bluuebunny/crossref_metadata_2025_split huggingface dataset:
Surface Melting Drives Fluctuations in Airborne Radar Penetration in West Central Greenland
Authors: Otosaka Inès N. , Shepherd Andrew , Casal Tânia G. D. , Coccia Alex , Davidson Malcolm , Di Bella Alessandro , Fettweis Xavier , Forsberg René , Helm Veit , Hogg Anna E. , Hvidegaard Sine M. , Lemos Adriano , Macedo Karlus , Kuipers Munneke Peter , Parrinello Tommaso , Simonsen Sebastian B. , Skourup Henriette , Sørensen Louise Sandberg 
Published: August 2020

AbstractGreenland Ice Sheet surface melting has increased since the 1990s, affecting the rheology and scattering properties of the near‐surface firn. We combine firn cores and modeled firn densities with 7 years of CryoVEx airborne Ku‐band (13.5 GHz) radar profiles to quantify the impact of melting on microwave radar penetration in West Central Greenland. Although annual layers are present in the Ku‐band radar profiles to depths up to 15 m below the ice sheet surface, fluctuations in summer melting strongly affect the degree of radar penetration. The extreme melting in 2012, for example, caused an abrupt 6.2 ± 2.4 m decrease in Ku‐band radar penetration. Nevertheless, retracking the radar echoes mitigates this effect, producing surface heights that agree to within 13.9 cm of coincident airborne laser measurements. We also examine 2 years of Ka‐band (34.5 GHz) airborne radar data and show that the degree of penetration is half that of coincident Ku‐band.
Evidence of Nonlinear Interactions Between Magnetospheric Electron Cyclotron Harmonic Waves
Authors: Gao Zhonglei , Zuo Pingbing , Feng Xueshang , Wang Yi , Jiang Chaowei , Wei Fengsi 
Published: August 2020

AbstractElectron cyclotron harmonic (ECH) waves play an important role in the magnetosphere‐ionosphere coupling. They are usually considered to be generated by the Bernstein‐mode instability with electron loss cone distributions. By analyzing the Van Allen Probes wave data, we present the direct evidence of the nonlinear interactions between ECH waves in the magnetosphere. Substorm‐injected electrons excite primary ECH waves in a series of structureless bands between multiples of the electron gyrofrequency. Nonlinear interactions between the primary ECH waves produce secondary waves at sum‐ and difference‐frequencies of the primary waves. Our results suggest that the nonlinear wave‐wave interactions can redistribute the primary ECH wave energy over a broader frequency range and hence potentially affect the magnetospheric electrons over a broader range of pitch angles and energies.
High‐Resolution Evidence for Middle Holocene East Asian Winter and Summer Monsoon Variations: Snapshots of Fossil Coral Records
Authors: Asami Ryuji , Yoshimura Natsumi , Toriyabe Hiroto , Minei Shogo , Shinjo Ryuichi , Hongo Chuki , Sakamaki Takashi , Fujita Kazuhiko 
Published: August 2020

AbstractWe first generated 53‐, 4‐, 10‐, and 9‐year‐long bimonthly time series of oxygen isotopes (δ18O), Sr/Ca, and U/Ca from 4.4 and 4.9 ka corals in Okinawa, Japan. Results indicate that the mid‐Holocene sea surface temperature (SST) and seawater δ18O were <1°C lower and 0.1–0.2‰ higher than today. Spectral analyses of reconstructed SST and seawater δ18O time series revealed East Asia monsoon (EAM) and El Niño/Southern Oscillation (ENSO) variations. Moreover, the latter had a dominant component of decade‐scale hydrologic variations. The Sr/Ca and U/Ca variations with removal of the ENSO component showed larger fluctuations by ~2°C in winter and ~0.6°C in summer at 4.4 ka relative to today, demonstrating (slightly) greater influences of the EAM variation on the northwestern subtropical Pacific in winter (summer) at that time, which are probably caused by enhancement of the EAM intensity and/or a southward migration of the latitudinal circulation.
Observed Hemispheric Asymmetry in Stratospheric Transport Trends From 1994 to 2018
Authors: Strahan Susan E. , Smale Dan , Douglass Anne R. , Blumenstock Thomas , Hannigan James W. , Hase Frank , Jones Nicholas B. , Mahieu Emmanuel , Notholt Justus , Oman Luke D. , Ortega Ivan , Palm Mathias , Prignon Maxime , Robinson John , Schneider Matthias , Sussmann Ralf , Velazco Voltaire A. 
Published: August 2020

AbstractTotal columns of the trace gases nitric acid (HNO3) and hydrogen chloride (HCl) are sensitive to variations in the lower stratospheric age of air, a quantity that describes transport time scales in the stratosphere. Analyses of HNO3 and HCl columns from the Network for the Detection of Atmospheric Composition Change panning 77°S to 79°N have detected changes in the extratropical stratospheric transport circulation from 1994 to 2018. The HNO3 and HCl analyses combined with the age of air from a simulation using the MERRA2 reanalysis show that the Southern Hemisphere lower stratosphere has become 1 month/decade younger relative to the Northern Hemisphere, largely driven by the Southern Hemisphere transport circulation. The analyses reveal multiyear anomalies with a 5‐ to 7‐year period driven by interactions between the circulation and the quasi‐biennial oscillation in tropical winds. This hitherto unrecognized variability is large relative to hemispheric transport trends and may bias ozone trend regressions.
Global Marine Isochore Estimates Using Machine Learning
Authors: Lee Taylor R. , Phrampus Benjamin J. , Obelcz Jeffrey , Wood Warren T. , Skarke Adam 
Published: August 2020

AbstractThe thickness normal to deposition (isopachs) and vertical thickness (isochores) of geological units is important for assessing various geologic processes. We present the first marine global sediment isochore estimates for five geological periods dating from middle Miocene (15.97 Ma) to present. We use sparsely distributed sediment depth vs. age observations from the Deep Sea Drilling Project and global maps of biological, oceanographic, geographic, and geological variables as training features in a k‐nearest neighbor regressor to estimate isochores. Results are compared to isochore estimates generated by applying a constant depositional rate from recent estimates of global total sediment thicknesses. Both models of isochore thickness exhibit consistent error. Results from a machine learning approach show major advantages, including results that are quantitative, easily updatable, and accompanied with uncertainty estimation. Final predictions can provide first‐order constraints on sediment deposition with geologic time, which is of timely importance for assessing past climate variability.
Using Seismic Noise Levels to Monitor Social Isolation: An Example From Rio de Janeiro, Brazil
Authors: Dias Fábio L. , Assumpção Marcelo , Peixoto Pedro S. , Bianchi Marcelo B. , Collaço Bruno , Calhau Jackson 
Published: August 2020

AbstractDecrease of seismic noise level, after reduction of traffic due to the COVID‐19 pandemic, has been observed worldwide. The possibility of using seismic noise as another proxy to estimate social isolation was tested with a station within Rio de Janeiro city. We used the isolation index measured from smartphone movement to calibrate the seismic noise levels and estimated an Isolation Seismic Index, ISI (% of the population at home), using the seismic noise energy. Noise levels best correlate with isolation measures in the frequency range 4–8 Hz. Small differences between the smartphone and the ISI indexes are interpreted as differences in social activities and noise sources. All mobility indexes are proxies to the actual isolation. Although ISI does not measure the number of people outside, it measures the number of noise sources (vehicles, trains, factories, etc.) and can be used as additional information to interpret anomalies in other proxies.
Step Increase in Eastern U.S. Precipitation Linked to Indian Ocean Warming
Authors: Strong Courtenay , McCabe Gregory J. , Weech Alexander 
Published: August 2020

AbstractA step increase in annual precipitation over the eastern United States in the early 1970s commenced five decades of invigorated hydroclimate, with ongoing impacts on streamflow and water resources. Despite its far‐reaching impacts, the dynamical origin of this change is unknown. Here analyses of a century of atmospheric and oceanic data trace the dynamics to changes in the Indian Ocean. Increases in fall precipitation contribute most strongly to the step increase, and the associated mechanism is emergence of a pan‐Pacific atmospheric wave emanating from deep convection over the warming Indian Ocean. Documentation of this fall teleconnection draws attention to projected anthropogenic increases in tropical oceanic heat content and their potential impacts on hydroclimate of the midlatitudes.
Dayside Auroral Observation Resulting From a Rapid Localized Compression of the Earth's Magnetic Field
Authors: Briggs Jennifer K. , Fasel Gerard J. , Silveira Marcos , Sibeck David G. , Lin Yu , Sigernes Fred 
Published: August 2020

AbstractThis paper presents observations of dayside auroral signatures resulting from a massive localized compression of the magnetopause, magnetosheath, and bow shock. On 10 December 2016, the Magnetospheric Multiscale Mission (MMS) spacecraft observed a compression of the entire magnetosheath and bow shock, which moved past the spacecraft in 1 min and 45 s. Shortly afterward, the resulting auroral signature was observed at the Kjell Henriksen Observatory located in Longyearbyen, Norway. The characteristics of this unique event have major ramifications for understanding dayside magnetospheric physics.
First Measurements of Low‐Frequency Sferics Associated With Terrestrial Gamma‐Ray Flashes Produced by Equatorial Thunderstorms
Authors: Zhang Hongbo , Lu Gaopeng , Lyu Fanchao , Ahmad Mohd Riduan , Qie Xiushu , Cummer Steven A. , Xiong Shaolin , Briggs Michael S. 
Published: August 2020

AbstractThe low‐frequency (LF) lightning sferics associated with terrestrial gamma‐ray flashes (TGFs) detected by Fermi Gamma‐ray Burst Monitor over equatorial thunderstorms have been recorded at a station in Melaka, Malaysia, since 2017. We examine the LF sferics of two Fermi TGFs, including one TGF‐associated lightning discharge at only about 28 km range. Both TGFs are related to the strongest pulse during the initial stage of their parent intracloud (IC) lightning, while in both cases, the light curve of gamma‐ray photon lags the major lightning pulse by approximately 100 μs. TGF occurred about 3 ms after the lightning initiation when the initial upward negative leader has ascended for about 2 km to reach the height of 10–11 km. Our analyses on a statistical basis show that TGF‐related lightning is mostly located in the strong convection of equatorial thunderstorms at the mature stage, and nearly half TGFs are not produced in the strongest convection region.
Intermittent Slip Along the Alto Tiberina Low‐Angle Normal Fault in Central Italy
Authors: Vuan A. , Brondi P. , Sugan M. , Chiaraluce L. , Di Stefano R. , Michele M. 
Published: August 2020

AbstractThe Alto Tiberina normal fault (ATF) in central Italy is a 50‐km‐long crustal structure that dips at a low angle (15–20°). Events on the fault plane are about 10 times less frequent than those located in its shallower syn‐ and antithetic hanging‐wall splays. To enhance ATF catalog and achieve a better understanding of the degree of coupling in the fault system, we apply a template matching technique in the 2010–2014 time window. We augment by a factor 5 the detections and decrease the completeness magnitude to negative values. Contrary to what previously observed on ATF, we highlight intermittent seismic activity and long‐lasting clusters interacting with sequences on the shallower splays. One of these episodes of prolonged seismic activity, detected at the end of 2013 on a 30‐km‐long ATF segment, suggest the ATF active role during an aseismic transient unraveled by geodetic data.
Evidence of Hydrological Intensification and Regime Change From Northern Alaskan Watershed Runoff
Authors: Arp C. D. , Whitman M. S. , Kemnitz R. , Stuefer S. L. 
Published: August 2020

AbstractSnowmelt‐dominated runoff regimes have defined northern Alaskan rivers. Discharge records from three watersheds within the National Petroleum Reserve in Alaska (NPR‐A) span 19 years and capture three notable periods of changing runoff. In the first, 2001–2008, mean annual runoff (MAR) averaged 90 mm, characterized by sharp snowmelt runoff and summer drought. Over the next 7 years, larger MAR averaged 120 mm driven by high and early snowmelt runoff. The most recent 4 years, 2016–2019, had even higher MAR of 163 mm with high and sustained late summer flows. Hydrograph analysis suggests a shift toward rainfall‐dominated runoff in the most recent period compared to snowmelt‐dominated hydrographs in the previous two. Declining sea ice appears closely linked to increasing late summer precipitation and a shift toward rainfall runoff. Future development in the NPR‐A will require continued hydrological monitoring and planning to mitigate flood and erosion hazards, permafrost degradation, and ecosystem impairment.
Momentum and Energy Predict the Backwater Rise Generated by a Large Wood Jam
Authors: Follett E. , Schalko I. , Nepf H. 
Published: August 2020

AbstractWood reintroduction is now considered an important aspect of stream restoration, due to ecohydraulic benefits associated with wood presence. Channel‐spanning wood jams create an upstream backwater, increasing flow heterogeneity, sediment deposition, and ecological productivity, but also flood risk. Backwater rise prediction is necessary to evaluate flood hazards in hydraulic models, improve design of engineered logjam projects, and compare jam effects across river systems. We present experimental results demonstrating that a jam can be modeled as a porous obstruction generating momentum loss proportional to the number, size, and packing density of the logs and the jam length. Energy and momentumconstraints are combined to predict backwater rise from unit discharge and a dimensionless structural parameter. This novel approach allows description of preexisting jams with a common metric. The model was used to demonstrate how backwater length, pool size, and upstream sediment deposition depend on jam structure and channel slope.
Reduced Poleward Transport Due to Stratospheric Heating Under Stratospheric Aerosols Geoengineering
Authors: Visioni Daniele , MacMartin Douglas G. , Kravitz Ben , Lee Walker , Simpson Isla R. , Richter Jadwiga H. 
Published: August 2020

AbstractBy injecting SO2 into the stratosphere at four latitudes (30°, 15°N/S), it might be possible not only to reduce global mean surface temperature but also to minimize changes in the equator‐to‐pole and inter‐hemispheric gradients of temperature, further reducing some of the impacts arising from climate change relative to equatorial injection. This can happen only if the aerosols are transported to higher latitudes by the stratospheric circulation, ensuring that a greater part of the solar radiation is reflected back to space at higher latitudes, compensating for the reduced sunlight. However, the stratospheric heating produced by these aerosols modifies the circulation and strengthens the stratospheric polar vortex which acts as a barrier to the transport of air toward the poles. We show how the heating results in a feedback where increasing injection rates lead to stronger high‐latitudinal transport barriers. This implies a potential limitation in the high‐latitude aerosol burden and subsequent cooling.
In Situ Observation of the Phase Transition Behavior of Shocked Baddeleyite
Authors: Takagi Sota , Kyono Atsushi , Nozawa Shunsuke , Kawai Nobuaki , Inukai Koji , Fukaya Ryo , Funamori Nobumasa , Adachi Shin‐ichi , Ichiyanagi Kouhei 
Published: August 2020

AbstractBaddeleyite (ZrO2) is used to infer shock pressures and understand the impact history of planetary bodies. Although the high‐pressure phase transition behavior of baddeleyite has been intensively investigated under hydrostatic conditions, there is little information on the dynamic response of phase transitions under shock‐loading conditions. We performed in situ X‐ray diffraction measurements on shock‐loaded baddeleyite using a synchrotron X‐ray pulse at beamline NW14A of the Photon Factory Advanced Ring, High Energy Accelerator Research Organization (KEK), Japan. A phase transition from monoclinic to orthorhombic‐I ZrO2 occurs at 3.3 GPa during shock compression and immediately returns to the monoclinic phase during subsequent release. Orthorhombic‐II ZrO2 is not observed up to 15 GPa. This study refines the phase transition behavior of baddeleyite under shock conditions up to 15 GPa, and the phase transition boundary is determined from direct observations. These findings improve the understanding of the shock history of planetary bodies.
Crater Morphometry on the Mafic Floor Unit at Jezero Crater, Mars: Comparisons to a Known Basaltic Lava Plain at the InSight Landing Site
Authors: Warner Nicholas H. , Schuyler Andrew J. , Rogers A. Deanne , Golombek Matthew P. , Grant John , Wilson Sharon , Weitz Cathy , Williams Nathan , Calef Fred 
Published: August 2020

AbstractA secondary objective for the Perseverance rover mission to Jezero crater, Mars, is to collect igneous rocks for analysis on Earth. The mafic crater floor unit (MFU) represents the best candidate. Ten‐meter‐scale craters on the MFU exhibit rocky ejecta, rims, and slopes that indicate resistant rock. The frequency distribution of these craters is, however, low. Comparisons of MFU craters to craters on a lava plain at the Interior Exploration using Seismic Investigation, Geodesy and Heat Transport mission (InSight) landing site reveal that the MFU lacks a granular regolith. Removal of regolith or exhumation of the MFU explains the rocky crater morphology and low density. Erosion rates, calculated using crater retention timescales of ~2.0 Ga for both locations, are 10−3 to 10−4 m/Myr. The rates derive from craters impacted into rocky materials on the MFU versus regolith at InSight. The difference in material strength, yet comparable erosion rates, requires more vigorous surface processes at Jezero relative to global averages on Mars.
Ocean Heat Storage Rate Unaffected by MOC Weakening in an Idealized Climate Model
Authors: Shatwell Peter , Czaja Arnaud , Ferreira David 
Published: August 2020

AbstractTo study the role of the Atlantic meridional overturning circulation (AMOC) in transient climate change, we perform an abrupt CO2‐doubling experiment using a coupled atmosphere‐ocean‐ice model with a simple geometry that separates the ocean into small and large basins. The small basin exhibits an overturning circulation akin to the AMOC. Over the simulated 200 years of change, it stores heat at a faster rate than the large basin by 0.6 ± 0.2 W m−2. We argue that this is due to the small basin MOC. However, we find that as the MOC weakens significantly, it has little impact on the small basin's heat storage rate. We suggest this is due to the effects of both compensating warming patterns and interbasin heat transports. Thus, although the presence of a MOC is important for enhanced heat storage, MOC weakening is surprisingly unimportant.
Generalized Substorm Current Wedge Model: Two Types of Dipolarizations in the Inner Magnetosphere
Authors: Ohtani S. , Motoba T. , Takahashi K. , Califf S. 
Published: August 2020

AbstractThe present study investigates dipolarization signatures in the inner magnetosphere using sharp geosynchronous dipolarizations as a reference. The results are summarized as follows: (1) The region of sharp and structured dipolarizations expands earthward while dipolarizations are sustained at geosynchronous orbit; (2) within 5 RE from Earth, dipolarization signatures are often smooth and gradual, resembling midlatitude positive bays, and they start simultaneously with substorm onsets; (3) off the equator (>0.5 RE), sharp dipolarizations often take place before geosynchronous dipolarizations. These results can be explained by a model current system with R1‐sense and R2‐sense current wedges (R1CW and R2CW) if (a) the R1CW, which is located outside, is more intense than the R2CW in total current, (b) the R1CW stays outside of geosynchronous orbit, and (c) the R2CW moves earthward. The model suggests that the region of sharp dipolarizations is confined between the two current wedges, and it expands earthward as the R2CW moves earthward (Result 1). Sufficiently earthward of the R2CW, the remote effect of the R1CW dominates that of the R2CW, and accordingly, magnetic disturbances resemble midlatitude positive bays (Result 2). Since the timing of sharp dipolarizations is determined by the passage of the R2CW, they take place earlier for outer flux tubes. Away from the magnetic equator, sharp dipolarizations can precede geosynchronous dipolarizations especially if the magnetic configuration is stretched (Result 3). Thus, this double‐current wedge model explains the variability of dipolarization signatures at different distances, and it may be regarded as a generalized substorm current wedge model.
Explicit Global Simulation of Gravity Waves in the Thermosphere
Authors: Becker Erich , Vadas Sharon L. 
Published: August 2020

AbstractWe present a new version of the high‐resolution Kühlungsborn Mechanistic general Circulation Model (KMCM) extended to z ∼ 450 km. This model is called HIAMCM (HI Altitude Mechanistic general Circulation Model) and explicitly simulates gravity waves (GWs) down to horizontal wavelengths of λh ∼ 165 km. We find predominant tertiary GWs in the winter thermosphere at middle/high latitudes. These GWs typically have horizontal wavelengths λh ∼ 300–1,100 km, ground‐based periods ∼ 25–90 min, and intrinsic horizontal phase speeds cIh ∼ 250–350 m s−1. Above z ∼ 200 km, the predominant GW horizontal propagation directions are roughly against the background winds from the diurnal tide; the GWs propagate mainly poleward at midnight, eastward at 6 local time (LT), equatorward at noon, and westward at 18 LT. Wintertime GWs at z ∼ 300 km having 165 km ≤ λh ≤ 330 km create a large hot spot over the Southern Andes/Antarctic Peninsula that agrees well with quiet time satellite measurements. Due to cancelation effects, the time‐averaged zonal mean Eliassen‐Palm flux divergence from the resolved GWs in the thermosphere is negligible compared to that of the tides and compared to the zonal component of the time‐averaged zonal mean ion drag. We also find that the thermospheric GWs dissipate mainly from macroturbulent diffusion and, above z ∼ 200 km, from molecular diffusion, whereas the tides dissipate mainly from ion drag. The averaged dissipative heating in the thermosphere due to tides is much stronger than that due to GWs.