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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 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

‘The Cloud is Not Not IT’: Ecological Change in Research Computing in the Cloud

Along with a number of other computing technologies, cloud computing services are increasingly being promoted as a way of enabling openness, reproducibility, and the acceleration of scientific work. While there have been a variety of studies of the cloud in terms of computing performance, there has been little empirical attention to the changes going on around cloud computing at the level of work and practice. Through a qualitative, ethnographic study, we follow a cosmology research group’s transition from a shared high performance computing cluster to a cloud computing service, and examine the cloud service as a coordinative artifact being integrated into a larger ecology of existing practices and artifacts. We find that the transition involves both change and continuity in the group’s coordinative work and maintenance work, and point out some of the effects this adoption has on the group’s larger set of practices. Finally, we discuss practical implications this has for the broader adoption of cloud computing in university-based scientific work.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2022

Crystalline phase transitions and water-soluble complexes of copper(Ⅰ) 2-hydroxyethanethiolate

We report the coordination polymer copper(I) 2-hydroxyethanethiolate, (CuSCH 2 CH 2 OH) n , though insoluble in all common solvents, dissolved readily in basic aqueous solutions of the thiolate anion (HOCH 2 CH 2 S – ) of 2-mercaptoethanol to form a single species: the tetranuclear cluster [Cu 4 (μ-SCH 2 CH 2 OH) 6 ] 2– . From this solution were grown X-ray quality single crystals of copper(I) 2-hydroxyethanethiolate. This compound underwent a hitherto unknown crystal phase transition at ca. 6 °C, from point group P2 1 2 1 2 1 to Pna2 1 , with noticeable changes in the geometry of the Cu-S layer and in the orientation of the alkylthiolate side chains. When the bulky base tetrabutylammonium hydroxide was employed in the aqueous thiolate solution used to dissolve (CuSCH 2 CH 2 OH) n , the water-soluble polynuclear copper(I) complex bis(tetrabutylammonium) hexakis(μ-2-hydroxyethanethiolato) tetracuprate(I), [(C 4 H 9 ) 4 N] 2 [Cu 4 (μ-SCH 2 CH 2 OH) 6 ], could be isolated as X-ray quality crystals. Structural characterization of this complex revealed a tetrahedral arrangement of copper(I) centers with thiolates bridging the edges of the tetrahedra. On standing, this complex degraded to a larger polynuclear Cu(I) sulfide cluster.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Vibrational Spectroscopy Reveals Effects of Electron Push–Pull and Solvent Polarity on Electron Delocalization in Radical Anions of Donor–Acceptor Compounds

Herein, the nature of excess electrons has been studied in donor–acceptor (D–A) compounds based on substituted triarylamines and a nitrile-functionalized fluorene, by changing the substituents on the triarylamines and also the solvent polarity. We observed that both electron push-pull capability at the distant location in the amine donor unit and solvation in solvents of varying polarity significantly affect the nitrile $ν(C≡N)$ vibrations of the fluorene acceptor unit in radical anions of these D–A compounds. Quantum calculations show that the push-pull capability translates the position of an excess electron while keeping its width relatively constant. On the other hand, solvation changes both, making an electron more compact in polar media. The current study points to the idea that solvation plays a more significant role in controlling the nature of excess electrons, while synthetic modification that influences electron push-pull capability enables further tuning.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Strong, thermo-reversible salogels with boronate ester bonds as thermal energy storage materials

Inorganic salt hydrates are promising phase change materials (PCMs) but suffer from low viscosity at temperatures above their melting point resulting in leakage problems during thermal storage applications. To achieve shape stabilization of one type of molten inorganic PCM – calcium nitrate tetrahydrate (CNH) – this work explored gelation of polyvinyl alcohol (PVA) in this solvent and the effect of dynamic boronate ester bonds on salogel strength. The occurrence of gelation of PVA in molten CNH but not in water is rationalized by the extremely high salt content and scarcity of hydration water in CNH, enabling intermolecular hydrogen bonding between PVA chains. While neat PVA salogels in CNH were weak, with a gel-to-sol transition temperature (T gel ) below room temperature, the addition of small amounts of borax (<~0.3 wt%) introduced dynamic covalent crosslinks and yielded salogels with T gel tunable over a wide temperature range from 7 to 70 °C. Here, the PVA/borax salogels were about one order of magnitude stronger than their well-known PVA/borax hydrogel counterparts, and, unlike PVA/borax hydrogels, were capable of retaining their shape and preventing leakage of molten CNH. Moreover, the salogels exhibited reversible and repeatable temperature-triggered gel-to-sol transitions and the ability to self-heal. The low polymer and crosslinker concentration also ensured that more than 95% of the heat of fusion of neat CNH was maintained in the salogels and was retained after twenty cycles of melting and crystallization, demonstrating the robust nature of these energy storage materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Untangling the sequence of events during the S 2 → S 3 transition in photosystem II and implications for the water oxidation mechanism

In oxygenic photosynthesis, light-driven oxidation of water to molecular oxygen is carried out by the oxygen-evolving complex (OEC) in photosystem II (PS II). Recently, we reported the room-temperature structures of PS II in the four (semi)stable S-states, S 1 , S 2 , S 3 , and S 0 , showing that a water molecule is inserted during the S 2 → S 3 transition, as a new bridging O(H)-ligand between Mn1 and Ca. To understand the sequence of events leading to the formation of this last stable intermediate state before O 2 formation, we recorded diffraction and Mn X-ray emission spectroscopy (XES) data at several time points during the S 2 → S 3 transition. At the electron acceptor site, changes due to the two-electron redox chemistry at the quinones, Q A and Q B , are observed. At the donor site, tyrosine Y Z and His190 H-bonded to it move by 50 µs after the second flash, and Glu189 moves away from Ca. This is followed by Mn1 and Mn4 moving apart, and the insertion of O X (H) at the open coordination site of Mn1. This water, possibly a ligand of Ca, could be supplied via a “water wheel”-like arrangement of five waters next to the OEC that is connected by a large channel to the bulk solvent. XES spectra show that Mn oxidation (τ of ∼350 µs) during the S 2 → S 3 transition mirrors the appearance of O X electron density. This indicates that the oxidation state change and the insertion of water as a bridging atom between Mn1 and Ca are highly correlated.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2023

Respiration is essential for aerobic growth of Zymomonas mobilis ZM4

ABSTRACT Zymomonas mobilis is an alpha-proteobacterium that is a promising platform for industrial scale production of biofuels due to its efficient ethanol fermentation and low biomass generation. Z. mobilis is aerotolerant and encodes a complete respiratory electron transport chain, but the benefit of respiration for growth in oxic conditions has never been confirmed, despite decades of research. Growth and ethanol production of wild-type Z. mobilis is poor in oxic conditions indicating that it does not benefit from oxidative phosphorylation. Additionally, in previous studies, aerobic growth improved significantly when respiratory genes were disrupted ( ndh ) or acquired point mutations ( cydA and cydB ), even if respiration was significantly reduced by these changes. Here, we obtained clean deletions of respiratory genes ndh and cydAB , individually and in combination, and showed, for the first time, that deletion of cydAB completely inhibited O 2 respiration and dramatically reduced growth in oxic conditions. Both respiration and aerobic growth were restored by expressing a heterologous, water-forming NADH oxidase, noxE . Oxygen can have many negative effects, including formation of reactive oxygen species (ROS) or directly inactivating oxygen sensitive enzymes. Our results suggest that the effect of molecular oxygen on enzymes had a greater negative impact on Z. mobilis than formation of ROS. This result shows that the main role of the electron transport chain in Z. mobilis is reducing the intracellular concentration of molecular oxygen, helping to explain why it is beneficial for Z. mobilis to use electron transport chain complexes that have little capacity to contribute to oxidative phosphorylation. IMPORTANCE A key to producing next-generation biofuels is to engineer microbes that efficiently convert non-food materials into drop-in fuels, and to engineer microbes effectively, we must understand their metabolism thoroughly. Zymomonas mobilis is a bacterium that is a promising candidate biofuel producer, but its metabolism remains poorly understood, especially its metabolism when exposed to oxygen. Although Z. mobilis respires with oxygen, its aerobic growth is poor, and disruption of genes related to respiration counterintuitively improves aerobic growth. This unusual result has sparked decades of research and debate regarding the function of respiration in Z. mobilis . Here, we used a new set of mutants to determine that respiration is essential for aerobic growth and likely protects the cells from damage caused by oxygen. We conclude that the respiratory pathway of Z. mobilis should not be deleted from chassis strains for industrial production because this would yield a strain that is intolerant of oxygen, which is more difficult to manage in industrial settings.

09 BIOMASS FUELS↗
DOE OSTI2021

Meanders as a scaling motif for understanding of floodplain soil microbiome and biogeochemical potential at the watershed scale

Abstract Background Biogeochemical exports from watersheds are modulated by the activity of microorganisms that function over micron scales. Here, we tested the hypothesis that meander-bound regions share a core microbiome and exhibit patterns of metabolic potential that broadly predict biogeochemical processes in floodplain soils along a river corridor. Results We intensively sampled the microbiomes of floodplain soils located in the upper, middle, and lower reaches of the East River, Colorado. Despite the very high microbial diversity and complexity of the soils, we reconstructed 248 quality draft genomes representative of subspecies. Approximately one third of these bacterial subspecies was detected across all three locations at similar abundance levels, and ~ 15% of species were detected in two consecutive years. Within the meander-bound floodplains, we did not detect systematic patterns of gene abundance based on sampling position relative to the river. However, across meanders, we identified a core floodplain microbiome that is enriched in capacities for aerobic respiration, aerobic CO oxidation, and thiosulfate oxidation with the formation of elemental sulfur. Given this, we conducted a transcriptomic analysis of the middle floodplain. In contrast to predictions made based on the prominence of gene inventories, the most highly transcribed genes were relatively rare amoCAB and nxrAB (for nitrification) genes, followed by genes involved in methanol and formate oxidation, and nitrogen and CO 2 fixation. Within all three meanders, low soil organic carbon correlated with high activity of genes involved in methanol, formate, sulfide, hydrogen, and ammonia oxidation, nitrite oxidoreduction, and nitrate and nitrite reduction. Overall, the results emphasize the importance of sulfur, one-carbon and nitrogen compound metabolism in soils of the riparian corridor. Conclusions The disparity between the scale of a microbial cell and the scale of a watershed currently limits the development of genomically informed predictive models describing watershed biogeochemical function. Meander-bound floodplains appear to serve as scaling motifs that predict aggregate capacities for biogeochemical transformations, providing a foundation for incorporating riparian soil microbiomes in watershed models. Widely represented genetic capacities did not predict in situ activity at one time point, but rather they define a reservoir of biogeochemical potential available as conditions change.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

High-temperature stable FBGs fabricated by a point-by-point femtosecond laser inscription for multi-parameter sensing

A high-temperature resistant fiber Bragg grating (FBG) with pronounced cladding modes is fabricated using a simple, fast, and flexible point-by-point femtosecond laser inscription. Cladding modes of different orders exhibit various sensitivities to changes in refractive index of the surrounding medium and temperature, while the Bragg wavelength is only dependent on the ambient temperature. By combining the resonance of cladding modes and the Bragg resonance, measurement of variations of temperature and surrounding refractive index can be achieved. Survivability of the cladding modes and the core mode in the fabricated FBG at elevated temperatures up to 1000°C is demonstrated.

Zhu, Chen (ORCID:0000000232567330)↗
DOE OSTI2024

Characteristics and sources of fluorescent aerosols in the central Arctic Ocean

The Arctic is sensitive to cloud radiative forcing. Due to the limited number of aerosols present throughout much of the year, cloud formation is susceptible to the presence of cloud condensation nuclei and ice nucleating particles (INPs). Primary biological aerosol particles (PBAP) contribute to INPs and can impact cloud phase, lifetime, and radiative properties. We present yearlong observations of hyperfluorescent aerosols (HFA), tracers for PBAP, conducted with a Wideband Integrated Bioaerosol Sensor, New Electronics Option during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition (October 2019–September 2020) in the central Arctic. We investigate the influence of potential anthropogenic and natural sources on the characteristics of the HFA and relate our measurements to INP observations during MOSAiC. Anthropogenic sources influenced HFA during the Arctic haze period. But surprisingly, we also found sporadic “bursts” of HFA with the characteristics of PBAP during this time, albeit with unclear origin. The characteristics of HFA between May and August 2020 and in October 2019 indicate a strong contribution of PBAP to HFA. Notably from May to August, PBAP coincided with the presence of INPs nucleating at elevated temperatures, that is, >–9°C, suggesting that HFA contributed to the “warm INP” concentration. The air mass residence time and area between May and August and in October were dominated by the open ocean and sea ice, pointing toward PBAP sources from within the Arctic Ocean. As the central Arctic changes drastically due to climate warming with expected implications on aerosol–cloud interactions, we recommend targeted observations of PBAP that reveal their nature (e.g., bacteria, diatoms, fungal spores) in the atmosphere and in relevant surface sources, such as the sea ice, snow on sea ice, melt ponds, leads, and open water, to gain further insights into the relevant source processes and how they might change in the future.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

Irradiated stainless steel 347 and irradiated, welded, and hydrogen charged Zircaloy-4 property data for the target solution vessel and support lines of SHINE

In support of the reactor-vessel development effort of SHINE medical technologies, irradiation of its structural materials and post-irradiation testing have been performed in the High Flux Isotope Reactor at Oak Ridge National Laboratory. This irradiation was performed on welded and unwelded AISI 347 steel and Zircaloy-4 samples at low temperature (~60°C) in contact with the reactor coolant. Hardness, Charpy, and tensile data have been taken from all cases of unirradiated specimens, while post-irradiation tensile and hardness data have been taken from AISI 347 base metal and Zircaloy-4 base metal and weld specimens so far. Additionally, 3-point bend data have been taken from explosion-welded specimens. The expected changes to hardness, yield strength, ultimate tensile strength, uniform elongation, and total elongation as a result of the low temperature and low dose neutron irradiation are reported here.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Sensing Electrical Networks Securely & Economically (SENSE)

The growing adoption of distributed energy resources (DERs) like battery energy storage systems and roof top solar/PV and the rapid penetration of electric vehicles (EVs), the electric grid is undergoing a major transformation with elevated stress on legacy grid assets. Despite a lot of expenditure to address these challenges, both in dollars and manpower, utilities have not been able to receive the value that was promised. The gains have been most visible at the transmission and substation level, especially where the main objective was improving operational and economic efficiency for the utility. Improving visibility and control at a few select points enhances the existing and established paradigm of centralized command and control. With changing load patterns, load types and the overall transition to an “active grid”, the centralized control and coordination paradigm gets challenged. To address the challenges, a new architecture and mechanism is needed, one that supports decentralized control and decision making, extracting value streams at the grid edge, particularly as the changes are fueled by transitions occurring in the distribution system. To address this, a communications and data processing platform, “GAMMA” was developed and demonstrated through the project. At the heart of the platform, are distributed, intelligent edge nodes with sensing and compute capabilities, that can record and analyze information locally. They are embedded in sensors and actuators specific to different distribution system applications. Phase 1 of the project focused on developing novel sensor technology that can be used for monitoring utility pole top distribution transformers. The sensors were designed with the objective of being low-cost, communicating with the GAMMA cloud using novel “delay-tolerant” networking using Bluetooth and a secure mobile application. They were non-intrusive in nature so that they can be installed quickly in the field, resulting in overall low cost of deployment and operations. Following the successful completion of Phase 1, the team manufactured 100 units for a field demonstration in Phase 2. The field demonstration was carried out on two real feeder systems with the local utility partner. In total, 100 sensors were installed and operated over a period of 6 months in the state of Georgia. The platform is operational end to end, with the cloud infrastructure deployed on a distributed, serverless environment that can serve multiple data streams, an analytics engine and a portal to securely view the data from multiple assets. The data collected through the GAMMA Mobile Phone app showcased the viability of the novel delay tolerant networking architecture, and the data processing algorithms developed through the course of the project, were successful in extracting important information about the overall network, improving the utility’s visibility and situational awareness in the distribution feeder.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2024

Transcriptomic Analysis of the CAM Species Kalanchoë fedtschenkoi Under Low- and High-Temperature Regimes

Temperature stress is one of the major limiting environmental factors that negatively impact global crop yields. Kalanchoë fedtschenkoi is an obligate crassulacean acid metabolism (CAM) plant species, exhibiting much higher water-use efficiency and tolerance to drought and heat stresses than C 3 or C 4 plant species. Previous studies on gene expression responses to low- or high-temperature stress have been focused on C 3 and C 4 plants. There is a lack of information about the regulation of gene expression by low and high temperatures in CAM plants. To address this knowledge gap, we performed transcriptome sequencing (RNA-Seq) of leaf and root tissues of K. fedtschenkoi under cold (8 °C), normal (25 °C), and heat (37 °C) conditions at dawn (i.e., 2 h before the light period) and dusk (i.e., 2 h before the dark period). Our analysis revealed differentially expressed genes (DEGs) under cold or heat treatment in comparison to normal conditions in leaf or root tissue at each of the two time points. In particular, DEGs exhibiting either the same or opposite direction of expression change (either up-regulated or down-regulated) under cold and heat treatments were identified. In addition, we analyzed gene co-expression modules regulated by cold or heat treatment, and we performed in-depth analyses of expression regulation by temperature stresses for selected gene categories, including CAM-related genes, genes encoding heat shock factors and heat shock proteins, circadian rhythm genes, and stomatal movement genes. Our study highlights both the common and distinct molecular strategies employed by CAM and C 3 /C 4 plants in adapting to extreme temperatures, providing new insights into the molecular mechanisms underlying temperature stress responses in CAM species.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2026

Performance Assessment of a Heat Pump Integrated with Thermal Energy Storage in Cooling & Heating Modes

Thermal Energy Storage (TES) systems have demonstrated significant potential to manage the fluctuating nature of energy demand and shifting peak loads. This technology has made significant recent advances, including new research on the integration of TES with residential and commercial Heating, Ventilation, and Air-Conditioning (HVAC) systems. In this paper, we present the use of a novel laboratory assessment framework for a prototype heat pump system integrated with latent heat phase change material-based TES. The assessment framework consists of 22 total test points (11 heating, 11 cooling) using psychrometric conditions from AHRI Standard 210/240 and includes transient conditions to evaluate TES charging and discharging modes. In addition, both continuous mode and cyclic mode were evaluated for charging and discharging to explore the benefits to TES heat transfer from repeated transient operation. The ambient conditions evaluated include heating mode charging at cold-climate heat pump conditions when winter-peaking utilities may most require demand reduction (H4Full conditions, 5°F/-15°C) and cooling mode charging at high ambient temperature that may occur during times of day when excess solar power generation results in low-cost electricity (105°F/40.6°C). The work demonstrates the demand reduction capabilities of integrating TES with HVAC, including up to 44% in cooling mode and 60% in heating mode compared to conventional HVAC equipment without TES.

25 ENERGY STORAGE↗
DOE OSTI2024

The long road to ignition: An eyewitness account

This paper reviews the many twists and turns in the long journey that culminated in ignition in late 2022 using the laser heated indirect-drive approach to imploding DT filled targets at the National Ignition Facility (NIF), located at the Lawrence Livermore National Laboratory (LLNL). We describe the early origins of the Laser Program at LLNL and key developments such as the paradigm shifting birth of high energy density physics (HEDP) studies with lasers, changes in choice of laser wavelength, and the development of key diagnostics and computer codes. Fulfilling the requirements of the multi-faceted Nova Technical Contract was a necessary condition for the approval of the NIF, but more importantly, the end of the Cold War and the cessation of nuclear testing were key catalysts in that approval, along with the ready-and-waiting field of HEDP. The inherent flexibility of the field of laser driven inertial confinement fusion played a fundamental role in achieving success at the NIF. We describe how the ultimately successful ignition target design evolved from the original “point design” target, through the lessons of experiment. All key aspects of that original design changed: The capsule's materials and size were changed; the hohlraum's materials, size, laser entrance hole size, and gas fills were also all changed, as were the laser pulse shapes that go along with all those changes. The philosophy to globally optimize performance for stability (by raising the adiabat and thus lowering the implosion convergence) was also key, as was progress in target fabrication, and in increasing NIF's energy output. The persistence of the research staff and the steadfast backing of our supporters were also necessary elements in this success. We gratefully acknowledge seven decades of researcher endeavors and four decades of the dedicated efforts of many hundreds of personnel across the globe who have participated in NIF construction, operation, target fabrication, diagnostic, and theoretical advances that have culminated in ignition.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Enhancement of superconductivity at a quantum critical point in (Ca x Sr 1- x ) 3 Rh 4 Sn 13

We report microscopic studies by muon spin spectroscopy of the superconducting properties as a function of chemical and hydrostatic pressure in the cubic ternary intermetallic (Ca x Sr 1-x ) 3 Rh 4 Sn 13 compounds. We find evidence of a quantum critical point at a critical pressure pc in the superconducting phase, where the superfluid density increases by a factor of two and the superconducting pairing strength displays a pronounced maximum. The enhancement of superconductivity is related to the structural phase transition at p c , which is accompanied by profound changes of the Fermi surface associated to the suppression of a charge density wave (CDW). The quantum critical point separates a superconducting phase coexisting with CDW from a pure superconducting phase, while in both phases superconductivity has a strong-coupling phonon-mediated BCS-like s-wave character. Together with the related isoelectronic compound (Ca x Sr 1-x ) 3 Ir 4 Sn 13 , this system shows that conventional BCS superconductors in the presence of competing orders may display behaviour and characteristics of unconventional superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Measuring Saturn's Electron Beam Energy Spectrum using Webb's Wedges

It is very difficult to measure the voltage of the load on the Saturn accelerator. Time-resolved measurements such as vacuum voltmeters and V-dot monitors are impractical at best and completely change the pulsed power behavior at the load at worst. We would like to know the load voltage of the machine so that we could correctly model the radiation transport and tune our x-ray unfold methodology and circuit simulations of the accelerator. Step wedges have been used for decades as a tool to measure the end - point energies of high energy particle beams. Typically, the technique is used for multi-megavolt accelerators, but we have adapted it to Saturn's modest <2 MV end-point energy and modified the standard bremsstrahlung x-ray source to extract the electron beam without changing the physics of the load region. We found clear evidence of high energy electrons >2 MV. We also attempted to unfold an electron energy spectrum using a machine learning algorithm and while these results come with large uncertainties, they qualitatively agree with PIC simulation results.

43 PARTICLE ACCELERATORS↗
DOE OSTI2020

Hydrometeor Shape Variability in Snowfall as Retrieved from Polarimetric Radar Measurements

A polarimetric radar–based method for retrieving atmospheric ice particle shapes is applied to snowfall measurements by a scanning Ka-band radar deployed at Oliktok Point, Alaska (70.495°N, 149.883°W). The mean aspect ratio, which is defined by the hydrometeor minor-to-major dimension ratio for a spheroidal particle model, is retrieved as a particle shape parameter. The radar variables used for aspect ratio profile retrievals include reflectivity, differential reflectivity, and the copolar correlation coefficient. The retrievals indicate that hydrometeors with mean aspect ratios below 0.2–0.3 are usually present in regions with air temperatures warmer than approximately from –17° to –15°C, corresponding to a regime that has been shown to be favorable for growth of pristine ice crystals of planar habits. Radar reflectivities corresponding to the lowest mean aspect ratios are generally between –10 and 10 dBZ. For colder temperatures, mean aspect ratios are typically in a range between 0.3 and 0.8. There is a tendency for hydrometeor aspect ratios to increase as particles transition from altitudes in the temperature range from –17° to –15°C toward the ground. Furthermore, this increase is believed to result from aggregation and riming processes that cause particles to become more spherical and is associated with areas demonstrating differential reflectivity decreases with increasing reflectivity. Aspect ratio retrievals at the lowest altitudes are consistent with in situ measurements obtained using a surface-based multiangle snowflake camera. Pronounced gradients in particle aspect ratio profiles are observed at altitudes at which there is a change in the dominant hydrometeor species, as inferred by spectral measurements from a vertically pointing Doppler radar.

54 ENVIRONMENTAL SCIENCES↗