emergence or suppression of convective self‐aggregation, resulting from a bifurcation of the dynamical system. The bifurcation occurs when a small dry patch forms in the domain and is sustained because it contributes to negative effective diffusivity of the circulation. Cloud‐radiation feedbacks and turbulence circulation interactions govern the formation of such dry patches, thereby modulating the bifurcation. This sensitive dependence on subgrid process models might be a fundamental barrier to climate predictability in light of inherent uncertainties in microscale processes. Because without the capability to include exact representations of those processes in climate models, slight differences in the different approximations used by modelers can lead to qualitative changes in climate predictions, at least for some processes.