bluuebunny/crossref_metadata_2025_split huggingface dataset:
Data Assimilation in Density‐Dependent Subsurface Flows via Localized Iterative Ensemble Kalman Filter
Authors: Xia Chuan‐An , Hu Bill X. , Tong Juxiu , Guadagnini Alberto 
Published: August 2018

AbstractParameter estimation in variable‐density groundwater flow systems is confronted with challenges of strong nonlinearity and heavy computational burden. Relying on a variant of the Henry problem, we evaluate the performance of a domain localization scheme of the iterative ensemble Kalman filter in the framework of data assimilation settings for variable‐density groundwater flows in a seawater intrusion scenario. The performance of the approach is compared against (a) the corresponding domain localization scheme of the ensemble Kalman filter in its standard formulation as well as (b) a covariance localization scheme of the latter. The equivalent freshwater head, hf, and salinity, Sa, are set as the target state variables. The randomly heterogeneous field of equivalent freshwater hydraulic conductivity, Kf, is considered as the system parameter field. Density‐independent and density‐driven flow settings are considered to evaluate the assimilation results using various methods and data. When only hf data are assimilated, all tested approaches perform generally well and a localization scheme embedded in the iterative ensemble Kalman filter appears to consistently outperform the domain localized version of the standard ensemble Kalman filter (EnKF) in a density‐driven scenario; Dirichlet boundary conditions tend to show a more pronounced negative effect on estimating Kf for density‐independent than for density‐dependent flow conditions; hf data are more informative in a density‐dependent than in a density‐independent setting. The sole use of Sa information does not yield satisfactory updates of hf for the covariance localization scheme of the standard EnKF, while the sole use of hf does. The domain localization scheme leads to difficulties in the attainment of global filter convergence when only Sa data are used. A covariance localization scheme associated with a standard EnKF can significantly alleviate this issue.
Floodplain Backwater Effect on Overtopping Induced Fluvial Dike Failure
Authors: Rifai Ismail , El Kadi Abderrezzak Kamal , Erpicum Sebastien , Archambeau Pierre , Violeau Damien , Pirotton Michel , Dewals Benjamin 
Published: August 2018

AbstractFluvial dikes (i.e., levees) can be damaged by overtopping flows, leading to breach formation and severe floods. Accurate prediction of the breach geometry and outflow hydrograph is crucial to achieve sound inundation risk assessment and mitigating measures. A set of laboratory experiments were performed to study overtopping induced fluvial dike breaching. The effects of floodplain tailwater on the breach expansion and outflow were investigated. Results are compared to previous experiments under different main channel inflow discharges combined with a free floodplain. The dike breaching process follows three stages: gradual start of overtopping and slow initiation of dike erosion, fast deepening and widening of the breach with highly transient flows, and flow quasi‐stabilization with or without sustained breach expansion. Using the recorded data, a simplified regression model relating the breach stabilization time and the final breach width to the floodplain confinement and inflow Froude number in the main channel is proposed.
How Data Set Characteristics Influence Ocean Carbon Export Models
Authors: Bisson K. M. , Siegel D. A. , DeVries T. , Cael B. B. , Buesseler K. O. 
Published: August 2018

AbstractOcean biological processes mediate the transport of roughly 10 petagrams of carbon from the surface to the deep ocean each year and thus play an important role in the global carbon cycle. Even so, the globally integrated rate of carbon export out of the surface ocean remains highly uncertain. Quantifying the processes underlying this biological carbon export requires a synthesis between model predictions and available observations of particulate organic carbon (POC) flux; yet the scale dissimilarities between models and observations make this synthesis difficult. Here we compare carbon export predictions from a mechanistic model with observations of POC fluxes from several data sets compiled from the literature spanning different space, time, and depth scales as well as using different observational methodologies. We optimize model parameters to provide the best match between model‐predicted and observed POC fluxes, explicitly accounting for sources of error associated with each data set. Model‐predicted globally integrated values of POC flux at the base of the euphotic layer range from 3.8 to 5.5 Pg C/year, depending on the data set used to optimize the model. Modeled carbon export pathways also vary depending on the data set used to optimize the model, as well as the satellite net primary production data product used to drive the model. These findings highlight the importance of collecting field data that average over the substantial natural temporal and spatial variability in carbon export fluxes, and advancing satellite algorithms for ocean net primary production, in order to improve predictions of biological carbon export.
Nonlinear Drift Resonance Between Charged Particles and Ultralow Frequency Waves: Theory and Observations
Authors: Li Li , Zhou Xu‐Zhi , Omura Yoshiharu , Wang Zi‐Han , Zong Qiu‐Gang , Liu Ying , Hao Yi‐Xin , Fu Sui‐Yan , Kivelson Margaret G. , Rankin Robert , Claudepierre Seth G. , Wygant John R. 
Published: August 2018

AbstractIn Earth's inner magnetosphere, electromagnetic waves in the ultralow frequency (ULF) range play an important role in accelerating and diffusing charged particles via drift resonance. In conventional drift resonance theory, linearization is applied under the assumption of weak wave‐particle energy exchange so particle trajectories are unperturbed. For ULF waves with larger amplitudes and/or durations, however, the conventional theory becomes inaccurate since particle trajectories are strongly perturbed. Here we extend the drift resonance theory into a nonlinear regime, to formulate nonlinear trapping of particles in a wave‐carried potential well, and predict the corresponding observable signatures such as rolled‐up structures in particle energy spectrum. After considering how this manifests in particle data with finite energy resolution, we compare the predicted signatures with Van Allen Probes observations. Their good agreement provides the first observational evidence for the occurrence of nonlinear drift resonance, highlighting the importance of nonlinear effects in magnetospheric particle dynamics under ULF waves.
Intense Electric Fields and Electron‐Scale Substructure Within Magnetotail Flux Ropes as Revealed by the Magnetospheric Multiscale Mission
Authors: Stawarz J. E. , Eastwood J. P. , Genestreti K. J. , Nakamura R. , Ergun R. E. , Burgess D. , Burch J. L. , Fuselier S. A. , Gershman D. J. , Giles B. L. , Le Contel O. , Lindqvist P.‐A. , Russell C. T. , Torbert R. B. 
Published: August 2018

AbstractThree flux ropes associated with near‐Earth magnetotail reconnection are analyzed using Magnetospheric Multiscale observations. The flux ropes are Earthward propagating with sizes from ∼3 to 11 ion inertial lengths. Significantly different axial orientations are observed, suggesting spatiotemporal variability in the reconnection and/or flux rope dynamics. An electron‐scale vortex, associated with one of the most intense electric fields (E) in the event, is observed within one of the flux ropes. This E is predominantly perpendicular to the magnetic field (B); the electron vortex is frozen‐in with E × B drifting electrons carrying perpendicular current and causing a small‐scale magnetic enhancement. The vortex is ∼16 electron gyroradii in size perpendicular to B and potentially elongated parallel to B. The need to decouple the frozen‐in vortical motion from the surrounding plasma implies a parallel E at the structure's ends. The formation of frozen‐in electron vortices within reconnection‐generated flux ropes may have implications for particle acceleration.
Evaluating the Contribution of Land‐Atmosphere Coupling to Heat Extremes in CMIP5 Models
Authors: Ukkola A. M. , Pitman A. J. , Donat M. G. , De Kauwe M. G. , Angélil O. 
Published: August 2018

AbstractLand‐atmosphere coupling can amplify heat extremes under declining soil moisture. Here we evaluate this coupling in 25 Coupled Model Intercomparison Project Phase 5 models using flux tower observations over Europe and North America. We compared heat extremes (2.5% of the hottest days of the year) and the evaporative fraction (EF; a measure of land surface dryness) on the day the heat extremes occurred. We found a negative relationship between the magnitude of heat extremes and EF in both models and observations in transitional regions, with the hottest temperatures occurring during the driest days, with a similar but less certain relationship in dry regions. Surprisingly, many models also showed an amplification of heat extremes by low EF in wet regions, a finding not supported by observations. Many models may therefore overamplify heat extremes over wet regions by overestimating the strength of land‐atmosphere coupling, with consequences for future projections of heat extremes.
Climate Constraints on Glaciation Over High‐Mountain Asia During the Last Glacial Maximum
Authors: Yan Qing , Owen Lewis A. , Wang Huijun , Zhang Zhongshi 
Published: August 2018

AbstractReasons for restricted and nonuniform glaciation over High‐Mountain Asia (HMA) during the global last glacial maximum (LGM; ~28‐23 ka) have been intensively studied but remain elusive. Using a 1‐km‐resolution ice sheet model, we show that glaciers across HMA exhibit high region variability in glaciation. Glaciers in western and southern HMA are the most sensitive to climate change and those in the interior are the least sensitive. Our model broadly reproduces the restricted glaciation across HMA during the LGM, although it overestimates the extent of glaciation over western and southern HMA as compared with reconstructions. Modeled decreases in precipitation hampers glacier growth over northern HMA, while insufficient cooling hampers glacier advance over eastern HMA for the LGM. Both reduced precipitation and insufficient cooling inhibit large‐scale glaciation over inner HMA. Moreover, climatic conditions conducive to glaciation across the entire HMA include a reduction in temperature of ~10 °C and an increase in precipitation, unlikely to have occurred during any Quaternary glacial maximum.
Temporal Dynamics of Aerodynamic Canopy Height Derived From Eddy Covariance Momentum Flux Data Across North American Flux Networks
Authors: Chu Housen , Baldocchi Dennis D. , Poindexter Cristina , Abraha Michael , Desai Ankur R. , Bohrer Gil , Arain M. Altaf , Griffis Timothy , Blanken Peter D. , O'Halloran Thomas L. , Thomas R. Quinn , Zhang Quan , Burns Sean P. , Frank John M. , Christian Dold , Brown Shannon , Black T. Andrew , Gough Christopher M. , Law Beverly E. , Lee Xuhui , Chen Jiquan , Reed David E. , Massman William J. , Clark Kenneth , Hatfield Jerry , Prueger John , Bracho Rosvel , Baker John M. , Martin Timothy A. 
Published: August 2018

AbstractAerodynamic canopy height (ha) is the effective height of vegetation canopy for its influence on atmospheric fluxes and is a key parameter of surface‐atmosphere coupling. However, methods to estimate ha from data are limited. This synthesis evaluates the applicability and robustness of the calculation of ha from eddy covariance momentum‐flux data. At 69 forest sites, annual ha robustly predicted site‐to‐site and year‐to‐year differences in canopy heights (R2 = 0.88, 111 site‐years). At 23 cropland/grassland sites, weekly ha successfully captured the dynamics of vegetation canopies over growing seasons (R2 > 0.70 in 74 site‐years). Our results demonstrate the potential of flux‐derived ha determination for tracking the seasonal, interannual, and/or decadal dynamics of vegetation canopies including growth, harvest, land use change, and disturbance. The large‐scale and time‐varying ha derived from flux networks worldwide provides a new benchmark for regional and global Earth system models and satellite remote sensing of canopy structure.
Dusty Space Plasma Diagnosis Using the Behavior of Polar Mesospheric Summer Echoes During Electron Precipitation Events
Authors: Mahmoudian A. , Senior A. , Scales W. A. , Kosch M. J. , Rietveld M. T. 
Published: August 2018

AbstractThe behavior of polar mesospheric summer echoes (PMSEs) during an electron precipitation event is investigated by including dusty plasma effects for the first time. The observational data recorded with the very high frequency (224 MHz) and ultrahigh frequency (930 MHz) radars at the European Incoherent SCATter Scientific Association on 10 and 11 July 2012 are presented. The observed radar echoes show that the PMSEs are both correlated and anticorrelated with the increased electron density associated with electron precipitation events on the two consecutive days. The experimental observations are compared with numerical simulations of the temporal evolution of PMSE with different background dusty plasma parameters during the electron precipitation event. Specifically, the effect of dust radius, dust density, recombination/photoionization rates, photo‐detachment current, and electron density enhancement ratio on the behavior of a PMSE layer and the associated dust charging process in the course of electron precipitation events is studied. It is observed that the ratio of electron density fluctuation amplitude δne to the plasma density (ne) plays a critical role in the appearance/disappearance of the layer. The simulation results revealed that the existence of PMSE is mainly determined by dust radius and dust density. The dusty plasma parameters associated with each event are estimated. The condensation nuclei of the ice particles such as proton hydrate clusters (H+(H2O)n) or meteoric smoke particles can be determined by employing the microphysical models along with the dusty plasma simulations. This can resolve any discrepancy in the description of the observed phenomena.
Nighttime Enhancements in the Midlatitude Ionosphere and Their Relation to the Plasmasphere
Authors: Li Quanhan , Hao Yongqiang , Zhang Donghe , Xiao Zuo 
Published: August 2018

AbstractIn situ electron density measurements by the CHAllenging Minisatellite Payload and the Defense Meteorological Satellites Program F17 satellites show that the midlatitude ionization at altitudes of ∼350 and 850 km is enhanced in the late evening. The enhancements increase to maximum around midnight and are clearly observed till early morning as the equatorial ionization decays to minimal level. They appear in the winter hemisphere during June and December solstices and in both hemispheres during equinox. The enhancements are well confined between ±30° and ±50° magnetic latitude, with the magnetic flux tubes of L = 1.3 − 2.4 connecting to the plasmasphere. Furthermore, coincident longitudinal variations exist in both the ionospheric enhancements and the plasmaspheric total electron content, especially during the solstice months. The coincidence may suggest essential plasma transport between the ionosphere and the plasmasphere. These facts support the idea that the plasmasphere provides extra plasma to the midlatitude ionosphere through downward plasma influx along the magnetic field lines to form the nighttime ionization enhancements when the sunlight is absent.
Response of Relativistic Electron Microbursts to the Arrival of High‐Speed Solar Wind Streams and its Relation to Flux Variation of Trapped Radiation Belt Electrons
Authors: Kurita S. , Miyoshi Y. , Blake J. B. , Friedel R. H. W. 
Published: August 2018

AbstractRelativistic electron microbursts are frequently observed during high‐speed solar wind stream (HSS) events. However, solar wind parameters, which are important for enhanced microburst precipitation, have not been well understood. We perform a superposed epoch analysis of microburst occurrence frequency during HSS events, considering the north/south component of the interplanetary magnetic field (IMF Bz) and solar wind speed. The IMF Bz dependence is investigated considering the Russell‐McPherron effect following the method used by Miyoshi and Kataoka (2008, https://doi.org/10.1029/2007JA012506). We find that microbursts are most frequently observed during faster (>500 km/s) solar wind speed streams with the IMF Bz shifted southward (fast SBZ‐HSS events), indicating that both fast solar wind speed and the southward IMF Bz are important for enhanced microburst precipitation. The occurrence frequency of chorus waves during the HSS events is investigated to gain an understanding of the different behavior of microburst activity in response to the HSS events. We show that relativistic electron microbursts are not always a good proxy for chorus waves, but a proxy for a subset of chorus that can resonate with MeV electrons. We also show that electron fluxes with energies from hundreds of keV to several MeV preferentially increase during the fast SBZ‐HSS events. These results indicate that observation of an enhanced occurrence of relativistic electron microbursts could be used as a proxy for the direct observation of the acceleration of MeV electrons by chorus waves in the heart of the outer radiation belt, not just a proxy for chorus wave activity in general.
Experimental Investigation of Total Bottom Shear Stress for Oscillatory Flows Over Sand Ripples
Authors: Yuan Jing , Wang Dongxu 
Published: August 2018

AbstractShoaling waves often produce vortex ripples on a sandy seabed, which significantly increases the local bottom shear stress. To improve the quantitative understanding of this phenomenon, a full‐scale experimental study is conducted using an oscillatory water tunnel. In the tests, sinusoidal oscillatory flows generate two‐dimensional uniform sand ripples from a 9‐m‐long movable bed made of coarse sand. The total bottom shear stress is estimated from the change of water pressure for driving the oscillatory flow. Flow around individual ripples is also measured using particle image velocimetry. After correction for imperfect flow generation and facility vibration, the leading three odd harmonics of total bottom shear stress are kept in the final measurements. The first harmonic is the dominant one and is generally in phase with the free‐stream velocity. Its amplitude decreases as ripples are washed‐off by increasing flow intensity. The third and the fifth harmonics are about 10–20% of the first harmonic in amplitude. Their presence makes the intraperiod variation of total bottom shear stress quite wavy with three peaks within one half period. Flow measurements suggest that these peaks are closely related to coherent vortex motions. For two selected tests, total bottom shear stress is estimated from velocity measurements via a control volume analysis. The results are in good agreement with those from the pressure‐based technique, which verifies both approaches. A new predictor for the equivalent sand grain roughness in Humbyrd's (2012, http://hdl.handle.net/1721.1/78234) formula for wave friction factor is calibrated based on the obtained measurements of maximum total bottom shear stress.
The Interannual Variability of the Breakdown of Fall Stratification on the New Jersey Shelf
Authors: Forsyth Jacob , Gawarkiewicz Glen , Andres Magdalena , Chen Ke 
Published: August 2018

AbstractDuring the seasonal evolution of stratification on the New Jersey shelf in the fall, strong thermal stratification that was established in the preceding summer is broken down through wind‐driven processes and surface cooling. Ten years of output from a Regional Ocean Modeling Systems run and a one‐dimensional mixed layer model is used here to examine the interannual variability in the strength of the stratification and in the processes that reduce stratification in fall. Our analysis shows that the strength of the stratification at the end of the summer is not correlated with the timing of shelf destratification. This indicates that processes that occur within the fall are more important for the timing of stratification breakdown than are the initial fall conditions. Furthermore, wind‐driven processes reduce a greater fraction of the stratification in each year than does the surface cooling during the fall. Winds affect the density gradients on the shelf through both changes to the temperature and salinity fields. Processes associated with the downwelling‐favorable winds are more effective than those during upwelling‐favorable winds in breaking down the vertical density gradients. In the first process, cross‐shelf advective fluxes during storms act to decrease stratification during downwelling‐favorable winds and increase stratification during upwelling‐favorable winds. Second, there is also enhanced velocity shear during downwelling‐favorable winds, which allows for more shear instabilities that break down stratification via mixing. Observational data and model output from Tropical Storm Ernesto compare favorably and suggest that downwelling‐favorable winds act through the mechanisms identified from the Regional Ocean Modeling Systems results.
Interannual Eddy Kinetic Energy Modulations in the Agulhas Return Current
Authors: Zhu Yanan , Qiu Bo , Lin Xiaopei , Wang Fan 
Published: August 2018

AbstractInterannual variability in the mesoscale eddy field in the Agulhas Return Current (ARC) of 32–42°S and 15–35°E is investigated based on satellite altimeter observations and state estimate from the Estimating the Circulation and Climate of the Ocean, Phase II from 1993 to 2016. It is found that the interannual modulation of eddy kinetic energy in the ARC region is externally mediated by the wind stress forcing that generates the westward propagating sea surface height anomalies across the South Indian Ocean subtropical gyre. The wind‐forced sea surface height anomalies influence the upstream Agulhas Current volume transports. By modulating the intensity of barotropic instability of the ARC mean flow centered around the retroflection region, the Agulhas Current inflow variability leads to the downstream interannual eddy kinetic energy fluctuations in the ARC region.
The Impact of Boreal Summer ENSO Events on Tropical Lower Stratospheric Ozone
Authors: Tweedy Olga V. , Waugh Darryn W. , Randel William J. , Abalos Marta , Oman Luke D. , Kinnison Doug E. 
Published: August 2018

AbstractThe interannual variability of tropical lower stratosphere ozone and its connections to sea surface temperatures in the equatorial Pacific are examined using a combination of chemistry climate model simulations, satellite observations, and reanalyses. The model simulations and observations show large differences in the magnitude of interannual variability in ozone between northern tropic (NT; EQ‐18° N) and southern tropic (EQ‐18° S) during boreal summer but small differences in winter. The interannual variability during boreal summer is highly correlated with summer sea surface temperatures in the eastern and central Pacific Ocean and El Niño–Southern Oscillation (ENSO) events. Larger variability in NT ozone is primarily due to meridional advection, connected to the changes in the onset date and strength of the Asian summer monsoon anticyclone. The Asian summer monsoon anticyclone forms earlier in a season and tends to be stronger during cold (La Niña) events leading to more isentropic transport of ozone from the extratropics into the NT, with the reverse for warm (El Niño) events.