This study investigated the relationship between the summer Arctic Dipole (AD) anomaly and the climatic variability in Eurasia during the period 1979–2021. It was found that the summer AD anomaly ...experienced a phase shift from frequent negative phases before 2006 to positive phases after 2007, as manifested by the shift of the center of the positive (negative) AD anomaly to Greenland (in the Laptev Sea and East Siberian Seas) in the more recent period (2007–2021) from the vicinity of the Kara Sea and Laptev Sea (the Canadian archipelago) in the earlier period (1979–2006). Before the mid-2000s, a wave train was shown in the middle troposphere of Eurasia, and this teleconnection pattern of atmospheric circulation could have resulted in local warm and wet (cool and dry) anomalies over northern Russia and East Asia (Western Europe and the Far east). Since the mid-2000s, the wave train has experienced a notable adjustment that was conducive to East Asian and Arctic cooling, displaying anticyclonic anomalies around northern Eurasia and two cyclonic anomalies centered near the Arctic and East Asia. The presence of a cold Arctic anomaly was found to enhance westerly winds at high latitudes by modulating the meridional temperature gradient (MTG) and impeding the southward propagation of cold Arctic air. Additionally, the warmth of northern Eurasia may have also resulted in a reduction in the MTG between northern Eurasia and the mid-lower latitudes, favoring a weakening of zonal winds over the central region of Eurasia. The increased upper-level westerly winds over southern East Asia implied a weakened East Asian Summer Monsoon, which inhibited precipitation in northeast China.
Irrigation activities have been recognized as one of the most potentially sustainable land management practices to tackle climate change. In this study, we investigated the impacts of present-day ...irrigation activities on regional temperature extremes changes in China using the state-of-the-art regional climate model RegCM-4.8.2, through two sets of long-term numerical experiments (i.e., with and without irrigation). The key findings are as follows. Firstly, by comparing the observations, RegCM4 has a good ability to reproduce both the spatial distribution and the volume of irrigation for provincial irrigation activities in China. In this context, by comparing the results of numerical experiments, we found that irrigation led to significant reductions (slightly changes) in the extreme indices associated with the warm (cold) tails of the maximum and minimum temperatures distributions, e.g., hot days, tropical nights (cold nights, frost days), particularly in the regions with intense irrigation (e.g., the North China Plain). Furthermore, these extreme indices were dominated by irrigation-induced shifts in mean temperatures, but the impacts of changes in temperature variability on extreme hot events were also non-negligible for the strongly irrigated regions. In addition, through attributing irrigation-induced changes in mean state temperatures, we revealed that the irrigation-induced repartitioning of the surface turbulent heat fluxes (i.e., sensible and latent heat) dominated the variations of temperatures and the associated extremes. However, region-specific land-atmosphere coupling features also led to differences in the irrigation-induced responses of surface energy components. We highlight the possible effects and mechanisms of irrigation on multiple types of extreme temperature indices, particularly the extreme hot events, which provide scientific clues for mitigating and adapting regional climate change.
Dynamic and continuous land use and cover change (LUCC) is one of the external forcing factors affecting regional climate in China. Based on the annual dynamic global land cover dataset derived from ...the Global Land Surface Satellite (GLASS-GLC), this paper modelled and investigated the effects of annual transient LUCC on precipitation over China using the regional climate model RegCM4.8.2. Analysis of the GLASS-GLC data revealed that considerable conversions of cropland to forest, grassland to cropland and bare ground to grassland had occurred during the last 3 decades (1984–2013) in southern, northeastern and northwestern China respectively. By comparing the differences between the two sets of experiments under the fixed LUCC and dynamic LUCC scenarios, the results showed that reforestation in southern China during the past 3 decades significantly enhanced local convective afternoon rainfall (CAR; ∼2 mm/day) during the post-flood season (POF; July-August-September). This reforestation effect on CAR also increased with the intensity of LUCC. However, the realistic LUCC effects were weak and negligible for other periods, regions and large-scale precipitation. Furthermore, we have identified two possible reasons that favored the occurrence of POFCAR, namely that the moisture and instability conditions required to trigger convections were both enhanced by reforestation. This was evidenced by increases (decreases) in convective available potential energy (lifting condensation level), increases in atmospheric water vapor content and declining tendencies of equivalent potential temperature with height. Overall, this study highlights the importance of reforestation impacts on the diurnal variations of the precipitation.
This study reveals a marked enhancement in the relationship between the variations in location of the winter East Asian Polar Front Jet (EAPJ) and the surface air temperature (SAT) in Eurasia since ...the mid-1990s. Before the mid-1990s, an evident wave train related to the meridional location of the EAPJ exhibited an anticyclonic anomaly over northern Europe and a cyclonic anomaly in northwestern Asia. With an equatorward shift of the EAPJ after the mid-1990s, the wave train experiences a notable adjustment that is conducive to East Asian cooling, displaying an anticyclonic anomaly around the Kara-Laptev Seas and a cyclonic anomaly near northeastern Asia. Arctic warming anomalies and sea ice loss contributed significantly to these decadal changes. Simulation experiments forced by observed Arctic sea-ice variability further confirm this result. Since the mid-1990s, Arctic sea ice loss (or Arctic warming anomaly) has contributed to a reduction in westerly winds in high latitudes by modulating the meridional temperature gradient. The deaccelerated winds intensify the Arctic cold air propagating to the south, enhancing the atmospheric baroclinicity and the westerly flow in the upper level at the south side of the EAPJ, favoring the southward shift of the EAPJ. With the equatorward shift of the EAPJ, the corresponding SAT anomalies in East Asia are more salient.
Developing high fidelity quantitative models of solid state reaction systems can be challenging, especially in deposition systems where, in addition to the multiple competing processes occurring ...simultaneously, the solid interacts with its atmosphere. In this work, we develop a model for the growth of a thin solid film where species from the atmosphere adsorb, diffuse, and react with the film. The model is mesoscale and describes an entire film with thickness on the order of microns. Because it is stochastic, the model allows us to examine inhomogeneities and agglomerations that would be impossible to characterize with deterministic methods. We demonstrate the modeling approach with the example of chalcopyrite Cu(InGa)(SeS)2 thin film growth via precursor reaction, which is a common industrial method for fabricating thin film photovoltaic modules. The model is used to understand how and why through-film variation in the composition of Cu(InGa)(SeS)2 thin films arises and persists. We believe that the model will be valuable as an effective quantitative description of many other materials systems used in semiconductors, energy storage, and other fast-growing industries.
Reducing carbon emissions is a looming challenge that will be required to limit global warming. One approach is to replace energy from fossil fuels with renewable electricity that has low carbon ...footprints. The continuous decrease of renewable electricity prices makes electrochemical processes very promising for environmentally friendly production of fuels and chemicals. One of the mature electrochemical processes is hydrogen (H₂) production from water electrolysis. If only excess solar/wind electricity is used to power water electrolyzers to produce green H₂, the intermittency of the electricity supply will require low-cost electrolyzer technologies. Emerging membraneless water electrolyzers offer an attractive approach to lowering the cost of H₂ production by eliminating membranes or diaphragms that are used in conventional water electrolyzers.
One aim of this dissertation is to understand the performance limits of membraneless water electrolyzers compared to the conventional designs. Another key electrochemical process to reduce carbon emissions is the conversion of carbon dioxide (CO₂) to value-added fuels and chemicals. CO₂ captured from air or flue gas needs to be extracted from carbon capture solution and pressurized before feeding to a conventional CO₂ electrolyzer. In order to avoid these energy intensive steps to make pressurized CO₂, there is a growing interest in developing membrane-based electrolyzers that can directly utilize the carbon capture solution to conduct electrochemical CO₂ conversion. This dissertation also explores a scalable membrane-free electrolyzer design that can convert carbon capture solution to syngas.
In Chapter 2, a parallel plate membraneless electrolyzer is used as a model system to demonstrate a combined experimental and modeling approach to explore its performance limits. This modeling framework quantitatively describes the trade-offs between efficiency, current density, electrode size, and product purity. Central to this work is the use of in situ high-speed videography (HSV) to monitor the width of H₂ bubble plumes produced downstream of parallel plate electrodes as a function of current density, electrode separation distance, and the Reynolds number (Re) associated with flowing 0.5 M H₂SO₄ electrolyte. These measurements reveal that the HSV-derived dimensionless bubble plume width serves as an excellent descriptor for correlating the aforementioned operating conditions with H₂ crossover rates. These empirical relationships, combined with electrochemical engineering design principles, provide a valuable framework for exploring performance limits and guiding the design of optimized membraneless electrolyzers. This framework shows that the efficiencies and current densities of optimized parallel plate membraneless electrolyzers constrained to H₂ crossover rates of 1% can exceed those of conventional alkaline electrolyzers but are lower than the efficiencies and current densities achieved by zero-gap polymer electrolyte membrane (PEM) electrolyzers.
Chapter 3 presents a packed bed membraneless electrolyzer (PBME) design for which liquid bicarbonate electrolyte flows sequentially through alternating porous flow-through anodes and cathodes. Within this design, hydrogen oxidation at porous anodes is used to produce protons that trigger in situ CO₂ release immediately upstream of porous cathodes, where electrochemical CO₂ reduction generates the desired product and returns the solution pH back towards its inlet value. By using the sequential flow-cell arrangement, the PBME offers the ability to mitigate large concentration overpotentials and non-uniform current distributions that naturally arise during scale-up of conventional membrane-based devices that rely on lateral flow of catholyte parallel to the surface of the electrodes. This study uses in situ colorimetric imaging to highlight the ability of PBME to rebalance pH across electrodes. In addition, results obtained with a multi-cell PBME “stack” demonstrate the scalability of this concept and reveal the ability to increase CO₂ utilization from 12.9% for a single-cell PBME up to 20.5% for a four-cell PBME operated under baseline conditions. Modeling results indicate that current utilization values >80% are theoretically possible for optimized multi-cell PBMEs operated at ambient pressure.
Chapter 4 demonstrates a stacked PBME design and an elevated pressure system. Hydrogen oxidation at the anode and hydrogen evolution at the cathode are conducted in this device at pressures up to 5 atm. The pressure of the system can be held at a constant value by the use of a back pressure regulator, and product gases can be collected with a gas sampling bag that’s directly connected to the back pressure regulator. The stereolithography 3D printing technology is used to fabricate components of the PBME from clear resin, which is suitable for the elevated pressure operation. Post-processing of the components makes surfaces transparent for imaging bubble dynamics in the device. Within this system, HOR current density of 120 mA cm-2 can be achieved at different pressures.
Finally, Chapter 5 provides concluding remarks and discusses future opportunities and challenges for membraneless electrolyzers for water electrolysis and electrochemical CO₂ conversion.
Abstract
Arctic sea ice loss is one of the most striking features in changing of the climate system and affects weather and climate in the midlatitudes through complex interactions and feedback ...processes. However, it is not clear whether Arctic sea ice loss has affected summer precipitation in East Asia. This study shows that summer regional precipitation in North China exhibits obvious features of phasic variations. Before 1998, positive and negative precipitation anomalies appeared alternately. While from 1999 to 2011, negative precipitation anomalies were dominant. Since 2012, increased summer precipitation has been observed frequently. A direct cause of increases of summer precipitation in North China is an enhancement of water vapour transport from South China and China's marginal seas, which can be attributed to the strengthening and northward shifting of the northwestern Pacific subtropical high and the spatial configuration with west low–east high and north low–south high in East Asian height fields. Although summer atmospheric circulation anomalies in high latitudes are dominant during the sea ice melting phase, their contributions to summer precipitation variations in North China are little. Simulation experiments show that Arctic sea ice loss can significantly affect the atmospheric circulation in high latitudes; however, it cannot be regarded as a major cause of observed recent increases in precipitation in North China. Additionally, Arctic sea ice loss has little impact on summer precipitation in the Yangtze River valley and South China.
Emerging membraneless electrolyzers offer an attractive approach to lowering the cost of hydrogen (H
2
) production from water electrolysis thanks to potential advantages in durability and ...manufacturability that are made possible by elimination of membranes or diaphragms from the device architecture. However, a fair comparison of the performance limits of membraneless electrolyzers to conventional electrolyzers is hindered by the early stage of research and absence of established design rules for the former. This task is made all the more difficult by the need to quantitatively describe multiphase flow between the electrodes in membraneless electrolyzers, which can have a huge impact on gas product purity. Using a parallel plate membraneless electrolyzer (PPME) as a model system, this study takes a combined experimental and modeling approach to explore its performance limits and quantitatively describe the trade-offs between efficiency, current density, electrode size, and product purity. Central to this work is the use of
in situ
high-speed videography (HSV) to monitor the width of H
2
bubble plumes produced downstream of parallel plate electrodes as a function of current density, electrode separation distance, and the Reynolds number (Re) associated with flowing 0.5 M H
2
SO
4
electrolyte. These measurements reveal that the HSV-derived dimensionless bubble plume width serves as an excellent descriptor for correlating the aforementioned operating conditions with H
2
crossover rates. These empirical relationships, combined with electrochemical engineering design principles, provide a valuable framework for exploring performance limits and guiding the design of optimized membraneless electrolyzers. Our analysis shows that the efficiencies and current densities of optimized PPMEs constrained to H
2
crossover rates of 1% can exceed those of conventional alkaline electrolyzers but are lower than the efficiencies and current densities achieved by zero-gap polymer electrolyte membrane (PEM) electrolyzers.
A framework is demonstrated to explore the performance limits and investigate the trade-offs between key performance metrics for membraneless electrolyzers.
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