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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Impact of gravity on hydrate saturation in gas‐rich environments

Abstract We extend a one‐dimensional analytical solution by including buoyancy‐driven flow to explore the impact of gravity on hydrate formation from gas injection into brine‐saturated sediments within the hydrate stability zone. This solution includes the fully coupled gas and liquid phase flow and the associated advective transport in a homogeneous system. We obtain the saturations assuming Darcy flow under combined pressure and gravity gradients; capillary forces are neglected. At a high gas supply rate, the overpressure gradient (gradient of water pressure deviation from the hydrostatic pressure) dominates the gas flow, and the hydrate saturation is independent of the flow rate and flow direction. At a low gas supply rate, the buoyancy (the drive for gas flow induced by the density difference between gas and liquid) dominates the gas flow, and the hydrate saturation depends on the flow rate and flow direction. Hydrate saturation is highest for upward flow, and lowest for downward flow. Hydrate saturation decreases with flow rate for upward flow, and increases with flow rate for downward flow. In all cases, hydrate saturation is constant behind the hydrate solidification front. Gas saturation is homogeneous and close to the residual value for upward flow at a low rate; gas flows at the rate it is supplied. Gas saturation is much greater than the residual value, and decreases from the gas inlet to the hydrate solidification front for downward flow at a very low rate. The effect of gravity is usually negligible in laboratory experiments, yet is significant in natural hydrate systems.

You, Kehua↗
DOE OSTI2016

River stage influences on uranium transport in a hydrologically dynamic groundwater-surface water transition zone

In this study, a well-field within a uranium (U) plume in the groundwater-surface water transition zone was monitored for a 3 year period for water table elevation and dissolved solutes. The plume discharges to the Columbia River, which displays a dramatic spring stage surge resulting from snowmelt. Groundwater exhibits a low hydrologic gradient and chemical differences with river water. River water intrudes the site in spring. Specific aims were to assess the impacts of river intrusion on dissolved uranium (U aq ), specific conductance (SpC), and other solutes, and to discriminate between transport, geochemical, and source term heterogeneity effects. Time series trends for U aq and SpC were complex and displayed large temporal and well-to-well variability as a result of water table elevation fluctuations, river water intrusion, and changes in groundwater flow directions. The wells were clustered into subsets exhibiting common behaviors resulting from the intrusion dynamics of river water and the location of source terms. Hot-spots in U aq varied in location with increasing water table elevation through the combined effects of advection and source term location. Heuristic reactive transport modeling with PFLOTRAN demonstrated that mobilized U aq was transported between wells and source terms in complex trajectories, and was diluted as river water entered and exited the groundwater system. While U aq time-series concentration trends varied significantly from year-to-year as a result of climate-caused differences in the spring hydrograph, common and partly predictable response patterns were observed that were driven by water table elevation, and the extent and duration of river water intrusion.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

N 2 O production and consumption from stable isotopic and concentration data in the Peruvian coastal upwelling system: N 2 O Production and Consumption off Peru

The ocean is an important source of nitrous oxide (N 2 O) to the atmosphere, yet the factors controlling N 2 O production and consumption in oceanic environments are still not understood nor constrained. We measured N 2 O concentrations and isotopomer ratios, as well as O 2 , nutrient and biogenic N 2 concentrations, and the isotopic compositions of nitrate and nitrite at several coastal stations during two cruises off the Peru coast (~5–16°S, 75–81°W) in December 2012 and January 2013. N 2 O concentrations varied from below equilibrium values in the oxygen deficient zone (ODZ) to up to 190nmol L -1 in surface waters. We used a 3-D-reaction-advection-diffusion model to evaluate the rates and modes of N 2 O production in oxic waters and rates of N 2 O consumption versus production by denitrification in the ODZ. Intramolecular site preference in N 2 O isotopomer was relatively low in surface waters (generally -3 to 14‰) and together with modeling results, confirmed the dominance of nitrifier-denitrification or incomplete denitrifier-denitrification, corresponding to an efflux of up to 0.6 Tg N yr -1 off the Peru coast. Other evidence, e.g., the absence of a relationship between ΔN 2 O and apparent O 2 utilization and significant relationships between nitrate, a substrate during denitrification, and N 2 O isotopes, suggest that N 2 O production by incomplete denitrification or nitrifier-denitrification decoupled from aerobic organic matter remineralization are likely pathways for extreme N 2 O accumulation in newly upwelled surface waters. We observed imbalances between N 2 O production and consumption in the ODZ, with the modeled proportion of N 2 O consumption relative to production generally increasing with biogenic N 2 . However, N 2 O production appeared to occur even where there was high N loss at the shallowest stations.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

Sea-ice thickness from field measurements in the northwestern Barents Sea: SEA-ICE THICKNESS, NW BARENTS SEA

The Barents Sea is one of the fastest changing regions of the Arctic, and has experienced the strongest decline in winter-time sea-ice area in the Arctic, at 22364% decade21. Sea-ice thickness in the Barents Sea is not well studied. We present two previously unpublished helicopter-borne electromagnetic(HEM) ice thickness measurements from the northwestern Barents Sea acquired in March 2003 and 2014.The HEM data are compared to ice thickness calculated from ice draft measured by ULS deployed between1994 and 1996. These data show that ice thickness varies greatly from year to year; influenced by the thermodynamic and dynamic processes that govern local formation vs long-range advection. In a year with a large inflow of sea-ice from the Arctic Basin, the Barents Sea ice cover is dominated by thick multiyear ice;as was the case in 2003 and 1995. In a year with an ice cover that was mainly grown in situ, the ice will be thin and mechanically unstable; as was the case in 2014. The HEM data allow us to explore the spatial and temporal variability in ice thickness. In 2003 the dominant ice class was more than 2 years old; and modal sea-ice thickness varied regionally from 0.6 to 1.4 m, with the thinner ice being either irst-year ice, or multiyear ice which had come into contact with warm Atlantic water. In 2014 the ice cover was predominantly locally grown ice less than 1 month old (regional modes of 0.5–0.8 m). These two situations represent two extremes of a range of possible ice thickness distributions that can present very different conditions for shipping traffic; or have a different impact on heat transport from ocean to atmosphere.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2016

Vertical dispersion in vegetated shear flows

Canopy layers control momentum and solute transport to and from the overlying water surface layer. These transfer mechanisms strongly dependent on canopy geometry, affect the amount of solute in the river, the hydrological retention and availability of dissolved solutes to organisms located in the vegetated layers, and are critical to improve water quality. In this work, we consider steady state transport in a vegetated channel under fully developed flow conditions. Under the hypothesis that the canopy layer can be described as an effective porous medium with prescribed properties, i.e., porosity and permeability, we model solute transport above and within the vegetated layer with an advection-dispersion equation with a spatially variable dispersion coefficient (diffusivity). By means of the Generalized Integral Transform Technique , we derive a semianalytical solution for the concentration field in submerged vegetated aquatic systems. We show that canopy layer’s permeability affects the asymmetry of the concentration profile, the effective vertical spreading behavior, and the magnitude of the peak concentration. Due to its analytical features, the model has a low computational cost. The proposed solution successfully reproduces previously published experimental data.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2016

Modeling the influence of preferential flow on the spatial variability and time-dependence of mineral weathering rates

Inferences of weathering rates from laboratory and field observations suggest significant scale and time-dependence. Preferential flow induced by heterogeneity (manifest as permeability variations or discrete fractures) has been suggested as one potential mechanism causing scale/time-dependence. In this paper, we present a quantitative evaluation of the influence of preferential flow on weathering rates using reactive transport modeling. Simulations were performed in discrete fracture networks (DFNs) and correlated random permeability fields (CRPFs), and compared to simulations in homogeneous permeability fields. The simulations reveal spatial variability in the weathering rate, multidimensional distribution of reactions zones, and the formation of rough weathering interfaces and corestones due to preferential flow. In the homogeneous fields and CRPFs, the domain-averaged weathering rate is initially constant as long as the weathering front is contained within the domain, reflecting equilibrium-controlled behavior. The behavior in the CRPFs was influenced by macrodispersion, with more spread-out weathering profiles, an earlier departure from the initial constant rate and longer persistence of weathering. DFN simulations exhibited a sustained time-dependence resulting from the formation of diffusion-controlled weathering fronts in matrix blocks, which is consistent with the shrinking core mechanism. A significant decrease in the domain-averaged weathering rate is evident despite high remaining mineral volume fractions, but the decline does not follow a math formula dependence, characteristic of diffusion, due to network scale effects and advection-controlled behavior near the inflow boundary. Finally, the DFN simulations also reveal relatively constant horizontally averaged weathering rates over a significant depth range, challenging the very notion of a weathering front.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

Theoretical analysis of non‐ G aussian heterogeneity effects on subsurface flow and transport

Abstract Much of the stochastic groundwater literature is devoted to the analysis of flow and transport in Gaussian or multi‐Gaussian log hydraulic conductivity (or transmissivity) fields, ( x being a position vector), characterized by one or (less frequently) a multiplicity of spatial correlation scales. Yet Y and many other variables and their (spatial or temporal) increments, Δ Y , are known to be generally non‐Gaussian. One common manifestation of non‐Gaussianity is that whereas frequency distributions of Y often exhibit mild peaks and light tails, those of increments are generally symmetric with peaks that grow sharper, and tails that become heavier, as separation scale or lag between pairs of Y values decreases. A statistical model that captures these disparate, scale‐dependent distributions of Y and in a unified and consistent manner has been recently proposed by us. This new “generalized sub‐Gaussian (GSG)” model has the form where is (generally, but not necessarily) a multiscale Gaussian random field and is a nonnegative subordinator independent of G . The purpose of this paper is to explore analytically, in an elementary manner, lead‐order effects that non‐Gaussian heterogeneity described by the GSG model have on the stochastic description of flow and transport. Recognizing that perturbation expansion of hydraulic conductivity diverges when Y is sub‐Gaussian, we render the expansion convergent by truncating Y 's domain of definition. We then demonstrate theoretically and illustrate by way of numerical examples that, as the domain of truncation expands, (a) the variance of truncated Y (denoted by Y t ) approaches that of Y and (b) the pdf (and thereby moments) of Y t increments approach those of Y increments and, as a consequence, the variogram of approaches that of . This in turn guarantees that perturbing to second order in (the standard deviation of Y t ) yields results which approach those we obtain upon perturbing to second order in even as the corresponding series diverges. Our analysis is rendered mathematically tractable by considering mean‐uniform steady state flow in an unbounded, two‐dimensional domain of mildly heterogeneous Y with a single‐scale function G having an isotropic exponential covariance. Results consist of expressions for (a) lead‐order autocovariance and cross‐covariance functions of hydraulic head, velocity, and advective particle displacement and (b) analogues of preasymptotic as well as asymptotic Fickian dispersion coefficients. We compare these theoretically and graphically with corresponding expressions developed in the literature for Gaussian Y . We find the former to differ from the latter by a factor k = ( denoting ensemble expectation) and the GSG covariance of longitudinal velocity to contain an additional nugget term depending on this same factor. In the limit as Y becomes Gaussian, k reduces to one and the nugget term drops out.

Riva, Monica↗
DOE OSTI2017

Thermal effect of climate change on groundwater-fed ecosystems

Groundwater temperature changes will lag surface temperature changes from a changing climate. Steady state solutions of the heat-transport equations are used to identify key processes that control the long-term thermal response of springs and other groundwater discharge to climate change, in particular changes in (1) groundwater recharge rate and temperature and (2) land-surface temperature transmitted through the vadose zone. Transient solutions are developed to estimate the time required for new thermal signals to arrive at ecosystems. The solution is applied to the volcanic Medicine Lake highlands, California, USA, and associated springs complexes that host groundwater-dependent ecosystems. In this system, upper basin groundwater temperatures are strongly affected only by recharge conditions. However, as the vadose zone thins away from the highlands, changes in the average annual land-surface temperature also influence groundwater temperatures. Transient response to temperature change depends on both the conductive time scale and the rate at which recharge delivers heat. Most of the thermal response of groundwater at high elevations will occur within 20 years of a shift in recharge temperatures, but the large lower elevation springs will respond more slowly, with about half of the conductive response occurring within the first 20 years and about half of the advective response to higher recharge temperatures occurring in approximately 60 years.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

Failure of Taylor's hypothesis in the atmospheric surface layer and its correction for eddy-covariance measurements

Taylors’ frozen turbulence hypothesis suggests that all turbulent eddies are advected by the mean streamwise velocity, without changes in their properties. This hypothesis has been widely invoked to compute Reynolds’ averaging using temporal turbulence data measured at a single point in space. However, in the atmospheric surface layer, the exact relationship between convection velocity and wavenumber k has not been fully revealed since previous observations were limited by either their spatial resolution or by the sampling length. Using Distributed Temperature Sensing (DTS), acquiring turbulent temperature fluctuations at high temporal and spatial frequencies, we computed convection velocities across wavenumbers using a phase spectrum method. We found that convection velocity decreases as k –1/3 at the higher wavenumbers of the inertial subrange instead of being independent of wavenumber as suggested by Taylor's hypothesis. We further corroborated this result using large eddy simulations. Applying Taylor's hypothesis thus systematically underestimates turbulent spectrum in the inertial subrange. As a result, a correction is proposed for point-based eddy-covariance measurements, which can improve surface energy budget closure and estimates of CO 2 fluxes.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

Subsurface Nonlinear Dynamical Heating and ENSO Asymmetry

Abstract This study presents evidence of interannual to interdecadal variabilities residing in the subsurface nonlinear dynamical heating (NDH) along the equatorial Pacific thermocline based on ocean reanalysis products. In association with a flattened thermocline and drastic halt of the Equatorial Undercurrent, the NDH increases sharply right after a peak of the strong El Niño, reducing the linear anomalous advective subsurface cooling by 30%. As the result, subsurface cold signals in the west are significantly weakened as it reaches the easternmost Pacific, causing less surface cooling in the east and thus weakening the subsequent La Niña. Therefore, the subsurface NDH contributes to asymmetry in both the magnitude and structure between El Niño and La Niña. The acute positive subsurface NDH associated with strong El Niño events leads to substantial variations in its decadal means and serves as a potential source for tropical Pacific decadal to interdecadal variabilities.

Hayashi, Michiya↗
DOE OSTI2018

Glacial cycles influence marine methane hydrate formation

Methane hydrates in fine-grained continental slope sediments often occupy isolated depth intervals surrounded by hydrate-free sediments. As they are not connected to deep gas sources, these hydrate deposits have been interpreted as sourced by in situ microbial methane. We investigate here the hypothesis that these isolated hydrate accumulations form preferentially in sediments deposited during Pleistocene glacial lowstands that contain relatively large amounts of labile particulate organic carbon, leading to enhanced microbial methanogenesis. To test this hypothesis, we apply an advection-diffusion-reaction model with a time-dependent organic carbon deposition controlled by glacioeustatic sea level variations. In the model, hydrate forms in sediments with greater organic carbon content deposited during the penultimate glacial cycle (~120-240 ka). As a result, the model predictions match hydrate-bearing intervals detected in three sites drilled on the northern Gulf of Mexico continental slope, supporting the hypothesis of hydrate formation driven by enhanced organic carbon burial during glacial lowstands.

58 GEOSCIENCES↗
DOE OSTI2018

Lagrangian Timescales of Southern Ocean Upwelling in a Hierarchy of Model Resolutions

Here, we study upwelling pathways and timescales of Circumpolar Deep Water (CDW) in a hierarchy of models using a Lagrangian particle tracking method. Lagrangian timescales of CDW upwelling decrease from 87 years to 31 years to 17 years as the ocean resolution is refined from 1° to 0.25° to 0.1°. We attribute some of the differences in timescale to the strength of the eddy fields, as demonstrated by temporally degrading high-resolution model velocity fields. Consistent with the timescale dependence, we find that an average Lagrangian particle completes 3.2 circumpolar loops in the 1° model in comparison to 0.9 loops in the 0.1° model. In conclusion, these differences suggest that advective timescales and thus interbasin merging of upwelling CDW may be overestimated by coarse-resolution models, potentially affecting the skill of centennial scale climate change projections.

58 GEOSCIENCES↗
DOE OSTI2017

A Fluid Pulse on the Hikurangi Subduction Margin: Evidence From a Heat Flux Transect Across the Upper Limit of Gas Hydrate Stability: Fluid Pulse on the Hikurangi Margin

A transect of seafloor heat probe measurements on the Hikurangi Margin shows a significant increase of thermal gradients upslope of the updip limit of gas hydrate stability at the seafloor. We interpret these anomalously high thermal gradients as evidence for a fluid pulse leading to advective heat flux, while endothermic cooling from gas hydrate dissociation depresses temperatures in the hydrate stability field. Previous studies predict a seamount on the subducting Pacific Plate to cause significant overpressure beneath our study area, which may be the source of the fluid pulse. Double-bottom simulating reflections are present in our study area and likely caused by uplift based on gas hydrate phase boundary considerations, although we cannot exclude a thermogenic origin. We suggest that uplift may be associated with the leading edge of the subducting seamount. Our results provide further evidence for the transient nature of fluid expulsion in subduction zones.

58 GEOSCIENCES↗
DOE OSTI2018

The Influence of Episodic Shallow Magma Degassing on Heat and Chemical Transport in Volcanic Hydrothermal Systems

Springs at La Soufrière of Guadeloupe have been monitored for nearly four decades since the phreatic eruption and associated seismic activity in 1976. We conceptualize degassing vapor/gas mixtures as square-wave sources of chloride and heat and apply a new semianalytic solution to demonstrate that chloride and heat pulses with the same timing and duration result in good matches between measured and simulated spring temperatures and concentrations. While the concentration of chloride pulses is variable, the local boiling temperature of 96°C was assigned to all thermal pulses. Because chloride is a conservative tracer, chloride breakthrough is only affected by one-dimensional advection and dispersion. The thermal tracer is damped and lagged relative to chloride due to conductive heat exchange with the overlying and underlying strata. Joint analysis of temperature and chloride allows estimation of the onset and duration of degassing pulses, refining the chronology of recent magmatic intrusion.

58 GEOSCIENCES↗
DOE OSTI2017

Tall Tower Vertical Profiles and Diurnal Trends of Ammonia in the Colorado Front Range: Tall Tower Vertical Profiles of Ammonia

Ammonia (NH 3 ) mixing ratios were measured between the surface and 280 m aboveground level from a moveable carriage at the Boulder Atmospheric Observatory tower in summer 2014 as part of the Front Range Air Pollution and Photochemistry Éxperiment. The campaign median mixing ratio was 3.3 ppb, ranging from below detection limits to 192 ppb. Median vertical profiles show an overall increase in NH 3 mixing ratios toward the surface of 6.7 ppb (89%) during the day and 3.9 ppb (141%) at night. In contrast to the overall increasing trend, some individual profiles show decreasing NH 3 in the lowest 10 m. This suggests that the local surface is capable of acting as either a source or a sink, depending on the relative amounts of NH 3 at the surface, and in advected air parcels. We further use this data set to investigate the variation in diurnal patterns of NH 3 as a function of height above the surface. At higher altitudes (100 ± 5 and 280 ± 5 m), NH 3 mixing ratios reach a gradual maximum during the day between 9:00 and 16:00 local time, likely driven by changes in source region. At lower altitudes (10 ± 5 m), the daytime maximum begins earlier at about 7:00 local time, followed by a sharper increase at 9:00 local time. At this height we also observe a peak in NH3 mixing ratios during the night, likely driven by the trapping of emitted NH 3 within the shallower nocturnal boundary layer.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

Investigating the dependence of SCM simulated precipitation and clouds on the spatial scale of large-scale forcing at SGP [Investigating the scale dependence of SCM simulated precipitation and cloud by using gridded forcing data at SGP]

Large-scale forcing data, such as vertical velocity and advective tendencies, are required to drive single-column models (SCMs), cloud-resolving models, and large-eddy simulations. Previous studies suggest that some errors of these model simulations could be attributed to the lack of spatial variability in the specified domain-mean large-scale forcing. This study investigates the spatial variability of the forcing and explores its impact on SCM simulated precipitation and clouds. A gridded large-scale forcing data during the March 2000 Cloud Intensive Operational Period at the Atmospheric Radiation Measurement program's Southern Great Plains site is used for analysis and to drive the single-column version of the Community Atmospheric Model Version 5 (SCAM5). When the gridded forcing data show large spatial variability, such as during a frontal passage, SCAM5 with the domain-mean forcing is not able to capture the convective systems that are partly located in the domain or that only occupy part of the domain. This problem has been largely reduced by using the gridded forcing data, which allows running SCAM5 in each subcolumn and then averaging the results within the domain. This is because the subcolumns have a better chance to capture the timing of the frontal propagation and the small-scale systems. As a result, other potential uses of the gridded forcing data, such as understanding and testing scale-aware parameterizations, are also discussed.

58 GEOSCIENCES↗
DOE OSTI2017

Diagnosing Cloud Biases in the GFDL AM3 Model With Atmospheric Classification

In this paper, we define a set of 21 atmospheric states, or recurring weather patterns, for a region surrounding the Atmospheric Radiation Measurement Program's Southern Great Plains site using an iterative clustering technique. The states are defined using dynamic and thermodynamic variables from reanalysis, tested for statistical significance with cloud radar data from the Southern Great Plains site, and are determined every 6 h for 14 years, creating a time series of atmospheric state. The states represent the various stages of the progression of synoptic systems through the region (e.g., warm fronts, warm sectors, cold fronts, cold northerly advection, and high-pressure anticyclones) with a subset of states representing summertime conditions with varying degrees of convective activity. We use the states to classify output from the NOAA/Geophysical Fluid Dynamics Laboratory AM3 model to test the model's simulation of the frequency of occurrence of the states and of the cloud occurrence during each state. The model roughly simulates the frequency of occurrence of the states but exhibits systematic cloud occurrence biases. Comparison of observed and model-simulated International Satellite Cloud Climatology Project histograms of cloud top pressure and optical thickness shows that the model lacks high thin cloud under all conditions, but biases in thick cloud are state-dependent. Frontal conditions in the model do not produce enough thick cloud, while fair-weather conditions produce too much. Finally, we find that increasing the horizontal resolution of the model improves the representation of thick clouds under all conditions but has little effect on high thin clouds. However, increasing resolution also changes the distribution of states, causing an increase in total cloud occurrence bias.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2018

Drivers of Seasonal Variability in Marine Boundary Layer Aerosol Number Concentration Investigated Using a Steady State Approach

Marine boundary layer (MBL) aerosol particles affect the climate through their interaction with MBL clouds. Although both MBL clouds and aerosol particles have pronounced seasonal cycles, the factors controlling seasonal variability of MBL aerosol particle concentration are not well-constrained. In this paper an aerosol budget is constructed representing the effects of wet deposition, free-tropospheric entrainment, primary surface sources, and advection on the MBL accumulation mode aerosol number concentration (N a ). These terms are further parameterized, and by assuming that on seasonal timescales N a is in steady state, the budget equation is rearranged to form a diagnostic equation for N a based on observable variables. Using data primarily collected in the subtropical northeast Pacific during the MAGIC campaign (Marine ARM (Atmospheric Radiation Measurement) GPCI (GCSS Pacific Cross-section Intercomparison) Investigation of Clouds), estimates of both mean summer and winter N a concentrations are made using the simplified steady-state model and seasonal mean observed variables, and are found to match well with the observed N a . To attribute the modeled difference between summer and winter aerosol concentrations to individual observed variables (e.g. precipitation rate, free-tropospheric aerosol number concentration), a local sensitivity analysis is combined with the seasonal difference in observed variables. This analysis shows that despite wintertime precipitation frequency being lower than summer, the higher winter precipitation rate accounted for approximately 60% of the modeled seasonal difference in N a , which emphasizes the importance of marine stratocumulus precipitation in determining MBL aerosol concentrations on longer time scales.

54 ENVIRONMENTAL SCIENCES↗