bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling centuries of estuarine morphodynamics in the Western Scheldt estuary
Authors: Dam G. , van der Wegen M. , Labeur R. J. , Roelvink D.
Published: April 2016

We hindcast a 110 year period (1860–1970) of morphodynamic behavior of the Western Scheldt estuary by means of a 2‐D, high‐resolution, process‐based model and compare results to a historically unique bathymetric data set. Initially, the model skill decreases for a few decades. Against common perception, the model skill increases after that to become excellent after 110 years. We attribute this to the self‐organization of the morphological system which is reproduced correctly by the numerical model. On time scales exceeding decades, the interaction between the major tidal forcing and the confinement of the estuary overrules other uncertainties. Both measured and modeled bathymetries reflect a trend of decreasing energy dissipation, less morphodynamic activity, and thus a more stable morphology over time, albeit that the estuarine adaptation time is long (approximately centuries). Process‐based models applied in confined environments and under constant forcing conditions may perform well especially on long (greater than decades) time scales.
Soft X‐ray irradiance measured by the Solar Aspect Monitor on the Solar Dynamic Observatory Extreme ultraviolet Variability Experiment
Authors: Lin C. Y. , Bailey S. M. , Jones A. , Woodraska D. , Caspi A. , Woods T. N. , Eparvier F. G. , Wieman S. R. , Didkovsky L. V.
Published: April 2016

The Solar Aspect Monitor (SAM) is a pinhole camera on the Extreme ultraviolet Variability Experiment (EVE) aboard the Solar Dynamics Observatory. SAM projects the solar disk onto the CCD through a metallic filter designed to allow only solar photons shortward of 7 nm to pass. Contamination from energetic particles and out‐of‐band irradiance is, however, significant in the SAM observations. We present a technique for isolating the 0.01–7 nm integrated irradiance from the SAM signal to produce the first results of broadband irradiance for the time period from May 2010 to May 2014. The results of this analysis agree with a similar data product from EVE's EUV SpectroPhotometer to within 25%. We compare our results with measurements from the Student Nitric Oxide Explorer Solar X‐ray Photometer and the Thermosphere Ionosphere Mesosphere Energetics and Dynamics Solar EUV Experiment at similar levels of solar activity. We show that the full‐disk SAM broadband results compared well to the other measurements of the 0.01–7 nm irradiance. We also explore SAM's capability toward resolving spatial contribution from regions of solar disk in irradiance and demonstrate this feature with a case study of several strong flares that erupted from active regions on 11 March 2011.
O+ ion conic and plasma sheet dynamics observed by Van Allen Probe satellites during the 1 June 2013 magnetic storm
Authors: Burke W. J. , Erickson P. J. , Yang J. , Foster J. , Wygant J. , Reeves G. , Kletzing C.
Published: April 2016

The Van Allen Probe satellites were near apogee in the late evening local time sector during the 1 June 2013 magnetic storm's main phase. About an hour after crossing the ring current's “nose structure” into the plasma sheet, the satellites encountered a quasiperiodic sequence of 0.08–3 keV O+ ions. Pitch angle distributions of this population consistently peaked nearly antiparallel to the local magnetic field. We interpret this population as O+ conics originating in the northern ionosphere. Sequences began as fairly steady state conic fluxes with energies in the ~ 80 to 100 eV range. Over about a half hour buildup phase, O+ energies peaked near 1 keV. During subsequent release phases lasting ~ 20 min, O+ energies returned to low‐energy starting points. We argue these observations reflect repeated formations and dissolutions of downward, magnetically aligned electric fields (ε||) layers trapping O+ conics between mirror points within heating layers below and electrostatic barriers above. Nearly identical variations were observed at the locations of both satellites during 9 of these 13 conic cycles. Phase differences between cycles were observed at both spacecraft during the remaining events. Most “buildup” to “release” phase transitions coincided with AL index minima. However, in situ magnetometer measurements indicate only weak dipolarizations of tail‐like magnetic fields. The lack of field‐aligned reflected O+ and tail‐like magnetic fields suggest that both ionospheres may be active. However, Southern Hemisphere origin conics cannot be observed since they would be isotropized and accelerated during neutral sheet crossings.
Observations of the migrating semidiurnal and quaddiurnal tides from the RAIDS/NIRS instrument
Authors: Azeem Irfan , Walterscheid Richard L. , Crowley Geoff , Bishop Rebecca L. , Christensen Andrew B.
Published: April 2016

In this paper we analyze temperature data from the Near‐Infrared Spectrometer (NIRS) instrument on Remote Atmospheric and Ionospheric Detection System experiment on the International Space Station and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) on the Thermosphere Ionosphere Mesosphere Energetics Dynamics satellite during June and July 2010 to investigate structures of the migrating semidiurnal (12 h) and quaddiurnal (6 h) tides in the upper mesosphere and lower thermosphere. Temperature measurements from the NIRS and SABER instruments allow us to examine the tides from the stratosphere to the lower thermosphere. We find that the amplitude of the migrating 6 h tide grows from ~5 K near 100 km altitude to ~30 K near 130 km. The amplitudes of the tide at altitudes accessible by NIRS are much larger than those previously reported at lower altitudes from the Aura Microwave Limb Sounder and the SABER instruments. The amplitude of the 12 h tide in the NIRS data shows two peaks in the lower thermosphere (between 95 and 130 km) with a maximum around 60 K occurring in the winter hemisphere near 20° latitude and a second maximum around 40 K occurring in the summer hemisphere near 30° latitude. The structure of the migrating terdiurnal (8 h) tide is also investigated in the NIRS data and shows increasing amplitude with altitude over a broad range of latitudes, roughly between 50°N and 30°S. Altitudinal variations seen in the 6, 8, and 12 h tides suggest an evolving mix of various Hough modes.
Chorus and chorus‐like emissions seen by the ionospheric satellite DEMETER
Authors: Parrot Michel , Santolík Ondřej , Nĕmec František
Published: April 2016

A lot of different emissions have been detected by the low‐altitude satellite DEMETER (Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions), and the aim of this paper is to study extremely low frequency (ELF) electromagnetic waves with elements drifting in frequency. It is shown that only some of them can be considered as usual chorus. These chorus elements are emitted in the equatorial plane, and their propagation analysis indicates that they are going downward at low altitudes in the ionosphere to be detected by the satellite. The study of one remarkable event recorded along the same orbit in both the Northern and the Southern Hemispheres on 8 May 2008 indicates that this propagation mechanism is reinforced at the location of the ionospheric trough, which corresponds to the plasmapause at higher altitudes. It has been observed that usual chorus elements at low frequencies are always in a frequency band which overlaps with a hiss band limited by a frequency cutoff close to the proton gyrofrequency. Other drifting elements can be attributed to emissions triggered by PLHR (power line harmonic radiation). It means that without a high‐resolution spectral analysis, chorus‐like elements triggered by PLHR can be wrongly considered as natural chorus. These drifting elements can also appear as filamentary structures emerging at the upper frequencies of a hiss band or quasiperiodic emissions. There are events where the elements even have certain similarities to quasiperiodic emissions. The difference between these elements and the chorus emissions will be emphasized.
Evolution of stress‐induced borehole breakout in inherently anisotropic rock: Insights from discrete element modeling
Authors: Duan K. , Kwok C. Y.
Published: April 2016

The aim of this study is to better understand the mechanisms controlling the initiation, propagation, and ultimate pattern of borehole breakouts in shale formation when drilled parallel with and perpendicular to beddings. A two‐dimensional discrete element model is constructed to explicitly represent the microstructure of inherently anisotropic rocks by inserting a series of individual smooth joints into an assembly of bonded rigid discs. Both isotropic and anisotropic hollow square‐shaped samples are generated to represent the wellbores drilled perpendicular to and parallel with beddings at reduced scale. The isotropic model is validated by comparing the stress distribution around borehole wall and along X axis direction with analytical solutions. Effects of different factors including the particle size distribution, borehole diameter, far‐field stress anisotropy, and rock anisotropy are systematically evaluated on the stress distribution and borehole breakout propagation. Simulation results reveal that wider particle size distribution results in the local stress perturbations which cause localization of cracks. Reduction of borehole diameter significantly alters the crack failure from tensile to shear and raises the critical pressure. Rock anisotropy plays an important role on the stress state around wellbore which lead to the formation of preferred cracks under hydrostatic stress. Far‐field stress anisotropy plays a dominant role in the shape of borehole breakout when drilled perpendicular to beddings while a secondary role when drilled parallel with beddings. Results from this study can provide fundamental insights on the underlying particle‐scale mechanisms for previous findings in laboratory and field on borehole stability in anisotropic rock.
Magnetotelluric investigation of the Vestfold Hills and Rauer Group, East Antarctica
Authors: Peacock J. R. , Selway K.
Published: April 2016

The Vestfold Hills and Rauer Group in East Antarctica have contrasting Archean to Neoproterozoic geological histories and are believed to be juxtaposed along a suture zone that now lies beneath the Sørsdal Glacier. Exact location and age of this suture zone are unknown, as is its relationship to regional deformation associated with the amalgamation of East Gondwana. To image the suture zone, magnetotelluric (MT) data were collected in Prydz Bay, East Antarctica, mainly along a profile crossing the Sørsdal Glacier and regions inland of the Vestfold Hills and Rauer Group islands. Time‐frequency analysis of the MT time series yielded three important observations: (1) Wind speeds in excess of ∼8 m/s reduce coherence between electric and magnetic fields due to charged wind‐blown particles of ice and snow. (2) Estimation of the MT transfer function is best between 1000 and 1400 UT when ionospheric Hall currents enhance the magnetic source field. (3) Nonplanar source field effects were minimal but detectable and removed from estimation of the MT transfer function. Inversions of MT data in 2‐D and 3‐D produce similar resistivity models, where structures in the preferred 3‐D resistivity model correlate strongly with regional magnetic data. The electrically conductive Rauer Group is separated from the less conductive Vestfold Hills by a resistive zone under the Sørsdal Glacier, which is interpreted to be caused by oxidation during suturing. Though a suture zone has been imaged, no time constrains on suturing can be made from the MT data.
The Cenozoic growth of the Qilian Shan in the northeastern Tibetan Plateau: A sedimentary archive from the Jiuxi Basin
Authors: Wang Weitao , Zhang Peizhen , Pang Jianzhang , Garzione Carmala , Zhang Huiping , Liu Caicai , Zheng Dewen , Zheng Wenjun , Yu Jingxing
Published: April 2016

Sedimentary deposits in Tibetan Basins archive the spatial‐temporal patterns of the deformation and surface uplift processes that created the area's high topography during the Cenozoic India‐Asia collision. In this study, new stratigraphic investigation of the Caogou section from the Jiuxi Basin in the northeasternmost part of Tibetan Plateau provides chronologic constraints on the deformation and northward growth of the plateau. Magnetostratigraphic analysis results suggest that the age of the studied ~1000 m thick section spans from ~24.2 Ma to 2.8 Ma. Detailed sedimentology and apatite fission track (AFT) analyses reveal that variations in the clast provenance, lithofacies, sediment accumulation rates, and AFT lag times occurred at ~13.5–10.5 Ma. We interpret these changes as in response to the initial uplift of the North Qilian Shan. In addition, paleomagnetic declination results from the section indicate a clockwise rotation of the Jiuxi Basin before ~13.5 Ma, which was followed by a subsequent counterclockwise rotation during 13.5–9 Ma. This reversal in rotation direction may be directly related to left‐lateral strike‐slip activity along the easternmost segment of the Altyn Tagh Fault. Combined with previous studies, we suggest that movement on the western part of the Altyn Tagh Fault was probably initiated during the Oligocene (>30 Ma) and that fault propagation to its eastern tip occurred during the middle‐late Miocene.
New insights into Phanerozoic tectonics of south China: Part 1, polyphase deformation in the Jiuling and Lianyunshan domains of the central Jiangnan Orogen
Authors: Li Jianhua , Dong Shuwen , Zhang Yueqiao , Zhao Guochun , Johnston Stephen T. , Cui Jianjun , Xin Yujia
Published: April 2016

The central Jiangnan Orogen, genetically formed by the Proterozoic Yangtze‐Cathaysia collision, presents as a composite structural feature in the Phanerozoic with multiple ductile and brittle fabrics whose geometries, kinematics, and ages are crucial to decipher the tectonic evolution of south China. New structural observations coupled with thermochronological and geochronological studies of these fabrics document four main stages of deformation. The earliest stage in early Paleozoic time (460–420 Ma) corresponds to combined E‐trending dextral and northwest directed thrust shearing that was variably partitioned in anastomosing high‐strain zones under greenschist‐facies conditions (~400–500°C), related to the continued Yangtze‐Cathaysia convergence externally driven by the suturing of south China with Australia. This event was heterogeneously overprinted by the second stage characterized by ~E‐oriented folding in middle Triassic time, geodynamically resulting from the continental collision of south China with Indochina and North China. The third stage was locally developed by northwest and southeast vergent thrusts that truncated ~E‐oriented folds in the Late Jurassic, due to northwestward subduction of the Paleo‐Pacific plate. The latest stage involved normal faulting and tectonic unroofing in Cretaceous time, which resulted in basin opening and reset footwall40Ar/39Ar ages in proximity to the Hengshan detachment fault, associated with roll‐back of the subducting Paleo‐Pacific plate.
Focusing of baroclinic tidal energy in a canyon
Authors: Vlasenko Vasiliy , Stashchuk Nataliya , Inall Mark E. , Porter Marie , Aleynik Dmitry
Published: April 2016

Strong three‐dimensional focusing of internal tidal energy in the Petite Sole Canyon in the Celtic Sea is analyzed using observational data and numerical modeling. In a deep layer (500–800 m) in the center of the canyon, shear variance was elevated by an order of magnitude. Corresponding large vertical oscillations of deep isotherms and a local maximum of horizontal velocity were replicated numerically using the MITgcm. The elevated internal tidal activity in the deep part of the canyon is explained in terms of the downward propagation and focusing of multiple internal tidal beams generated at the shelf break. The near‐circular shape of the canyon head and steep bottom topography throughout the canyon (steeper than the tidal beam) create favorable conditions for the lens‐like focusing of tidal energy in the canyon's center. Observations and modeling show that the energy focusing greatly intensifies local diapycnal mixing that leads to local formation of a baroclinic eddy.
Turbulent flow field and air entrainment in laboratory plunging breaking waves
Authors: Na Byoungjoon , Chang Kuang‐An , Huang Zhi‐Cheng , Lim Ho‐Joon
Published: April 2016

This paper presents laboratory measurements of turbulent flow fields and void fraction in deep‐water plunging breaking waves using imaging and optical fiber techniques. Bubble‐size distributions are also determined based on combined measurements of velocity and bubble residence time. The most excited mode of the local intermittency measure of the turbulent flow and its corresponding length scale are obtained using a wavelet‐based method and found to correlate with the swirling strength and vorticity. Concentrated vortical structures with high intermittency are observed near the lower boundaries of the aerated rollers where the velocity shear is high; the length scale of the deduced eddies ranges from 0.05 to 0.15 times the wave height. The number of bubbles with a chord length less than 2 mm demonstrates good correlation with the swirling strength. The power‐law scaling and the Hinze scale of the bubbles determined from the bubble chord length distribution compare favorably with existing measurements. The turbulent dissipation rate, accounting for void fraction, is estimated using mixture theory. When void fraction is not considered, the turbulent dissipation rate is underestimated by more than 70% in the initial impinging and the first splash‐up roller. A significant discrepancy of approximately 67% between the total energy dissipation rate and the turbulence dissipation rate is found. Of this uncounted dissipation, 23% is caused by bubble‐induced dissipation.
Infragravity waves on fringing reefs in the tropical Pacific: Dynamic setup
Authors: Becker J. M. , Merrifield M. A. , Yoon H.
Published: April 2016

Cross‐shore pressure and current observations from four fringing reefs of lengths ranging from 135 to 420 m reveal energetic low‐frequency (∼0.001–0.05 Hz) motions. The spatial structure and temporal amplitudes of an empirical orthogonal function analysis of the pressure measurements suggest the dominant low‐frequency variability is modal. Incoming and outgoing linear flux estimates also support partially standing modes on the reef flat during energetic events. A cross‐covariance analysis suggests that breakpoint forcing excites these partially standing modes, similar to previous findings at other steep reefs. The dynamics of Symonds et al. (1982) with damping are applied to a step reef, with forcing obtained by extending a point break model of Vetter et al. (2010) for breaking wave setup to the low‐frequency band using the shoaled envelope of the incident free surface elevation. A one parameter, linear analytical model for the reef flat free surface elevation is presented, which describes between 75% and 97% of the variance of the observed low‐frequency shoreline significant wave height for all reefs considered over a range of conditions. The linear model contains a single dimensionless parameter that is the ratio of the inertial to dissipative time scales, and the observations from this study exhibit more low‐frequency variability when the dissipative time scale is greater than the inertial time scale for the steep reefs considered.
Hygroscopicity and CCN activity of atmospheric aerosol particles and their relation to organics: Characteristics of urban aerosols in Nagoya, Japan
Authors: Kawana Kaori , Nakayama Tomoki , Mochida Michihiro
Published: April 2016

The size‐resolved distributions of hygroscopic growth factor g and the ratios of cloud condensation nuclei (CCN) to condensation nuclei of atmospheric aerosols were investigated in Nagoya, Japan. The average of the distributions of g at 85% relative humidity was bimodal. The size‐resolved mean κ derived from g showed an increasing trend with diameter: 0.17–0.33 at 24–359 nm. The κ values calculated from CCN activation curves were 37% higher than those derived from g. Only 9% of the 37% difference is explained by the difference in the κ of inorganics under subsaturated and supersaturated conditions, suggesting a contribution of organics to the remaining 28% difference. The size‐averaged κ of organics (κorg) was calculated as 0.14 and 0.19 by two different methods. The number fractions of CCN predicted from the hygroscopicity data over the range of 24–359 nm are loosely consistent with those observed if the size‐ and time‐averaged g is applied to all particles (differences: −30% to +10%). This consistency improves if size‐ and time‐resolved g and g distribution are used (differences: −19% to −3%). Whereas the number fractions of CCN predicted from the composition data are greatly underestimated if organics are assumed to be insoluble (differences: −64% to −45%), they are more consistent if κorg of 0.14 or 0.19 is applied (differences: −10% to +14%). The results demonstrate the importance of the dependence of the g of particles on time and particle size and the hygroscopicity of organics for CCN number concentrations in the urban atmosphere.
Why do oceanic negative cloud‐to‐ground lightning exhibit larger peak current values?
Authors: Chronis T. , Koshak W. , McCaul E.
Published: April 2016

This study examines the temporal (monthly) and spatial climatology (2004–2010) of the first return stroke of the cloud‐to‐ground (CG) lightning flash peak current (Ip) across various land/water boundaries over the contiguous United States. Four regions are examined: the Gulf of Mexico (region 1), the Florida peninsula (region 2), Lake Michigan (region 3), and part of the U.S. Mid‐Atlantic (region 4). The crosss across the coastlines of regions 1, 2, and 4 show a gradual oceanward increase in the mean negative polarity CG peak current values (−Ip). This transition along the respective land/ocean boundaries is not sharp but gradual. In direct contrast with ocean, there is no consistent behavior in −Ip values as we move from land out across the fresh water of Lake Michigan (region 3). Meanwhile, the positive CG flash peak current (+Ip) values do not exhibit a consistent variation across any coastal boundary. For region 1, the −Ip values increase as we move toward the coast (southwards) especially during the wet season (June–October). This finding is in direct contrast with studies that documented winter as the season of maximum −Ip values. The zonal and seasonal variations of −Ip values across region 4 are not quite as pronounced, but the oceanic −Ip values are still larger than over the adjoining landmass. We explore in turn which up to date hypotheses pertinent to the oceanic −Ip enhancement are supported or refuted by our findings. It is concluded that the oceanic −Ip enhancement is not an artifact related to CG detection or Ip retrieval methods, nor is it likely related to the cloud top heights or CG activity. The study cannot refute the role of electrical conductivity and its contribution to CG leader attachment processes. However, given the observed “blurred transition” of the Ip values across the coastlines this paper suggests that likely the main physical mechanism is acting on the thundercloud potential. The recently suggested role of sodium chloride (NaCl) but also the role of ice crystal size (implicated herein), as possible modulators of the thundercloud potential, exhibit distinct pros and cons. Their candidacy is supported by their strong physical links to the electrostatic charging and thundercloud electric potential buildup but also by the exhibited blurred −Ip transition across the coastlines. In contrast, the suggested mechanisms cannot individually explain the observed −Ip enhancement in terms of season, NaCl concentrations, and absence of similar behavior in the respective +Ip values.
Planning, implementation, and scientific goals of the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) field mission
Authors: Toon Owen B. , Maring Hal , Dibb Jack , Ferrare Richard , Jacob Daniel J. , Jensen Eric J. , Luo Z. Johnny , Mace Gerald G. , Pan Laura L. , Pfister Lenny , Rosenlof Karen H. , Redemann Jens , Reid Jeffrey S. , Singh Hanwant B. , Thompson Anne M. , Yokelson Robert , Minnis Patrick , Chen Gao , Jucks Kenneth W. , Pszenny Alex
Published: April 2016

The Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) field mission based at Ellington Field, Texas, during August and September 2013 employed the most comprehensive airborne payload to date to investigate atmospheric composition over North America. The NASA ER‐2, DC‐8, and SPEC Inc. Learjet flew 57 science flights from the surface to 20 km. The ER‐2 employed seven remote sensing instruments as a satellite surrogate and eight in situ instruments. The DC‐8 employed 23 in situ and five remote sensing instruments for radiation, chemistry, and microphysics. The Learjet used 11 instruments to explore cloud microphysics. SEAC4RS launched numerous balloons, augmented AErosol RObotic NETwork, and collaborated with many existing ground measurement sites. Flights investigating convection included close coordination of all three aircraft. Coordinated DC‐8 and ER‐2 flights investigated the optical properties of aerosols, the influence of aerosols on clouds, and the performance of new instruments for satellite measurements of clouds and aerosols. ER‐2 sorties sampled stratospheric injections of water vapor and other chemicals by local and distant convection. DC‐8 flights studied seasonally evolving chemistry in the Southeastern U.S., atmospheric chemistry with lower emissions of NOx and SO2 than in previous decades, isoprene chemistry under high and low NOx conditions at different locations, organic aerosols, air pollution near Houston and in petroleum fields, smoke from wildfires in western forests and from agricultural fires in the Mississippi Valley, and the ways in which the chemistry in the boundary layer and the upper troposphere were influenced by vertical transport in convective clouds.
The impact of cloud vertical profile on liquid water path retrieval based on the bispectral method: A theoretical study based on large‐eddy simulations of shallow marine boundary layer clouds
Authors: Miller Daniel J. , Zhang Zhibo , Ackerman Andrew S. , Platnick Steven , Baum Bryan A.
Published: April 2016

Passive optical retrievals of cloud liquid water path (LWP), like those implemented for Moderate Resolution Imaging Spectroradiometer (MODIS), rely on cloud vertical profile assumptions to relate optical thickness (τ) and effective radius (re) retrievals to LWP. These techniques typically assume that shallow clouds are vertically homogeneous; however, an adiabatic cloud model is plausibly more realistic for shallow marine boundary layer cloud regimes. In this study a satellite retrieval simulator is used to perform MODIS‐like satellite retrievals, which in turn are compared directly to the large‐eddy simulation (LES) output. This satellite simulator creates a framework for rigorous quantification of the impact that vertical profile features have on LWP retrievals, and it accomplishes this while also avoiding sources of bias present in previous observational studies. The cloud vertical profiles from the LES are often more complex than either of the two standard assumptions, and the favored assumption was found to be sensitive to cloud regime (cumuliform/stratiform). Confirming previous studies, drizzle and cloud top entrainment of dry air are identified as physical features that bias LWP retrievals away from adiabatic and toward homogeneous assumptions. The mean bias induced by drizzle‐influenced profiles was shown to be on the order of 5–10 g/m2. In contrast, the influence of cloud top entrainment was found to be smaller by about a factor of 2. A theoretical framework is developed to explain variability in LWP retrievals by introducing modifications to the adiabatic re profile. In addition to analyzing bispectral retrievals, we also compare results with the vertical profile sensitivity of passive polarimetric retrieval techniques.
Global distribution of deep convection reaching tropopause in 1 year GPM observations
Authors: Liu Nana , Liu Chuntao
Published: April 2016

To characterize and quantify tropopause‐reaching deep convection, 1 year of Global Precipitation Mission (GPM) Ku band radar echoes are surveyed in relation to several reference levels derived from the ERA‐Interim reanalysis data set. Consistent with the observations of the Tropical Rainfall Measuring Mission over the tropics, the GPM has detected tropopause‐reaching deep convection dominantly over tropical land, especially over Panama and Central Africa. At middle and high latitudes, tropopause‐reaching convective storms are mainly found over land in the Northern Hemisphere during the summer. Compared to those in the tropics, convective cores at middle and high latitudes have relatively larger sizes at the tropopause, especially those over central North America. The zonal distributions of the occurrences of 15 dBZ and 20 dBZ radar echoes at the tropopause show two comparable maxima, one in the tropics and the other in northern middle‐high latitudes. This implies that the convection penetrating the tropopause at northern middle‐high latitudes is as frequent as those over the tropics. It is important to understand their role in the vertical transport of trace gases between the troposphere and the stratosphere.
Bias adjustment of satellite‐based precipitation estimation using gauge observations: A case study in Chile
Authors: Yang Zhongwen , Hsu Kuolin , Sorooshian Soroosh , Xu Xinyi , Braithwaite Dan , Verbist Koen M. J.
Published: April 2016

Satellite‐based precipitation estimates (SPEs) are promising alternative precipitation data for climatic and hydrological applications, especially for regions where ground‐based observations are limited. However, existing satellite‐based rainfall estimations are subject to systematic biases. This study aims to adjust the biases in the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks–Cloud Classification System (PERSIANN‐CCS) rainfall data over Chile, using gauge observations as reference. A novel bias adjustment framework, termed QM‐GW, is proposed based on the nonparametric quantile mapping approach and a Gaussian weighting interpolation scheme. The PERSIANN‐CCS precipitation estimates (daily, 0.04°×0.04°) over Chile are adjusted for the period of 2009–2014. The historical data (satellite and gauge) for 2009–2013 are used to calibrate the methodology; nonparametric cumulative distribution functions of satellite and gauge observations are estimated at every 1°×1° box region. One year (2014) of gauge data was used for validation. The results show that the biases of the PERSIANN‐CCS precipitation data are effectively reduced. The spatial patterns of adjusted satellite rainfall show high consistency to the gauge observations, with reduced root‐mean‐square errors and mean biases. The systematic biases of the PERSIANN‐CCS precipitation time series, at both monthly and daily scales, are removed. The extended validation also verifies that the proposed approach can be applied to adjust SPEs into the future, without further need for ground‐based measurements. This study serves as a valuable reference for the bias adjustment of existing SPEs using gauge observations worldwide.
Sensitivity of Amazonian TOA flux diurnal cycle composite monthly variability to choice of reanalysis
Authors: Dodson J. Brant , Taylor Patrick C.
Published: April 2016

Amazonian deep convection experiences a strong diurnal cycle driven by the cycle in surface sensible heat flux, which contributes to a significant diurnal cycle in the top of the atmosphere (TOA) radiative flux. Even when accounting for seasonal variability, the TOA flux diurnal cycle varies significantly on the monthly timescale. Previous work shows evidence supporting a connection between variability in the convective and radiative cycles, likely modulated by variability in monthly atmospheric state (e.g., convective instability). The hypothesized relationships are further investigated with regression analysis of the radiative diurnal cycle and atmospheric state using additional meteorological variables representing convective instability and upper tropospheric humidity. The results are recalculated with three different reanalyses to test the reliability of the results. The radiative diurnal cycle sensitivity to upper tropospheric humidity is about equal in magnitude to that of convective instability. In addition, the results are recalculated with the data subdivided into the wet and dry seasons. Overall, clear‐sky radiative effects have a dominant role in radiative diurnal cycle variability during the dry season. Because of this, even in a convectively active region, the clear‐sky radiative effects must be accounted for in order to fully explain the monthly variability in diurnal cycle. Finally, while there is general agreement between the different reanalysis‐based results when examining the full data time domain (without regard to time of year), there are significant disagreements when the data are divided into wet and dry seasons. The questionable reliability of reanalysis data is a major limitation.
Evaluating and improving cloud phase in the Community Atmosphere Model version 5 using spaceborne lidar observations
Authors: Kay Jennifer E. , Bourdages Line , Miller Nathaniel B. , Morrison Ariel , Yettella Vineel , Chepfer Helene , Eaton Brian
Published: April 2016

Spaceborne lidar observations from the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite are used to evaluate cloud amount and cloud phase in the Community Atmosphere Model version 5 (CAM5), the atmospheric component of a widely used state‐of‐the‐art global coupled climate model (Community Earth System Model). By embedding a lidar simulator within CAM5, the idiosyncrasies of spaceborne lidar cloud detection and phase assignment are replicated. As a result, this study makes scale‐aware and definition‐aware comparisons between model‐simulated and observed cloud amount and cloud phase. In the global mean, CAM5 has insufficient liquid cloud and excessive ice cloud when compared to CALIPSO observations. Over the ice‐covered Arctic Ocean, CAM5 has insufficient liquid cloud in all seasons. Having important implications for projections of future sea level rise, a liquid cloud deficit contributes to a cold bias of 2–3°C for summer daily maximum near‐surface air temperatures at Summit, Greenland. Over the midlatitude storm tracks, CAM5 has excessive ice cloud and insufficient liquid cloud. Storm track cloud phase biases in CAM5 maximize over the Southern Ocean, which also has larger‐than‐observed seasonal variations in cloud phase. Physical parameter modifications reduce the Southern Ocean cloud phase and shortwave radiation biases in CAM5 and illustrate the power of the CALIPSO observations as an observational constraint. The results also highlight the importance of using a regime‐based, as opposed to a geographic‐based, model evaluation approach. More generally, the results demonstrate the importance and value of simulator‐enabled comparisons of cloud phase in models used for future climate projection.
Review on climate change on the Tibetan Plateau during the last half century
Authors: Kuang Xingxing , Jiao Jiu Jimmy
Published: April 2016

The Tibetan Plateau (TP) is undergoing significant warming since the 1950s. During the past two decades, extensive research has been conducted to investigate the climate change on the plateau. This review presents an overview of recent progress on climate change on the TP with the aim of providing a comprehensive understanding of changes in climate variables. Long‐term observation data from meteorological stations presented by the published literature were used to show the trends in various climate variables. The TP is overall getting warmer and wetter during the past decades. Temperature is significantly increased, especially since the 1980s. The overall warming rate ranges from 0.16 to 0.67°C decade−1 since the 1950s during different periods. The TP shows a uniform warming trend with the most significant warming in the northern part. Precipitation is slightly increased, and the spatial pattern of changes in precipitation is variable. The annual precipitation is increasing in most areas of the TP. Some subregions are becoming wetter, while some subregions are becoming drier. Pan evaporation, reference evapotranspiration, and potential evapotranspiration have been found to decrease since the 1960s. Actual evapotranspiration is significantly increased since the 1960s. Wind speed and sunshine duration increased up to the 1970s and then decreased significantly afterwards. Relative humidity fluctuated up and down to the end of the 1990s and appeared to decrease afterwards. Vapor pressure deficit shows an overall increasing trend since the 1970s. Causes of changes in the climate variables are presented, and future research directions are recommended.
Orbital evidence for more widespread carbonate‐bearing rocks on Mars
Authors: Wray James J. , Murchie Scott L. , Bishop Janice L. , Ehlmann Bethany L. , Milliken Ralph E. , Wilhelm Mary Beth , Seelos Kimberly D. , Chojnacki Matthew
Published: April 2016

Carbonates are key minerals for understanding ancient Martian environments because they are indicators of potentially habitable, neutral‐to‐alkaline water and may be an important reservoir for paleoatmospheric CO2. Previous remote sensing studies have identified mostly Mg‐rich carbonates, both in Martian dust and in a Late Noachian rock unit circumferential to the Isidis basin. Here we report evidence for older Fe‐ and/or Ca‐rich carbonates exposed from the subsurface by impact craters and troughs. These carbonates are found in and around the Huygens basin northwest of Hellas, in western Noachis Terra between the Argyre basin and Valles Marineris, and in other isolated locations spread widely across the planet. In all cases they cooccur with or near phyllosilicates, and in Huygens basin specifically they occupy layered rocks exhumed from up to ~5 km depth. We discuss factors that might explain their observed regional distribution, arguments for why carbonates may be even more widespread in Noachian materials than presently appreciated and what could be gained by targeting these carbonates for further study with future orbital or landed missions to Mars.
Quantification of the bed load effects on turbulent open‐channel flows
Authors: Liu Detian , Liu Xiaofeng , Fu Xudong , Wang Guangqian
Published: April 2016

With a computational model combining large eddy simulation and a discrete element model, detailed quantification of the bed load effects on turbulent open‐channel flows is presented. The objective is the revelation of bed load particle impact on the mean flow properties and coherent structures. Two comparative numerical experiments with mobile and immobile beds are conducted. Mean properties (e.g., velocity and Reynolds stress profiles) show good agreement with experimental data. Comparing the mobile and immobile cases, the effective bed position is nearly the same, whereas the equivalent sand roughness is changed. The flow experiences higher bottom shear stress over immobile bed. To quantify impact on turbulent structures, a revised quadrant analysis is performed to calculate four key parameters of ejection and sweep events (duration, maximum shear stress, transported momentum, and period). Results show that the ejection and sweep events have comparable importance in the outer region. However, sweep becomes dominant in the near‐wall region. The motion of particles enhances the sweep dominance by breaking up the ejection structures and decreasing their occurrence ratio. The results also suggest that the ejection events are easier to be influenced by the particle motions because they originate from the near‐wall region. The duration, maximum shear stress, and transported momentum decrease close to the bed. The period remains relatively constant in the outer region but decreases near the bed. Visualization of the coherent structure reveals that the instantaneous particle motion has strong correlation with the bursting cycle events.
Extensive nitrogen loss from permeable sediments off North‐West Africa
Authors: Sokoll Sarah , Lavik Gaute , Sommer Stefan , Goldhammer Tobias , Kuypers Marcel M. M. , Holtappels Moritz
Published: April 2016

The upwelling area off North‐West Africa is characterized by high export production, high nitrate and low oxygen concentration in bottom waters. The underlying sediment consists of sands that cover most of the continental shelf. Due to their permeability sands allow for fast advective pore water transport and can exhibit high rates of nitrogen (N) loss via denitrification as reported for anthropogenically eutrophied regions. However, N loss from sands underlying naturally eutrophied waters is not well studied, and in particular, N loss from the North‐West African shelf is poorly constrained. During two research cruises in April/May 2010/2011, sediment was sampled along the North‐West African shelf and volumetric denitrification rates were measured in sediment layers down to 8 cm depth using slurry incubations with 15N‐labeled nitrate. Areal N loss was calculated by integrating volumetric rates down to the nitrate penetration depth derived from pore water profiles. Areal N loss was neither correlated with water depth nor with bottom water concentrations of nitrate and oxygen but was strongly dependent on sediment grain size and permeability. The derived empirical relation between benthic N loss and grains size suggests that pore water advection is an important regulating parameter for benthic denitrification in sands and further allowed extrapolating rates to an area of 53,000 km2 using detailed sediment maps. Denitrification from this region amounts to 995 kt yr−1 (average 3.6 mmol m−2 d−1) which is 4 times higher than previous estimates based on diffusive pore water transport. Sandy sediments cover 50–60% of the continental shelf and thus may contribute significantly to the global benthic N loss.
Daytime sensible heat flux estimation over heterogeneous surfaces using multitemporal land‐surface temperature observations
Authors: Castellví F. , Cammalleri C. , Ciraolo G. , Maltese A. , Rossi F.
Published: April 2016

Equations based on surface renewal (SR) analysis to estimate the sensible heat flux (H) require as input the mean ramp amplitude and period observed in the ramp‐like pattern of the air temperature measured at high frequency. A SR‐based method to estimate sensible heat flux (HSR‐LST) requiring only low‐frequency measurements of the air temperature, horizontal mean wind speed, and land‐surface temperature as input was derived and tested under unstable conditions over a heterogeneous canopy (olive grove). HSR‐LST assumes that the mean ramp amplitude can be inferred from the difference between land‐surface temperature and mean air temperature through a linear relationship and that the ramp frequency is related to a wind shear scale characteristic of the canopy flow. The land‐surface temperature was retrieved by integrating in situ sensing measures of thermal infrared energy emitted by the surface. The performance of HSR‐LST was analyzed against flux tower measurements collected at two heights (close to and well above the canopy top). Crucial parameters involved in HSR‐LST, which define the above mentioned linear relationship, were explained using the canopy height and the land surface temperature observed at sunrise and sunset. Although the olive grove can behave as either an isothermal or anisothermal surface, HSR‐LST performed close to H measured using the eddy covariance and the Bowen ratio energy balance methods. Root mean square differences between HSR‐LST and measured H were of about 55 W m−2. Thus, by using multitemporal thermal acquisitions, HSR‐LST appears to bypass inconsistency between land surface temperature and the mean aerodynamic temperature. The one‐source bulk transfer formulation for estimating H performed reliable after calibration against the eddy covariance method. After calibration, the latter performed similar to the proposed SR‐LST method.
Linking snowfall and snow accumulation to generate spatial maps of SWE and snow depth
Authors: Broxton Patrick D. , Dawson Nicholas , Zeng Xubin
Published: April 2016

It is critically important but challenging to estimate the amount of snow on the ground over large areas due to its strong spatial variability. Point snow data are used to generate or improve (i.e., blend with) gridded estimates of snow water equivalent (SWE) by using various forms of interpolation; however, the interpolation methodologies often overlook the physical mechanisms for the snow being there in the first place. Using data from the Snow Telemetry and Cooperative Observer networks in the western United States, we show that four methods for the spatial interpolation of peak of winter snow water equivalent (SWE) and snow depth based on distance and elevation can result in large errors. These errors are reduced substantially by our new method, i.e., the spatial interpolation of these quantities normalized by accumulated snowfall from the current or previous water years. Our method results in significant improvement in SWE estimates over interpolation techniques that do not consider snowfall, regardless of the number of stations used for the interpolation. Furthermore, it can be used along with gridded precipitation and temperature data to produce daily maps of SWE over the western United States that are comparable to existing estimates (which are based on the assimilation of much more data). Our results also show that not honoring the constraint between SWE and snowfall when blending in situ data with gridded data can lead to the development and propagation of unrealistic errors.
Streamflow response to increasing precipitation extremes altered by forest management
Authors: Kelly Charlene N. , McGuire Kevin J. , Miniat Chelcy Ford , Vose James M.
Published: April 2016

Increases in extreme precipitation events of floods and droughts are expected to occur worldwide. The increase in extreme events will result in changes in streamflow that are expected to affect water availability for human consumption and aquatic ecosystem function. We present an analysis that may greatly improve current streamflow models by quantifying the impact of the interaction between forest management and precipitation. We use daily long‐term data from paired watersheds that have undergone forest harvest or species conversion. We find that interactive effects of climate change, represented by changes in observed precipitation trends, and forest management regime, significantly alter expected streamflow most often during extreme events, ranging from a decrease of 59% to an increase of 40% in streamflow, depending upon management. Our results suggest that vegetation might be managed to compensate for hydrologic responses due to climate change to help mitigate effects of extreme changes in precipitation.