bluuebunny/crossref_metadata_2025 huggingface dataset:
High‐Resolution Rayleigh Wave Group Velocity Variation Beneath Greenland
Authors: Pourpoint Maeva , Anandakrishnan Sridhar , Ammon Charles J.
Published: January 2018

We present a high‐resolution group velocity model of Greenland from the analysis of fundamental mode Rayleigh waves. Regional and teleseismic events recorded by the Greenland Ice Sheet Monitoring Network seismic network were used and we developed a group velocity correction method to estimate the dispersion within our region of study. The global dispersion model GDM52 from Ekström (2011, https://doi.org/10.1111/j.1365‐246X.2011.05225.x) was used to calculate group delays from the earthquake to the boundaries of our study area. An iterative reweighted generalized least squares approach was then used to invert for the regional group velocity variations between periods of 25 s and 180 s. The group delay correction method helps alleviate the limitations of the sparse Greenland seismic network in a region with poor seismicity. Both the ray coverage and resolution of our model are significantly better than similar studies of Greenland using two‐station methods. Spike tests suggest that features as small as 200 km can be resolved across Greenland. Our dispersion maps are consistent with previous studies and reveal many signatures of known geologic features including known sedimentary basins in Baffin Bay, the West and East Greenland flood basalt provinces, the East and South Greenland Archean blocks. Our model also contains two prominent features: a deep high‐velocity anomaly extending from northwestern to southwestern Greenland that could be the signature of a cratonic root and a low‐velocity anomaly in central eastern Greenland that correlates with the Icelandic plume track and could be associated with lithospheric thinning and upwelling of hot asthenosphere material.
Directional Wave Spectra Observed During Intense Tropical Cyclones
Authors: Collins C. O. , Potter H. , Lund B. , Tamura H. , Graber H. C.
Published: January 2018

Two deep‐sea moorings were deployed 780 km off the coast of southern Taiwan for 4–5 months during the 2010 typhoon season. Directional wave spectra, wind speed and direction, and momentum fluxes were recorded on two Extreme Air‐Sea Interaction buoys during the close passage of Severe Tropical Storm Dianmu and three tropical cyclones (TCs): Typhoon Fanapi, Super Typhoon Megi, and Typhoon Chaba. Conditions sampled include significant wave heights up to 11 m and wind speeds up to 26 m s−1. Details varied for large‐scale spectral structure in frequency and direction but were mostly bimodal. The modes were generally composed of a swell system emanating from the most intense storm region and local wind‐seas. The peak systems were consistently young, meaning actively forced by winds, when the storms were close. During the peaks of the most intense passages—Chaba at the northern mooring and Megi at the southern—the bimodal seas coalesced. During Chaba, the swell and wind‐sea coupling directed the high frequency waves and the wind stress away from the wind direction. A spectral wave model was able reproduce many of the macrofeatures of the directional spectra.
Light Absorption by Suspended Particles in the Red Sea: Effect of Phytoplankton Community Size Structure and Pigment Composition
Authors: Kheireddine Malika , Ouhssain Mustapha , Organelli Emanuele , Bricaud Annick , Jones Burton H.
Published: January 2018

The light absorption properties of phytoplankton (aph(λ)) and nonalgal particles (anap(λ)) associated with phytoplankton pigments were analyzed across the Red Sea, in the upper 200 m depth, between October 2014 and August 2016. The contribution by nonalgal particles to the total particulate light absorption (aph(λ) + anap(λ)) was highly variable (23 ± 17% at 440 nm) and no relationship between anap(440) and chlorophyllaconcentration, [TChla], was observed. Phytoplankton‐specific phytoplankton absorption coefficients at 440 and 676 nm for a given [TChla], aph*(440), and(676) were slightly higher than those derived from average relationships for open ocean waters within the surface layer as well as along the water column. Variations in the concentration of photosynthetic and photoprotective pigments were noticeable by changes in phytoplankton community size structure as well as in(λ). This study revealed that a higher proportion of picophytoplankton and an increase in photoprotective pigments (mainly driven by zeaxanthin) tended to be responsible for the higher(λ) values found in the Red Sea as compared to other oligotrophic regions with similar [TChla]. Understanding this variability across the Red Sea may help improve the accuracy of biogeochemical parameters, such as [TChla], derived from in situ measurements and ocean color remote sensing at a regional scale.
Assessing Flood Risk Under Sea Level Rise and Extreme Sea Levels Scenarios: Application to the Ebro Delta (Spain)
Authors: Sayol J. M. , Marcos M.
Published: January 2018

This study presents a novel methodology to estimate the impact of local sea level rise and extreme surges and waves in coastal areas under climate change scenarios. The methodology is applied to the Ebro Delta, a valuable and vulnerable low‐lying wetland located in the northwestern Mediterranean Sea. Projections of local sea level accounting for all contributions to mean sea level changes, including thermal expansion, dynamic changes, fresh water addition and glacial isostatic adjustment, have been obtained from regionalized sea level projections during the 21st century. Particular attention has been paid to the uncertainties, which have been derived from the spread of the multi‐model ensemble combined with seasonal/inter‐annual sea level variability from local tide gauge observations. Besides vertical land movements have also been integrated to estimate local relative sea level rise. On the other hand, regional projections over the Mediterranean basin of storm surges and wind‐waves have been used to evaluate changes in extreme events. The compound effects of surges and extreme waves have been quantified using their joint probability distributions. Finally, offshore sea level projections from extreme events superimposed to mean sea level have been propagated onto a high resolution digital elevation model of the study region in order to construct flood hazards maps for mid and end of the 21st century and under two different climate change scenarios. The effect of each contribution has been evaluated in terms of percentage of the area exposed to coastal hazards, which will help to design more efficient protection and adaptation measures.
Impacts of Mesoscale Eddies on the Vertical Nitrate Flux in the Gulf Stream Region
Authors: Zhang Shuwen , Curchitser Enrique N. , Kang Dujuan , Stock Charles A. , Dussin Raphael
Published: January 2018

The Gulf Stream (GS) region has intense mesoscale variability that can affect the supply of nutrients to the euphotic zone (Zeu). In this study, a recently developed high‐resolution coupled physical‐biological model is used to conduct a 25‐year simulation in the Northwest Atlantic. The Reynolds decomposition method is applied to quantify the nitrate budget and shows that the mesoscale variability is important to the vertical nitrate supply over the GS region. The decomposition, however, cannot isolate eddy effects from those arising from other mesoscale phenomena. This limitation is addressed by analyzing a large sample of eddies detected and tracked from the 25‐year simulation. The eddy composite structures indicate that positive nitrate anomalies within Zeu exist in both cyclonic eddies (CEs) and anticyclonic eddies (ACEs) over the GS region, and are even more pronounced in the ACEs. Our analysis further indicates that positive nitrate anomalies mostly originate from enhanced vertical advective flux rather than vertical turbulent diffusion. The eddy‐wind interaction‐induced Ekman pumping is very likely the mechanism driving the enhanced vertical motions and vertical nitrate transport within ACEs. This study suggests that the ACEs in GS region may play an important role in modulating the oceanic biogeochemical properties by fueling local biomass production through the persistent supply of nitrate.
Winter Counter‐Wind Transport in the Inner Southwestern Yellow Sea
Authors: Wu Hui , Gu Jinghua , Zhu Ping
Published: January 2018

Coastal currents generally flow downshelf with land on the right side (Northern Hemisphere) under the geostrophic balance, and are often strengthened by downwelling‐favorable winds. However, the recent mooring observation in the inner southwestern Yellow Sea showed that coastal transport direction can be substantially changed by tidal forcing. In the survey, the tidal‐averaged transports at two out of three sites remained northward (i.e., in the upshelf direction) and opposite the downwelling‐favorable northerly wind, except during a brief neap tide period. Numerical experiments showed that the incoming Poincaré wave tide from the East China Sea plays a key role in forming this counter‐wind transport system. This tidal wave produces a shoreward tidal stress south of 33.5°N in the inner southwestern Yellow Sea, driving an upshelf transport under the Earth's rotation. Counterpropagating tidal waves from the East China Sea and the northern Yellow Sea collide in coastal water in 32.5–34°N, which produce a standing tidal wave and therefore a mean sea‐surface setup with alongshore and cross‐shelf scales of both >100 km. This sea‐surface setup causes an alongshore sea surface gradient, which veers the upshelf transport to the offshore direction under geostrophic balance. The strong tidal current increases the tidal‐mean bottom resistance in the SCW, thus reduces the wind‐driven current to a magnitude smaller than the tide‐induced residual transport velocity. Therefore, upshelf transport persists in the inner southwestern Yellow Sea, and the Changjiang River Estuary becomes a major source area for the inner southwestern Yellow Sea.
Correlation Lengths for Estimating the Large‐Scale Carbon and Heat Content of the Southern Ocean
Authors: Mazloff M. R. , Cornuelle B. D. , Gille S. T. , Verdy A.
Published: January 2018

The spatial correlation scales of oceanic dissolved inorganic carbon, heat content, and carbon and heat exchanges with the atmosphere are estimated from a realistic numerical simulation of the Southern Ocean. Biases in the model are assessed by comparing the simulated sea surface height and temperature scales to those derived from optimally interpolated satellite measurements. While these products do not resolve all ocean scales, they are representative of the climate scale variability we aim to estimate. Results show that constraining the carbon and heat inventory between 35°S and 70°S on time‐scales longer than 90 days requires approximately 100 optimally spaced measurement platforms: approximately one platform every 20° longitude by 6° latitude. Carbon flux has slightly longer zonal scales, and requires a coverage of approximately 30° by 6°. Heat flux has much longer scales, and thus a platform distribution of approximately 90° by 10° would be sufficient. Fluxes, however, have significant subseasonal variability. For all fields, and especially fluxes, sustained measurements in time are required to prevent aliasing of the eddy signals into the longer climate scale signals. Our results imply a minimum of 100 biogeochemical‐Argo floats are required to monitor the Southern Ocean carbon and heat content and air‐sea exchanges on time‐scales longer than 90 days. However, an estimate of formal mapping error using the current Argo array implies that in practice even an array of 600 floats (a nominal float density of about 1 every 7° longitude by 3° latitude) will result in nonnegligible uncertainty in estimating climate signals.
Inversion of In Situ Light Absorption and Attenuation Measurements to Estimate Constituent Concentrations in Optically Complex Shelf Seas
Authors: Ramírez‐Pérez M. , Twardowski M. , Trees C. , Piera J. , McKee D.
Published: January 2018

A deconvolution approach is presented to use spectral light absorption and attenuation data to estimate the concentration of the major nonwater compounds in complex shelf sea waters. The inversion procedure requires knowledge of local material‐specific inherent optical properties (SIOPs) which are determined from natural samples using a bio‐optical model that differentiates between Case I and Case II waters and uses least squares linear regression analysis to provide optimal SIOP values. A synthetic data set is used to demonstrate that the approach is fundamentally consistent and to test the sensitivity to injection of controlled levels of artificial noise into the input data. Self‐consistency of the approach is further demonstrated by application to field data collected in the Ligurian Sea, with chlorophyll (Chl), the nonbiogenic component of total suspended solids (TSSnd), and colored dissolved organic material (CDOM) retrieved with RMSE of 0.61 mg m−3, 0.35 g m−3, and 0.02 m−1, respectively. The utility of the approach is finally demonstrated by application to depth profiles of in situ absorption and attenuation data resulting in profiles of optically significant constituents with associated error bar estimates. The advantages of this procedure lie in the simple input requirements, the avoidance of error amplification, full exploitation of the available spectral information from both absorption and attenuation channels, and the reasonably successful retrieval of constituent concentrations in an optically complex shelf sea.
Dynamic Topography and Sea Level Anomalies of the Southern Ocean: Variability and Teleconnections
Authors: Armitage Thomas W. K. , Kwok Ron , Thompson Andrew F. , Cunningham Glenn
Published: January 2018

This study combines sea surface height (SSH) estimates of the ice‐covered Southern Ocean with conventional open‐ocean SSH estimates from CryoSat‐2 to produce monthly composites of dynamic ocean topography (DOT) and sea level anomaly (SLA) on a 50 km grid spanning 2011–2016. This data set reveals the full Southern Ocean SSH seasonal cycle for the first time; there is an antiphase relationship between sea level on the Antarctic continental shelf and the deeper basins, with coastal SSH highest in autumn and lowest in spring. As a result of this pattern of seasonal SSH variability, the barotropic component of the Antarctic Slope Current (ASC) has speeds that are regionally up to twice as fast in the autumn. Month‐to‐month circulation variability of the Ross and Weddell Gyres is strongly influenced by the local wind field, and is correlated with the local wind curl (Ross: −0.58; Weddell: −0.67). SSH variability is linked to both the Southern Oscillation and the Southern Annular Mode, dominant modes of southern hemisphere climate variability. In particular, during the strong 2015–2016 El Niño, a sustained negative coastal SLA of up to −6 cm, implying a weakening of the ASC, was observed in the Pacific sector of the Southern Ocean. The ability to examine sea level variability in the seasonally ice‐covered regions of the Southern Ocean—climatically important regions with an acute sparsity of data—makes this new merged sea level record of particular interest to the Southern Ocean oceanography and glaciology communities.
Seismic Oceanography's Failure to Flourish: A Possible Solution
Authors: Ruddick B. R.
Published: January 2018

A recent paper in Journal of Geophysical Research: Oceans used multichannel seismic observations to map estimates of internal wave mixing in the Gulf of Mexico, finding greatly enhanced mixing over the slope region. These results suggest that the ocean margins may supply the mixing required to close the global thermohaline circulation, and the techniques demonstrated here might be used to map mixing over much of the world's continental shelves. The use of multichannel seismics to image ocean phenomena is nearly 15 years old, and despite the initial promise, the techniques have not become as broadly used as initially expected. We discuss possible reasons for this, and suggest an alternative approach that might gain broader success.
Frozen‐Droplets Aggregation at Temperature Below −40°C
Authors: Pedernera Débora Analía , Ávila Eldo E.
Published: January 2018

The onset of the aggregation process of frozen droplets was investigated in laboratory settings. The experiments were conducted in a cloud chamber controlled at temperatures cooler than −40°C, where pure water droplets freeze spontaneously without the need for ice nucleating particles. We present laboratory evidence supporting that the aggregation process can occur for frozen droplet sizes around 10 μm in diameter and at concentrations observed in the cloud chamber of 70 ± 20 cm−3, which can be found in some regions of anvil cirrus. The characteristics and morphology of the aggregates were examined in detail. Additional experiments performed with electrically charged droplets show that the aggregation processes can be significantly accelerated, suggesting that the mechanism of collision and adhesion could be related to electrical forces generated by different charge distributions or dipole interactions between the interacting ice surfaces. The current work aims at advancing our fundamental understanding of the aggregation process of frozen droplets, which is necessary for understanding the cloud microphysical processes.
Impact of Grain Shape and Multiple Black Carbon Internal Mixing on Snow Albedo: Parameterization and Radiative Effect Analysis
Authors: He Cenlin , Liou Kuo‐Nan , Takano Yoshi , Yang Ping , Qi Ling , Chen Fei
Published: January 2018

We quantify the effects of grain shape and multiple black carbon (BC)‐snow internal mixing on snow albedo by explicitly resolving shape and mixing structures. Nonspherical snow grains tend to have higher albedos than spheres with the same effective sizes, while the albedo difference due to shape effects increases with grain size, with up to 0.013 and 0.055 for effective radii of 1,000 μm at visible and near‐infrared bands, respectively. BC‐snow internal mixing reduces snow albedo at wavelengths < ~1.5 μm, with negligible effects at longer wavelengths. Nonspherical snow grains show less BC‐induced albedo reductions than spheres with the same effective sizes by up to 0.06 at ultraviolet and visible bands. Compared with external mixing, internal mixing enhances snow albedo reduction by a factor of 1.2–2.0 at visible wavelengths depending on BC concentration and snow shape. The opposite effects on albedo reductions due to snow grain nonsphericity and BC‐snow internal mixing point toward a careful investigation of these two factors simultaneously in climate modeling. We further develop parameterizations for snow albedo and its reduction by accounting for grain shape and BC‐snow internal/external mixing. Combining the parameterizations with BC‐in‐snow measurements in China, North America, and the Arctic, we estimate that nonspherical snow grains reduce BC‐induced albedo radiative effects by up to 50% compared with spherical grains. Moreover, BC‐snow internal mixing enhances the albedo effects by up to 30% (130%) for spherical (nonspherical) grains relative to external mixing. The overall uncertainty induced by snow shape and BC‐snow mixing state is about 21–32%.
Modulating Effects of Mesoscale Oceanic Eddies on Sea Surface Temperature Response to Tropical Cyclones Over the Western North Pacific
Authors: Ma Zhanhong , Fei Jianfang , Huang Xiaogang , Cheng Xiaoping
Published: January 2018

The impact of mesoscale oceanic eddies on the temporal and spatial characteristics of sea surface temperature (SST) response to tropical cyclones is investigated in this study based on composite analysis of cyclone‐eddy interactions over the western North Pacific. The occurrence times of maximum cooling, recovery time, and spatial patterns of SST response are specially evaluated. The influence of cold‐core eddies (CCEs) renders the mean occurrence time of maximum SST cooling to become about half a day longer than that in eddy‐free condition, while warm‐core eddies (WCEs) have little effect on this facet. The recovery time of SST cooling also takes longer in presence of CCEs, being overall more pronounced for stronger or slower tropical cyclones. The effect of WCEs on the recovery time is again not significant. The modulation of maximum SST decrease by WCEs for category 2–5 storms is found to be remarkable in the subtropical region but not evident in the tropical region, while the role of CCEs is remarkable in both regions. The CCEs are observed to change the spatial characteristics of SST response, with enhanced SST decrease initially at the right side of storm track. During the recovery period the strengthened SST cooling by CCEs propagates leftward gradually, with a feature similar as both the westward‐propagating eddies and the recovery of cold wake. These results underscore the importance of resolving mesoscale oceanic eddies in coupled numerical models to improve the prediction of storm‐induced SST response.
Numerical Investigation on Absorption Enhancement of Black Carbon Aerosols Partially Coated With Nonabsorbing Organics
Authors: Zhang Xiaolin , Mao Mao , Yin Yan , Wang Bin
Published: January 2018

This study numerically evaluates the effects of aerosol microphysics, including coated volume fraction of black carbon (BC), shell/core ratio, and size distribution, on the absorption enhancement (Eab) of polydisperse BC aggregates partially coated by organics, which is calculated by the exact multiple‐sphere T‐matrix method. The coated volume fraction of BC plays a substantial role in determining the absorption enhancement of partially coated BC aggregates, which typically have an Eab in the range of ~1.0–2.0 with a larger value for larger coated volume fraction of BC as the shell/core ratio, BC geometry, and size distribution are fixed. The shell/core ratio, BC geometry, and size distribution have little impact on the Eab of coated BC with small coated volume fraction of BC, while they become significant for large coated volume fraction of BC. The Eab of partially coated BC particles can be slightly less than 1.0 for the large BC in the accumulation mode exhibiting large shell/core ratio and small coated volume fraction of BC, indicating that the absorption shows even slight decrease relative to uncoated BC particles. For partially coated BC aggregates in the accumulation and coarse modes, the refractive index uncertainties of BC result in the Eab differences of less than 9% and 2%, respectively, while those of organics can induce larger variations with the maximum differences up to 22% and 18%, respectively. Our study indicates that accounting for particle coating microphysics, particularly the coated volume fraction of BC, can potentially help to understand the differences in observations of largely variable absorption enhancements over various regions.
Spring Snow‐Albedo Feedback Analysis Over the Third Pole: Results From Satellite Observation and CMIP5 Model Simulations
Authors: Guo Hui , Wang Xiaoyi , Wang Tao , Ma Yaoming , Ryder James , Zhang Taotao , Liu Dan , Ding Jinzhi , Li Yue , Piao Shilong
Published: January 2018

The snow‐albedo feedback is a crucial component in high‐altitude cryospheric change but is poorly quantified over the Third Pole, encompassing the Karakoram and Tibetan Plateau. Here we present an analysis of present‐day and future spring snow‐albedo feedback over the Third Pole, using a 28 year satellite‐based albedo and the latest climate model simulations. We show that present‐day spring snow‐albedo feedback strength is primarily determined by the decrease in albedo due to snow metamorphosis, rather than that due to reduced snow cover in the Karakoram, but not found in Southeastern Tibet. We further demonstrate an emergent relationship between snow‐albedo feedback from the seasonal cycle and that from climate change across models. Combined with contemporary satellite‐based snow‐albedo feedback from seasonal cycle, this relationship enables us to estimate that the feedback strength for the Karakoram with a relatively high glaciated area is −2.42 ± 0.48% K−1 under an unmitigated scenario, which is much stronger than that for Southeastern Tibet (−1.64 ± 0.48% K−1) and for the Third Pole (−0.89 ± 0.44% K−1), respectively. Moreover, it is noteworthy that the magnitude of the constrained strength is only half of the unconstrained model estimate for the Third Pole, suggesting that current climate models generally overestimate the feedback of spring snow change to temperature change based on the unmitigated scenario.
Hydrological Cycle in the Heihe River Basin and Its Implication for Water Resource Management in Endorheic Basins
Authors: Li Xin , Cheng Guodong , Ge Yingchun , Li Hongyi , Han Feng , Hu Xiaoli , Tian Wei , Tian Yong , Pan Xiaoduo , Nian Yanyun , Zhang Yanlin , Ran Youhua , Zheng Yi , Gao Bing , Yang Dawen , Zheng Chunmiao , Wang Xusheng , Liu Shaomin , Cai Ximing
Published: January 2018

Endorheic basins around the world are suffering from water and ecosystem crisis. To pursue sustainable development, quantifying the hydrological cycle is fundamentally important. However, knowledge gaps exist in how climate change and human activities influence the hydrological cycle in endorheic basins. We used an integrated ecohydrological model, in combination with systematic observations, to analyze the hydrological cycle in the Heihe River Basin, a typical endorheic basin in arid region of China. The water budget was closed for different landscapes, river channel sections, and irrigation districts of the basin from 2001 to 2012. The results showed that climate warming, which has led to greater precipitation, snowmelt, glacier melt, and runoff, is a favorable factor in alleviating water scarcity. Human activities, including ecological water diversion, cropland expansion, and groundwater overexploitation, have both positive and negative effects. The natural oasis ecosystem has been restored considerably, but the overuse of water in midstream and the use of environmental flow for agriculture in downstream have exacerbated the water stress, resulting in unfavorable changes in surface‐ground water interactions and raising concerns regarding how to fairly allocate water resources. Our results suggest that the water resource management in the region should be adjusted to adapt to a changing hydrological cycle, cropland area must be reduced, and the abstraction of groundwater must be controlled. To foster long‐term benefits, water conflicts should be handled from a broad socioeconomic perspective. The findings can provide useful information on endorheic basins to policy makers and stakeholders around the world.
Signatures of Lithospheric Flexure and Elevated Heat Flow in Stereo Topography at Coronae on Venus
Authors: O'Rourke Joseph G. , Smrekar Suzanne E.
Published: January 2018

Signatures of lithospheric flexure were previously identified at a dozen or more large coronae on Venus. Thin plate models fit to topographic profiles return elastic parameters, allowing derivation of mechanical thickness and surface heat flows given an assumed yield strength envelope. However, the low resolution of altimetry data from the NASA Magellan mission has hindered studying the vast majority of coronae, particularly those less than a few hundred kilometers in diameter. Here we search for flexural signatures around 99 coronae over ∼20% of the surface in Magellan altimetry data and stereo‐derived topography that was recently assembled from synthetic aperture radar images. We derive elastic thicknesses of ∼2 to 30 km (mostly ∼5 to 15 km) with Cartesian and axisymmetric models at 19 coronae. We discuss the implications of low values that were also noted in earlier gravity studies. Most mechanical thicknesses are estimated as <19 km, corresponding to thermal gradients >24 K km−1. Implied surface heat flows >95 mW m−2—twice the global average in many thermal evolution models—imply that coronae are major contributors to the total heat budget or Venus is cooling faster than expected. Binomial statistics show that “Type 2” coronae with incomplete fracture annuli are significantly less likely to host flexural signatures than “Type 1” coronae with largely complete annuli. Stress calculations predict extensional faulting where nearly all profiles intersect concentric fractures. We failed to identify systematic variations in flexural parameters based on type, geologic setting, or morphologic class. Obtaining quality, high‐resolution topography from a planetwide survey is vital to verifying our conclusions.
Nonlocal Sediment Transport on Steep Lateral Moraines, Eastern Sierra Nevada, California, USA
Authors: Doane Tyler H. , Furbish David Jon , Roering Joshua J. , Schumer Rina , Morgan Daniel J.
Published: January 2018

Recent work has highlighted the significance of long‐distance particle motions in hillslope sediment transport. Such motions imply that the flux at a given hillslope position is appropriately described as a weighted function of surrounding conditions that influence motions reaching the given position. Although the idea of nonlocal sediment transport is well grounded in theory, limited field evidence has been provided. We test local and nonlocal formulations of the flux and compare their ability to reproduce land surface profiles of steep moraines in California. We show that nonlocal and nonlinear models better reproduce evolved land surface profiles, notably the amount of lowering and concavity near the moraine crest and the lengthening and straightening of the depositional apron. The analysis provides the first estimates of key parameters that set sediment entrainment rates and travel distances in nonlocal formulations and highlights the importance of correctly specifying the entrainment rate when modeling land surface evolution. Moraine evolution associated with nonlocal and nonlinear transport formulations, when described in terms of the evolution of the Fourier transform of the moraine surface, displays a distinct behavior involving growth of certain wave numbers, in contrast to the decay of all wave numbers associated with linear transport. Nonlinear and nonlocal formulations share key mathematical elements yielding a nonlinear relation between the flux and the land surface slope.
Comparison of Big‐Leaf, Two‐Big‐Leaf, and Two‐Leaf Upscaling Schemes for Evapotranspiration Estimation Using Coupled Carbon‐Water Modeling
Authors: Luo Xiangzhong , Chen Jing M. , Liu Jane , Black T. Andrew , Croft Holly , Staebler Ralf , He Liming , Arain M. Altaf , Chen Bin , Mo Gang , Gonsamo Alemu , McCaughey Harry
Published: January 2018

Evapotranspiration (ET) is commonly estimated using the Penman‐Monteith equation, which assumes that the plant canopy is a big leaf (BL) and the water flux from vegetation is regulated by canopy stomatal conductance (Gs). However, BL has been found to be unsuitable for terrestrial biosphere models built on the carbon‐water coupling principle because it fails to capture daily variations of gross primary productivity (GPP). A two‐big‐leaf scheme (TBL) and a two‐leaf scheme (TL) that stratify a canopy into sunlit and shaded leaves have been developed to address this issue. However, there is a lack of comparison of these upscaling schemes for ET estimation, especially on the difference between TBL and TL. We find that TL shows strong performance (r2 = 0.71, root‐mean‐square error = 0.05 mm/h) in estimating ET at nine eddy covariance towers in Canada. BL simulates lower annual ET and GPP than TL and TBL. The biases of estimated ET and GPP increase with leaf area index (LAI) in BL and TBL, and the biases of TL show no trends with LAI. BL miscalculates the portions of light‐saturated and light‐unsaturated leaves in the canopy, incurring negative biases in its flux estimation. TBL and TL showed improved yet different GPP and ET estimations. This difference is attributed to the lower Gs and intercellular CO2 concentration simulated in TBL compared to their counterparts in TL. We suggest to use TL for ET modeling to avoid the uncertainty propagated from the artificial upscaling of leaf‐level processes to the canopy scale in BL and TBL.
Geological and Chemical Factors that Impacted the Biological Utilization of Cobalt in the Archean Eon
Authors: Moore Eli K. , Hao Jihua , Prabhu Anirudh , Zhong Hao , Jelen Ben I. , Meyer Mike , Hazen Robert M. , Falkowski Paul G.
Published: January 2018

The geosphere and biosphere coevolved and influenced Earth's biological and mineralogical diversity. Changing redox conditions influenced the availability of different transition metals, which are essential components in the active sites of oxidoreductases, proteins that catalyze electron transfer reactions across the tree of life. Despite its relatively low abundance in the environment, cobalt (Co) is a unique metal in biology due to its importance to a wide range of organisms as the metal center of vitamin B12 (aka cobalamin, Cbl). Cbl is vital to multiple methyltransferase enzymes involved in energetically favorable metabolic pathways. It is unclear how Co availability is linked to mineral evolution and weathering processes. Here we examine important biological functions of Co, as well as chemical and geological factors that may have influenced the utilization of Co early in the evolution of life. Only 66 natural minerals are known to contain Co as an essential element. However, Co is incorporated as a minor element in abundant rock‐forming minerals, potentially representing a reliable source of Co as a trace element in marine systems due to weathering processes. We developed a mineral weathering model that indicates that dissolved Co was potentially more bioavailable in the Archean ocean under low S conditions than it is today. Mineral weathering, redox chemistry, Co complexation with nitrogen‐containing organics, and hydrothermal environments were crucial in the incorporation of Co in primitive metabolic pathways. These chemical and geological characteristics of Co can inform the biological utilization of other trace metals in early forms of life.
MODIS‐Based Estimates of Global Terrestrial Ecosystem Respiration
Authors: Ai Jinlong , Jia Gensuo , Epstein Howard E. , Wang Hesong , Zhang Anzhi , Hu Yonghong
Published: January 2018

Terrestrial ecosystem respiration (Reco) represents a large carbon source from land to atmosphere and is highly spatiotemporally heterogeneous across scales. Upscaling of field‐measured respiration data using remote sensing information is urgently needed for understanding regional and global patterns of ecosystem respiration. Using Moderate Resolution Imaging Spectroradiometer (MODIS) data with resolutions of 1 km and 8 days and flux measurements from 171 sites (total of 812 site years) across the world from 2000 to 2014, we developed a semiempirical, yet physiologically based, remote sensing model, which can simulate Reco observed across most biomes with a small margin of error (R2 = 0.55, root‐mean‐square error = 1.67 gCm−2d−1, efficiency = 0.46, and mean bias error = 0.18 gCm−2d−1). The reference respiration at the annual mean nighttime land surface temperature (LST) can be well represented by MODIS enhanced vegetation index and LST. A comprehensive comparison of six respiration‐temperature (R‐T) models shows that the more physiologically based R‐T model (extended Arrhenius model) may be most suitable for estimating the respiration rate at higher latitudes. Integrating an effect of vegetation change on Reco in different biomes effectively improves estimates of Reco in almost all of the biomes.
An Environmentally Forced Tropical Cyclone Hazard Model
Authors: Lee Chia‐Ying , Tippett Michael K. , Sobel Adam H. , Camargo Suzana J.
Published: January 2018

A new statistical‐dynamical model is developed for estimating the long‐term hazard of rare, high impact tropical cyclones events globally. There are three components representing the complete storm lifetime: an environmental index‐based genesis model, a beta‐advection track model, and an autoregressive intensity model. All three components depend upon the local environmental conditions, including potential intensity, relative sea surface temperature, 850 and 250 hPa steering flow, deep‐layer mean vertical shear, 850 hPa vorticity, and midlevel relative humidity. The hazard model, using 400 realizations of a 32 year period (approximately 3,000 storms per realization), captures many aspects of tropical cyclone statistics, such as genesis and track density distribution. Of particular note, it simulates the observed number of rapidly intensifying storms, a challenging issue in tropical cyclone modeling and prediction. Using the return period curve of landfall intensity as a measure of local tropical cyclone hazard, the model reasonably simulates the hazard in the western north Pacific (coastal regions of the Philippines, China, Taiwan, and Japan) and the Caribbean islands. In other regions, the observed return period curve can be captured after a local landfall frequency adjustment that forces the total number of landfalls to be the same as that observed while allowing the model to freely simulate the distribution of intensities at landfall.
Concurrent Deformation in the Longmen Shan and the Sichuan Basin: A Critical Wedge Captured by Limit Analysis
Authors: Zhang Zhen , Zhang Huai , Wang Liangshu , Cheng Huihong , Shi Yaolin , Leroy Yves M.
Published: January 2018

The southern Longmen Shan region has seismically active faults in its western part, the Beichuan fault (BCF) and the Pengguan fault (PGF), both rooting on a deep décollement, and has flat topography in its eastern part that marks the current deformation front due to thrusting from a shallow décollement. The concurrent faulting at the rear and at the front is separated by the flat Sichuan Basin. The objective of this contribution is to propose a mechanical approach based on the limit analysis theory to estimate the frictional properties that could explain how this heterogeneous wedge could be critical in the sense that faulting occurs at various places concurrently. A simple prototype with bilinear topography is first proposed to construct analytical solutions. The activations of the BCF, the PGF, and of the upper décollement up to the deformation front are examples of collapse mechanisms captured analytically by the limit analysis. Deformation maps are constructed in the space of the friction angles of the upper and lower décollements, identifying the domain of activation of the various collapse mechanisms. The critical conditions in terms of friction angles for the concomitant activation of these various collapse mechanisms are established. A more realistic prototype is then analyzed with the numerical code OptumG2, which is also based on the limit analysis theory. The deformation maps established for the first prototype are then applied to evaluate the critical conditions of the complex prototype with a limited number of numerical simulations.
Evidence for 1.5 km of Uplift of the Central Anatolian Plateau's Southern Margin in the Last 450 kyr and Implications for Its Multiphased Uplift History
Authors: Öğretmen Nazik , Cipollari Paola , Frezza Virgilio , Faranda Costanza , Karanika Konstantina , Gliozzi Elsa , Radeff Giuditta , Cosentino Domenico
Published: January 2018

At the southern margin of the Central Anatolian Plateau (CAP), marine deposits that overlie the Central Tauride units at up to 2 km of elevation were used to constrain the onset of uplift to the middle‐late Miocene. This study demonstrates that much younger marine deposits cap the southern margin. We recognize the Last Common Occurrence ofNeogloboquadrinaspp. (sin) (0.61 Ma) andPseudoemiliania lacunosa(0.467 Ma), which points to an early middle Pleistocene age. The benthic fauna indicates an epibathyal marine environment (400 to 500 m paleodepth), with an associated paleocoastline now at ~1,500 to 1,600 m above sea level. Our new results imply uplift rates of up to 3.21–3.42 mm/yr for the CAP southern margin since the deposition of the young marine units. In the area, the evaluation of late Pleistocene and Holocene uplift rates of ~1 mm/yr points to a post early middle Pleistocene short‐lived period of rapid uplift of the CAP southern margin, which can correlate the short‐lived surface uplift signal in numerical models of slab breakoff. Overall, this work demonstrates that the majority of the modern topography at the CAP southern margin (1,500 to 1,600 m) was only recently acquired, pointing to the absence of a significant orographic barrier along the southern plateau margin prior to 500 ka. The multiphased uplift recognized at the CAP southern margin by previous authors, as well as the fast uplift rate documented in this work, can be linked to lithosphere delamination and subsequent slab breakoff during the Arabian‐Anatolian continental collision.
Minimized Capillary End Effect During CO2 Displacement in 2‐D Micromodel by Manipulating Capillary Pressure at the Outlet Boundary in Lattice Boltzmann Method
Authors: Kang Dong Hun , Yun Tae Sup
Published: January 2018

We propose a new outflow boundary condition to minimize the capillary end effect for a pore‐scale
displacement simulation. The Rothman‐Keller lattice Boltzmann method with multi‐relaxation time is implemented to manipulate a nonflat wall and inflow‐outflow boundaries with physically acceptable fluid properties in 2‐D microfluidic chip domain. Introducing a mean capillary pressure acting at
‐water interface to the nonwetting fluid at the outlet effectively prevents
injection pressure from suddenly dropping upon
breakthrough such that the continuous
invasion and the increase of
saturation are allowed. This phenomenon becomes most pronounced at capillary number of logCa = −5.5, while capillary fingering and massive displacement of
prevail at low and high capillary numbers, respectively. Simulations with different domain length in homogeneous and heterogeneous domains reveal that capillary pressure and
saturation near the inlet are reproducible compared with those with a proposed boundary condition. The residual
saturation uniquely follows the increasing tendency with increasing capillary number, corroborated by experimental evidences. The determination of the mean capillary pressure and its sensitivity are also discussed. The proposed boundary condition is commonly applicable to other pore‐scale simulations to accurately capture the spatial distribution of nonwetting fluid and corresponding displacement ratio.
Modeling of Future Changes in Seasonal Snowpack and Impacts on Summer Low Flows in Alpine Catchments
Authors: Jenicek Michal , Seibert Jan , Staudinger Maria
Published: January 2018

It is expected that an increasing proportion of the precipitation will fall as rain in alpine catchments in the future. Consequently, snow storage is expected to decrease, which, together with changes in snowmelt rates and timing, might cause reductions in spring and summer low flows. The objectives of this study were (1) to simulate the effect of changing snow storage on low flows during the warm seasons and (2) to relate drought sensitivity to the simulated snow storage changes at different elevations. The Swiss Climate Change Scenarios 2011 data set was used to derive future changes in air temperature and precipitation. A typical bucket‐type catchment model, HBV‐light, was applied to 14 mountain catchments in Switzerland to simulate streamflow and snow in the reference period and three future periods. The largest relative decrease in annual maximum SWE was simulated for elevations below 2,200 m a.s.l. (60–75% for the period 2070–2099) and the snowmelt season shifted by up to 4 weeks earlier. The relative decrease in spring and summer minimum runoff that was caused by the relative decrease in maximum SWE (i.e., elasticity), reached 40–90% in most of catchments for the reference period and decreased for the future periods. This decreasing elasticity indicated that the effect of snow on summer low flows is reduced in the future. The fraction of snowmelt runoff in summer decreased by more than 50% at the highest elevations and almost disappeared at the lowest elevations. This might have large implications on water availability during the summer.
Uncertainty Quantification and Global Sensitivity Analysis of Subsurface Flow Parameters to Gravimetric Variations During Pumping Tests in Unconfined Aquifers
Authors: Maina Fadji Zaouna , Guadagnini Alberto
Published: January 2018

We study the contribution of typically uncertain subsurface flow parameters to gravity changes that can be recorded during pumping tests in unconfined aquifers. We do so in the framework of a Global Sensitivity Analysis and quantify the effects of uncertainty of such parameters on the first four statistical moments of the probability distribution of gravimetric variations induced by the operation of the well. System parameters are grouped into two main categories, respectively, governing groundwater flow in the unsaturated and saturated portions of the domain. We ground our work on the three‐dimensional analytical model proposed by Mishra and Neuman (2011), which fully takes into account the richness of the physical process taking place across the unsaturated and saturated zones and storage effects in a finite radius pumping well. The relative influence of model parameter uncertainties on drawdown, moisture content, and gravity changes are quantified through (a) the Sobol' indices, derived from a classical decomposition of variance and (b) recently developed indices quantifying the relative contribution of each uncertain model parameter to the (ensemble) mean, skewness, and kurtosis of the model output. Our results document (i) the importance of the effects of the parameters governing the unsaturated flow dynamics on the mean and variance of local drawdown and gravity changes; (ii) the marked sensitivity (as expressed in terms of the statistical moments analyzed) of gravity changes to the employed water retention curve model parameter, specific yield, and storage, and (iii) the influential role of hydraulic conductivity of the unsaturated and saturated zones to the skewness and kurtosis of gravimetric variation distributions. The observed temporal dynamics of the strength of the relative contribution of system parameters to gravimetric variations suggest that gravity data have a clear potential to provide useful information for estimating the key hydraulic parameters of the system.
Regional Variability of Nitrate Fluxes in the Unsaturated Zone and Groundwater, Wisconsin, USA
Authors: Green Christopher T. , Liao Lixia , Nolan Bernard T. , Juckem Paul F. , Shope Christopher L. , Tesoriero Anthony J. , Jurgens Bryant C.
Published: January 2018

Process‐based modeling of regional
fluxes to groundwater is critical for understanding and managing water quality, but the complexity of
reactive transport processes makes implementation a challenge. This study introduces a regional vertical flux method (VFM) for efficient estimation of reactive transport of
in the vadose zone and groundwater. The regional VFM was applied to 443 well samples in central‐eastern Wisconsin. Chemical measurements included O2,
, N2 from denitrification, and atmospheric tracers of groundwater age including carbon‐14, chlorofluorocarbons, tritium, and tritiogenic helium. VFM results were consistent with observed chemistry, and calibrated parameters were in‐line with estimates from previous studies. Results indicated that (1) unsaturated zone travel times were a substantial portion of the transit time to wells and streams, (2) since 1945 fractions of applied N leached to groundwater have increased for manure‐N, possibly due to increased injection of liquid manure, and decreased for fertilizer‐N, and (3) under current practices and conditions, approximately 60% of the shallow aquifer will eventually be affected by downward migration of
, with denitrification protecting the remaining 40%. Recharge variability strongly affected the unsaturated zone lag times and the eventual depth of the
front. Principal components regression demonstrated that VFM parameters and predictions were significantly correlated with hydrogeochemical landscape features. The diverse and sometimes conflicting aspects of N management (e.g., limiting N volatilization versus limiting N losses to groundwater) warrant continued development of large‐scale holistic strategies to manage water quality and quantity.