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Geophysical Research Letters

Bradley J Thomson, Debra L Buczkowski, Larry S Crumpler, Kimberly D Seelos, Caleb I Fassett
The origin of the sedimentary mound within Gale crater, the landing site for the Mars Science Laboratory rover Curiosity , remains enigmatic. Here we examine the total potential contribution of fluvial material by conducting a volume-based analysis. On the basis of these results, the mound can be divided into three zones: a lower, intermediate, and upper zone. The top boundary of the lowermost zone is defined by maximal contribution of water-lain sediments, which are ~13 to 20% of the total mound volume. The upper zone is defined by the elevation of the unbreached rim to the north (-2...
May 28, 2019: Geophysical Research Letters
Ian Joughin, Benjamin E Smith, Christian G Schoof
The choice of the best basal friction law to use in ice-sheet models remains a source of uncertainty in projections of sea level. The parameters in commonly used friction laws can produce a broad range of behavior and are poorly constrained. Here we use a time series of elevation and speed data to examine the simulated transient response of Pine Island Glacier, Antarctica, to a loss of basal traction as its grounding line retreats. We evaluate a variety of friction laws, which produces a diversity of responses, to determine which best reproduces the observed speedup when forced with the observed thinning...
May 16, 2019: Geophysical Research Letters
Antonio Genova, Sander Goossens, Erwan Mazarico, Frank G Lemoine, Gregory A Neumann, Weijia Kuang, Terence J Sabaka, Steven A Hauck, David E Smith, Sean C Solomon, Maria T Zuber
Geodetic analysis of radio tracking measurements of the MESSENGER spacecraft while in orbit about Mercury has yielded new estimates for the planet's gravity field, tidal Love number, and pole coordinates. The derived right ascension ( α = 281.0082° ± 0.0009°; all uncertainties are 3 standard deviations) and declination ( δ =61.4164° ± 0.0003°) of the spin pole place Mercury in the Cassini state. Confirmation of the equilibrium state with an estimated mean (whole-planet) obliquity ϵ of 1.968 ± 0.027 arcmin enables the confident determination of the planet's normalized polar moment of inertia (0...
April 16, 2019: Geophysical Research Letters
A Malagón-Romero, A Luque
We investigate the emergence of space stems ahead of negative leaders. These are luminous spots that appear ahead of an advancing leader mediating the leader's stepped propagation. We show that space stems start as regions of locally depleted conductivity that form in the streamers of the corona around the leader. An attachment instability enhances the electric field leading to strongly inhomogeneous, bright, and locally warmer regions ahead of the leader that explain the existing observations. Since the attachment instability is only triggered by fields above 10 kV/cm and internal electric fields are lower in positive than in negative streamers, our results explain why, although common in negative leaders, space stems, and stepping are hardly observed if not absent in positive leaders...
April 16, 2019: Geophysical Research Letters
Benjamin M Sanderson, Cameron Wobus, Dave Mills, Claire Zarakas, Allison Crimmins, Marcus C Sarofim, Chris Weaver
The changing risk of extreme precipitation is difficult to project. Events are rare by definition, and return periods of heavy precipitation events are often calculated assuming a stationary climate. Furthermore, ensembles of climate model projections are not large enough to fully categorize the tails of the distribution. To address this, we cluster the contiguous United States into self-similar hydroclimates to estimate changes in the expected frequency of extremely rare events under scenarios of global mean temperature change...
April 16, 2019: Geophysical Research Letters
Zhiqiang Cui, Zhaoxia Pu, Vijay Tallapragada, Robert Atlas, Christopher S Ruf
The NASA Cyclone Global Navigation Satellite System (CYGNSS) was launched in December 2016, providing an unprecedented opportunity to obtain ocean surface wind speeds including wind estimates over the hurricane inner-core region. This study demonstrates the influence of assimilating an early version of CYGNSS observations of ocean surface wind speeds on numerical simulations of two notable landfalling hurricanes, Harvey and Irma (2017). A research version of the National Centers for Environmental Prediction operational Hurricane Weather Research and Forecasting model and the Gridpoint Statistical Interpolation-based hybrid ensemble three-dimensional variational data assimilation system are used...
March 16, 2019: Geophysical Research Letters
Hariprasad D Alwe, Dylan B Millet, Xin Chen, Jonathan D Raff, Zachary C Payne, Kathryn Fledderman
Formic acid (HCOOH) is among the most abundant carboxylic acids in the atmosphere, but its budget is poorly understood. We present eddy flux, vertical gradient, and soil chamber measurements from a mixed forest and apply the data to better constrain HCOOH source/sink pathways. While the cumulative above-canopy flux was downward, HCOOH exchange was bidirectional, with extended periods of net upward and downward flux. Net above-canopy fluxes were mostly upward during warmer/drier periods. The implied gross canopy HCOOH source corresponds to 3% and 38% of observed isoprene and monoterpene carbon emissions and is 15× underestimated in a state-of-science atmospheric model (GEOS-Chem)...
March 16, 2019: Geophysical Research Letters
James Braithwaite, Lars Stixrude
Ab initio molecular dynamics simulations predict that CaSiO3 perovskite melts at 5600 K at 136 GPa, and 6400 K at 300 GPa, significantly higher than MgSiO3 perovskite. The entropy of melting (1.8 kB per atom) is much larger than that of many silicates at ambient pressure and of simple liquids and varies little with pressure. The volume of melting decreases rapidly with increasing pressure, to 3 % at 136 GPa, producing a melting slope that diminishes rapidly with pressure. We determine the melting temperature via the ZW method, combining the Z method, for which we clarify the theoretical basis, with a waiting time analysis...
February 28, 2019: Geophysical Research Letters
Heewon Moon, Benoit P Guillod, Lukas Gudmundsson, Sonia I Seneviratne
Soil moisture-precipitation feedbacks in a large ensemble of global climate model simulations are evaluated. A set of three metrics are used to assess the sensitivity of afternoon rainfall occurrence to morning soil moisture in terms of their spatial, temporal, and heterogeneity characteristics. Positive (negative) spatial feedback indicates that the afternoon rainfall occurs more frequently over wetter (drier) land surface than its surroundings. Positive (negative) temporal feedback indicates preference over temporally wetter (drier) conditions, and positive (negative) heterogeneity feedback indicates preference over more spatially heterogeneous (homogeneous) soil moisture conditions...
February 16, 2019: Geophysical Research Letters
E Boulard, M Harmand, F Guyot, G Lelong, G Morard, D Cabaret, S Boccato, A D Rosa, R Briggs, S Pascarelli, G Fiquet
Recent experiments have demonstrated the existence of previously unknown iron oxides at high pressure and temperature including newly discovered pyrite-type FeO2 and FeO2 Hx phases stable at deep terrestrial lower mantle pressures and temperatures. In the present study, we probed the iron oxidation state in high-pressure transformation products of Fe3+ OOH goethite by in situ X-ray absorption spectroscopy in laser-heated diamond-anvil cell. At pressures and temperatures of ~91 GPa and 1,500-2,350 K, respectively, that is, in the previously reported stability field of FeO2 Hx , a measured shift of -3...
February 16, 2019: Geophysical Research Letters
Ryan S Padrón, Lukas Gudmundsson, Sonia I Seneviratne
Future changes in multidecadal mean water availability, represented as the difference between precipitation and evapotranspiration, remain highly uncertain in ensemble simulations of climate models. Here we identify a physically meaningful relationship between present-day mean precipitation and projected changes in water availability. This suggests that the uncertainty can be reduced by conditioning the ensemble on observed precipitation, which is achieved through a novel probabilistic approach that uses Approximate Bayesian Computation...
January 28, 2019: Geophysical Research Letters
Maria Koroni, Hanneke Paulssen, Jeannot Trampert
We analyze the sensitivity of PP precursor traveltimes that are often used to infer lateral variation in the depths of the 410- and 660-km discontinuities in the mantle. Previous results were inconclusive due to complex wave phenomena, such as multiple energy conversions and focusing/defocusing, that hamper their interpretation. Using spectral-element synthetics and Fréchet derivatives calculated with adjoint methods, we compute sensitivity kernels for volumetric and boundary parameters in a 1-D model for representative epicentral distances of past studies, and a dominant period of 11-25 s...
January 28, 2019: Geophysical Research Letters
Andreas Fichtner, Andrea Zunino
We present a method to explore the effective nullspace of nonlinear inverse problems without Monte Carlo sampling. This is based on the construction of an artificial Hamiltonian system where a model is treated as a high-dimensional particle. Depending on its initial momentum and mass matrix, the particle evolves along a trajectory that traverses the effective nullspace, thereby producing a series of alternative models that are consistent with observations and their uncertainties. Variants of the nullspace shuttle enable hypothesis testing, for example, by adding features or by producing smoother or rougher models...
January 28, 2019: Geophysical Research Letters
Masafumi Imai, Thomas K Greathouse, William S Kurth, G Randall Gladstone, Corentin K Louis, Philippe Zarka, Scott J Bolton, John E P Connerney
Observations of Jovian broadband kilometric (bKOM) radiation and ultraviolet (UV) auroras were acquired with the Waves and Juno-UVS instruments for ∼2 hr over the northern and southern polar regions during Juno's perijoves 4, 5, and 6 passes (PJ4, PJ5, and PJ6). During all six time periods, Juno traversed auroral magnetic field lines connecting to the UV main auroral ovals, matching the estimates of bKOM radio source footprints. The localized bKOM radio sources for the PJ4 north pass map to magnetic field lines having distances of 10 to 12 Jovian radii (R J ) at the magnetic equator, whereas the extended bKOM radio sources for the other events map to field lines extending to 20-61 R J ...
January 28, 2019: Geophysical Research Letters
R W Ebert, T K Greathouse, G Clark, F Allegrini, F Bagenal, S J Bolton, J E P Connerney, G R Gladstone, M Imai, V Hue, W S Kurth, S Levin, P Louarn, B H Mauk, D J McComas, C Paranicas, J R Szalay, M F Thomsen, P W Valek, R J Wilson
We compare electron and UV observations mapping to the same location in Jupiter's northern polar region, poleward of the main aurora, during Juno perijove 5. Simultaneous peaks in UV brightness and electron energy flux are identified when observations map to the same location at the same time. The downward energy flux during these simultaneous observations was not sufficient to generate the observed UV brightness; the upward energy flux was. We propose that the primary acceleration region is below Juno's altitude, from which the more intense upward electrons originate...
January 16, 2019: Geophysical Research Letters
N Žagar, D Jelić, M J Alexander, E Manzini
A new measure of subseasonal variability is introduced that provides a scale-dependent estimation of vertically and meridionally integrated atmospheric variability in terms of the normal modes of linearized primitive equations. Applied to the ERA-Interim data, the new measure shows that subseasonal variability decreases for larger zonal wave numbers. Most of variability is due to balanced (Rossby mode) dynamics but the portion associated with the inertio-gravity (IG) modes increases as the scale reduces. Time series of globally integrated variability anomalies in ERA-Interim show an increase in variability after year 2000...
December 16, 2018: Geophysical Research Letters
E Klein, Z Duputel, D Zigone, C Vigny, J-P Boy, C Doubre, G Meneses
We detected a long-term transient deformation signal between 2014 and 2016 in the Atacama region (Chile) using survey Global Positioning System (GPS) observations. Over an ∼150 km along-strike region, survey GPS measurements in 2014 and 2016 deviate significantly from the interseismic trend estimated using previous observations. This deviation from steady state deformation is spatially coherent and reveals a horizontal westward diverging motion of several centimeters, along with a significant uplift. It is confirmed by continuous measurements of recently installed GPS stations...
November 28, 2018: Geophysical Research Letters
M R Agius, N Harmon, C A Rychert, S Tharimena, J-M Kendall
Accurate marine sediment characteristics, for example, thickness and seismic velocity, are important for constraining sedimentation rates with implications for climate variations and for seismic imaging of deeper structures using ocean bottom seismic deployments. We analyze P-to-S seismic phase conversions from the sediment-crust boundary recorded by the Passive Imaging of the Lithosphere-Asthenosphere Boundary (PI-LAB) experiment to infer the sediment thickness across the Mid-Atlantic Ridge covering 0- to 80-Myr-old seafloor...
November 28, 2018: Geophysical Research Letters
Martin G Mlynczak, Linda A Hunt, B Thomas Marshall, James M Russell
Observations of thermospheric infrared radiative cooling by carbon dioxide (CO2 ) and nitric oxide (NO) from 2002 to 2018 are presented. The time span covers more than 6,000 days including most of solar cycle (SC) 23 and the entirety of SC 24 to date. Maxima of infrared cooling rate profiles (nW/m3 ) are smaller during SC 24 than SC 23, indicating a cooler thermosphere. Rates of global infrared power (W) from CO2 are now at levels observed during the deep solar minimum of 2009. Rates of NO power are still larger than those observed during 2009 and are being maintained at an elevated level by geomagnetic activity...
November 16, 2018: Geophysical Research Letters
C J Smith, R J Kramer, G Myhre, P M Forster, B J Soden, T Andrews, O Boucher, G Faluvegi, D Fläschner, Ø Hodnebrog, M Kasoar, V Kharin, A Kirkevåg, J-F Lamarque, J Mülmenstädt, D Olivié, T Richardson, B H Samset, D Shindell, P Stier, T Takemura, A Voulgarakis, D Watson-Parris
Rapid adjustments are responses to forcing agents that cause a perturbation to the top of atmosphere energy budget but are uncoupled to changes in surface warming. Different mechanisms are responsible for these adjustments for a variety of climate drivers. These remain to be quantified in detail. It is shown that rapid adjustments reduce the effective radiative forcing (ERF) of black carbon by half of the instantaneous forcing, but for CO2 forcing, rapid adjustments increase ERF. Competing tropospheric adjustments for CO2 forcing are individually significant but sum to zero, such that the ERF equals the stratospherically adjusted radiative forcing, but this is not true for other forcing agents...
November 16, 2018: Geophysical Research Letters
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