Abstract This article reports a facile method for the synthesis of Pd‐Ru nanocages by activating the galvanic replacement reaction between Pd nanocrystals and a Ru(III) precursor with I ‐ ions. The as‐synthesized nanocages feature a hollow interior, ultrathin wall of ≈2.5 nm in thickness, and a cubic shape. Our quantitative study suggests that the reduction rate of the Ru(III) precursor can be substantially accelerated upon the introduction of I ‐ ions and then retarded as the ratio of I ‐ /Ru 3+ is increased. The Pd‐Ru nanocages take an alloy structure, with the Ru atoms in the nanocages crystallized in a face‐centered cubic structure instead of the hexagonal close‐packed phase taken by bulk Ru. Using Pd nanocubes with different edge lengths, the dimensions of the nanocages in the range of 6−18 nm can readily be tuned. When tested as catalysts toward the electro‐oxidation of ethylene glycol and glycerol, respectively, the Pd‐Ru cubic nanocages prepared from 18 nm Pd cubes exhibit 5.1‐ and 6.2‐fold enhancements in terms of mass activity relative to the commercial Pd/C. After 1000 cycles of accelerated durability test, the mass activities of the nanocages are still 3.3 and 3.7 times as high as that of the pristine commercial Pd/C catalyst, respectively.

Zhao, Ming↗

DOE OSTI2020

Convective Transition Statistics over Tropical Oceans for Climate Model Diagnostics: GCM Evaluation

To assess deep convective parameterizations in a variety of GCMs and examine the fast-time-scale convective transition, a set of statistics characterizing the pickup of precipitation as a function of column water vapor (CWV), PDFs and joint PDFs of CWV and precipitation, and the dependence of the moisture–precipitation relation on tropospheric temperature is evaluated using the hourly output of two versions of the GFDL Atmospheric Model, version 4 (AM4), NCAR CAM5 and superparameterized CAM (SPCAM). The 6-hourly output from the MJO Task Force (MJOTF)/GEWEX Atmospheric System Study (GASS) project is also analyzed. Contrasting statistics produced from individual models that primarily differ in representations of moist convection suggest that convective transition statistics can substantially distinguish differences in convective representation and its interaction with the large-scale flow, while models that differ only in spatial–temporal resolution, microphysics, or ocean–atmosphere coupling result in similar statistics. Most of the models simulate some version of the observed sharp increase in precipitation as CWV exceeds a critical value, as well as that convective onset occurs at higher CWV but at lower column RH as temperature increases. While some models quantitatively capture these observed features and associated probability distributions, considerable intermodel spread and departures from observations in various aspects of the precipitation–CWV relationship are noted. For instance, in many of the models, the transition from the low-CWV, nonprecipitating regime to the moist regime for CWV around and above critical is less abrupt than in observations. Additionally, some models overproduce drizzle at low CWV, and some require CWV higher than observed for strong precipitation. For many of the models, it is particularly challenging to simulate the probability distributions of CWV at high temperature.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2020

Azimuthally averaged wind and thermodynamic structures of tropical cyclones in global climate models and their sensitivity to horizontal resolution

Characteristics of tropical cyclones (TCs) in global climate models (GCMs) are known to be influenced by details of the model configurations, including horizontal resolution and parameterization schemes. Understanding model-to-model differences in TC characteristics is a prerequisite for reducing uncertainty in future TC activity projections by GCMs. This study performs a process-level examination of TC structures in eight GCM simulations that span a range of horizontal resolutions from 1° to 0.25°. A recently developed set of process-oriented diagnostics is used to examine the azimuthally averaged wind and thermodynamic structures of the GCM-simulated TCs. Results indicate that the inner-core wind structures of simulated TCs are more strongly constrained by the horizontal resolutions of the models than are the thermodynamic structures of those TCs. As expected, the structures of TC circulations become more realistic with smaller horizontal grid spacing, such that the radii of maximum wind (RMW) become smaller, and the maximum vertical velocities occur off the center. However, the RMWs are still too large, especially at higher intensities, and there are rising motions occurring at the storm centers, inconsistently with observations. The distributions of precipitation, moisture, radiative and surface turbulent heat fluxes around TCs are diverse, even across models with similar horizontal resolutions. At the same horizontal resolution, models that produce greater rainfall in the inner-core regions tend to simulate stronger TCs. Further, when TCs are weak, the radial gradient of net column radiative flux convergence is comparable to that of surface turbulent heat fluxes, emphasizing the importance of cloud-radiative feedbacks during the early developmental phases of TCs.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2019

Evaluation of Modeled Precipitation in Oceanic Extratropical Cyclones Using IMERG

Using a high-spatial- and high-temporal-resolution precipitation dataset, Integrated Multi-satellite Retrievals for GPM (IMERG), extratropical cyclone precipitation is evaluated in two reanalyses and two climate models. Based on cyclone-centered composites, all four models overestimate precipitation in the western subsiding and dry side of the cyclones, and underestimate the precipitation in the eastern ascending and moist side. By decomposing the composites into frequency of occurrence and intensity (mean precipitation rate when precipitating), the analysis reveals a tendency for all four models to overestimate frequency and underestimate intensity, with the former issue dominating in the western half and the latter in the eastern half of the cyclones. Differences in frequency are strongly dependent on cyclone environmental moisture, while the differences in intensity are strongly impacted by the strength of ascent within the cyclone. There are some uncertainties associated with the observations: IMERG might underreport frozen precipitation and possibly exaggerate rates in vigorously ascending regions. Nevertheless, the analysis suggests that all models produce extratropical cyclone precipitation too often and too lightly. These biases have consequences when evaluating the changes in precipitation characteristics with changes in cyclone properties: the models disagree on the magnitude of the change in precipitation intensity with a change in environmental moisture and in precipitation frequency with a change in cyclone strength. This complicates accurate predictions of precipitation changes in a changing climate.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2019

Moist Static Energy Budget Analysis of Tropical Cyclone Intensification in High-Resolution Climate Models

Abstract Tropical cyclone intensification processes are explored in six high-resolution climate models. The analysis framework employs process-oriented diagnostics that focus on how convection, moisture, clouds, and related processes are coupled. These diagnostics include budgets of column moist static energy and the spatial variance of column moist static energy, where the column integral is performed between fixed pressure levels. The latter allows for the quantification of the different feedback processes responsible for the amplification of moist static energy anomalies associated with the organization of convection and cyclone spinup, including surface flux feedbacks and cloud-radiative feedbacks. Tropical cyclones (TCs) are tracked in the climate model simulations and the analysis is applied along the individual tracks and composited over many TCs. Two methods of compositing are employed: a composite over all TC snapshots in a given intensity range, and a composite over all TC snapshots at the same stage in the TC life cycle (same time relative to the time of lifetime maximum intensity for each storm). The radiative feedback contributes to TC development in all models, especially in storms of weaker intensity or earlier stages of development. Notably, the surface flux feedback is stronger in models that simulate more intense TCs. This indicates that the representation of the interaction between spatially varying surface fluxes and the developing TC is responsible for at least part of the intermodel spread in TC simulation.

Wing, Allison A.↗

DOE OSTI2019

Understand the Direct Effect of CO 2 Increase on Tropical Circulation and TC Activity: Land Surface Warming Versus Direct Radiative Forcing

Abstract The direct effect of increased CO 2 involves contributions from both land warming and direct radiative forcing. Here, their relative impacts on tropical circulation and tropical cyclones (TCs) are quantified by increasing CO 2 over land and ocean separately in a high‐resolution atmosphere‐only model. It is found that land warming induces substantial vertical velocity changes over nearby oceans and such local changes are compensated by opposite motions within tropical ascending regions. The cloud mask effect leads to smaller CO 2 ‐induced radiative heating over ascending regions. Such inhomogeneity in radiative forcing dominates compensated changes in surface fluxes and gross moist stability, leading to the slowdown of the tropical overturning circulation. TC activity is tightly linked with the large‐scale ascent because of the influence on the atmospheric humidity. Regional circulation changes caused by land warming strongly suppresses TC activity over the northwest Pacific, while weakened ascent from direct radiative forcing causes an overall TC reduction.

Zhou, Wenyu↗

DOE OSTI2019

Clinical significance and immunobiology of IL-21 in autoimmunity

Not Available

Long, Di↗

DOE OSTI2018

Estimation of bare soil evaporation for different depths of water table in the wind-blown sand area of the Ordos Basin, China

Soil surface evaporation is a significant component of the hydrological cycle, occurring at the interface between the atmosphere and vadose zone, but it is affected by factors such as groundwater level, soil properties, solar radiation and others. In order to understand the soil evaporation characteristics in arid regions, a field experiment was conducted in the Ordos Basin, central China, and high accuracy sensors of soil moisture, moisture potential and temperature were installed in three field soil profiles with water-table depths (WTDs) of about 0.4, 1.4 and 2.2 m. Soil-surface-evaporation values were estimated by observed data combined with Darcy’s law. Results showed that: (1) soil-surface-evaporation rate is linked to moisture content and it is also affected by air temperature. When there is sufficient moisture in the soil profile, soil evaporation increases with rising air temperature. For a WTD larger than the height of capillary rise, the soil evaporation is related to soil moisture content, and when air temperature is above 25 °C, the soil moisture content reduces quickly and the evaporation rate lowers; (2) phreatic water contributes to soil surface evaporation under conditions in which the WTD is within the capillary fringe. This indicates that phreatic water would not participate in soil evaporation for a WTD larger than the height of capillary rise. This finding developed further the understanding of phreatic evaporation, and this study provides valuable information on recognized soil evaporation processes in the arid environment.

58 GEOSCIENCES↗

DOE OSTI2018

Evaluation of Planetary Boundary Layer Simulation in GFDL Atmospheric General Circulation Models

This study describes the performance of two Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric general circulation models (AGCMs) in simulating the climatologies of planetary boundary layer (PBL) parameters, with a particular focus on the diurnal cycles. The two models differ solely in the PBL parameterization: one uses a prescribed K-profile parameterization (KPP) scheme with an entrainment parameterization, and the other employs a turbulence kinetic energy (TKE) scheme. The models are evaluated through comparison with the reanalysis ensemble, which is generated from European Centre for Medium-Range Weather Forecasts (ECMWF) twentieth-century reanalysis (ERA-20C), ERA-Interim, NCEP CFSR, and NASA MERRA, and the following systematic biases are identified. The models exhibit widespread cold biases in the high latitudes, and the biases are smaller when the KPP scheme is used. The diurnal cycle amplitudes are underestimated in most dry regions, and the model with the TKE scheme simulates larger amplitudes. For the near-surface winds, the models underestimate both the daily means and the diurnal amplitudes; the differences between the models are relatively small compared to the biases. The role of the PBL schemes in simulating the PBL parameters is investigated through the analysis of vertical profiles. The Sahara, which is suitable for focusing on the role of vertical mixing in dry PBLs, is selected for a detailed analysis. It reveals that compared to the KPP scheme, the heat transport is weaker with the TKE scheme in both convective and stable PBLs as a result of weaker vertical mixing, resulting in larger diurnal amplitudes. Lack of nonlocal momentum transport from the nocturnal low-level jets to the surfaces appears to explain the underestimation of the near-surface winds in the models.

58 GEOSCIENCES↗

DOE OSTI2018

Synthesis of Ru Icosahedral Nanocages with a Face-Centered-Cubic Structure and Evaluation of Their Catalytic Properties

Owing to the presence of {111} facets, twin boundaries, and strain fields on the surface, noble-metal nanocrystals with an icosahedral shape have been reported with stellar performance toward an array of catalytic reactions. Here, we report the successful synthesis of Ru icosahedral nanocages with a face-centered cubic (fcc) structure by conformally coating Pd icosahedral seeds with ultrathin Ru shells, followed by selective removal of the Pd cores via chemical etching. We discovered that the presence of bromide ions was critical to the layer-by-layer deposition of Ru atoms. According to in situ XRD, the fcc structure in the Ru nanocages could be retained up to 300 °C before it was transformed into the conventional hexagonal close-packed (hcp) structure. Additionally, the icosahedral shape of the Ru nanocages could be largely preserved up to 300 °C. The Ru icosahedral nanocages with twin boundaries on the surface exhibited greatly enhanced activities toward both the reduction of 4-nitrophenol and decomposition of hydrazine than their cubic and octahedral counterparts. When benchmarked against the parental Pd@Ru core–shell nanocrystals, all the Ru nanocages displayed superior catalytic activities. First-principles density functional theory calculations also suggest that the fcc-Ru icosahedral nanocages containing residual Pd atoms are more promising than the conventional hcp-Ru solid nanoparticles in catalyzing nitrogen reduction for ammonia synthesis. With the subsurface impurities of Pd, the twin boundary regions of the icosahedral nanocages are able to stabilize the N 2 dissociation transition state, reducing the overall reaction barrier and promoting the competition with the N 2 desorption process.

36 MATERIALS SCIENCE↗

DOE OSTI2018

The GFDL Global Atmosphere and Land Model AM4.0/LM4.0: 1. Simulation Characteristics With Prescribed SSTs

In this two–part paper, a description is provided of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). This version, with roughly 100 km horizontal resolution and 33 levels in the vertical, contains an aerosol model that generates aerosol fields from emissions and a “light” chemistry mechanism designed to support the aerosol model but with prescribed ozone. In Part 1, the quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode—with prescribed sea surface temperatures (SSTs) and sea–ice distribution—is described and compared with previous GFDL models and with the CMIP5 archive of AMIP simulations. Here, the model's Cess sensitivity (response in the top–of–atmosphere radiative flux to uniform warming of SSTs) and effective radiative forcing are also presented. In Part 2, the model formulation is described more fully and key sensitivities to aspects of the model formulation are discussed, along with the approach to model tuning.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2018

The GFDL Global Atmosphere and Land Model AM4.0/LM4.0: 2. Model Description, Sensitivity Studies, and Tuning Strategies

Here, in Part 2 of this two–part paper, documentation is provided of key aspects of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode has been provided in Part 1. Part 2 provides documentation of key components and some sensitivities to choices of model formulation and values of parameters, highlighting the convection parameterization and orographic gravity wave drag. The approach taken to tune the model's clouds to observations is a particular focal point. Care is taken to describe the extent to which aerosol effective forcing and Cess sensitivity have been tuned through the model development process, both of which are relevant to the ability of the model to simulate the evolution of temperatures over the last century when coupled to an ocean model.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2017

Facile Synthesis of Ru-Based Octahedral Nanocages with Ultrathin Walls in a Face-Centered Cubic Structure

Noble-metal nanocages with ultrathin (less than 2 nm) walls and well-defined facets have received great interest owing to their remarkable utilization efficiency of atoms and facet-dependent catalytic activities toward various reactions. Here, we report the synthesis of Ru-based octahedral nanocages covered by {111} facets, together with ultrathin walls in a face-centered cubic (fcc) structure rather than the hexagonal close-packed (hcp) of bulk Ru. The involvement of slow injection for the Ru(III) precursor, the introduction of KBr, and the use of elevated temperature were all instrumental to the formation of Pd@Ru core–shell octahedra with a conformal, uniform shell and a smooth surface. The {111} facets were well preserved during the selective removal of the Pd cores via wet etching, even when the Ru walls were only five atomic layers in thickness. Through in situ XRD, we demonstrated that the fcc structure of the Ru nanocages was stable up to 300 °C. We also used first-principles, self-consistent density functional theory calculations to study the adsorption and dissociation of N2 as a means to predict the catalytic performance toward ammonia synthesis. Our results suggested that the small proportions of Pd atoms left behind in the walls during etching could play a key role in stabilizing the adsorption of N2 as well as in reducing the activation energy barrier to N2 dissociation.

Zhao, Ming↗