Yakun Liu

The effect of anthropogenic aerosols on lightning is one of the least understood aspects of human‐induced climate change. Global aerosol clearly diminished during the COVID pandemic by 7.6%. A pronounced decrease in global lightning activity in the range 3.0%–5.8% is identified from various detection systems during this natural experiment. The Maritime Continent lightning chimney shows the largest reduction of 7.0% in aerosol accompanied by a lightning drop of 15%. The COVID period in 2020 also experiences a transition from pre‐COVID El Niño to a strong and sustained La Niña. Compensation for ENSO forcing of lightning activity is implemented to disclose the distinct responses of three global lightning chimneys to competing thermodynamic and aerosol effects. Our observational findings indicate a marked influence of aerosol on a global scale by virtue of the extraordinary COVID‐induced aerosol alteration.

流注放电被应用于消毒杀菌、臭氧生产等领域，其中二次流注放电过程对臭氧有效生产持续时间和效率影响明显，然而氧浓度对二次流注放电过程及目标产物产量的影响还不清楚。为此，开发不同氧浓度下针-板电极二次流注发展过程的流体分析模型，解决高氧浓度下流注放电模拟的非物理分支（Branch）问题，分析氧浓度对二次正流注光发射特性的影响，研究不同氧浓度下的阴极转移电荷量和激发态氧原子 O(³P) 产量，并与实验数据进行对比。结果表明，氧浓度由 20%增加至 90%后，二次流注放电通道电子密度平均降低 90%，电场强度变化3P) 单位能量产率上升 64%，同时放电时间缩短会使产量降低 50%，但激发态氧原子 O(³P) 单位能量产率的提高优于单次降产量。氧浓度增大引发氧分子二、三体吸附效应增强和电子密度下降是单次放电产量下降的原因，电子与氧分子碰撞概率提升是单位能量产率上升的原因。

The effect of anthropogenic aerosols on lightning is one of the least understood aspects of human-induced climate change. Global aerosol clearly diminished during the COVID pandemic by 7.6%. A pronounced decrease in global lightning activity in the range 3.0%-5.8% is identified from various detection systems during this natural experiment. The Maritime Continent lightning chimney shows the largest reduction of 7.0% in aerosol accompanied by a lightning drop of 15%. The COVID period in 2020 also experiences a transition from pre-COVID El Niño to a strong and sustained La Niña. Compensation for ENSO forcing of lightning activity is implemented to disclose the distinct responses of three global lightning chimneys to competing thermodynamic and aerosol effects. Our observational findings indicate a marked influence of aerosol on a global scale by virtue of the extraordinary COVID-induced aerosol alteration.

The high-temperature superconducting (HTS) cable technology has promising application and is gradually adopted in the power grid. However, the short-circuit faults in HTS cables may cause catastrophic consequences and pose a serious threat to the safe operation of the power grid. This work established an equivalent circuit model based on the internal structures of the cold dielectric tri-axial three phase HTS cables. Compared with conventional counterparts, the HTS cable has distinct dynamic impedance characteristics influenced by its current flow and temperature response. Based on the interactive simulation method of PSCAD and MATLAB, the electrical and thermal performance of HTS cables is obtained with considering the quench process under different short-circuit fault states. The instantaneous change of current and quench resistance of superconducting tapes can serve as a reference for fault warning design of HTS cables.

The importance of lightning has long been recognized from the point of view of climate‐related phenomena. However, the detailed investigation of lightning on global scales is currently hindered by the incomplete and spatially uneven detection efficiency of ground‐based global lightning detection networks and by the restricted spatio‐temporal coverage of satellite observations. We are developing different methods for investigating global lightning activity based on Schumann resonance (SR) measurements. SRs are global electromagnetic resonances of the Earth‐ionosphere cavity maintained by the vertical component of lightning. Since charge separation in thunderstorms is gravity‐driven, charge is typically separated vertically in thunderclouds, so every lightning flash contributes to the measured SR field. This circumstance makes SR measurements very suitable for climate‐related investigations. In this study, 19 days of global lightning activity in January 2019 are analyzed based on SR intensity records from 18 SR stations and the results are compared with independent lightning observations provided by ground‐based (WWLLN, GLD360, and ENTLN) and satellite‐based (GLM, LIS/OTD) global lightning detection. Daily average SR intensity records from different stations exhibit strong similarity in the investigated time interval. The inferred intensity of global lightning activity varies by a factor of 2–3 on the time scale of 3–5 days which we attribute to continental‐scale temperature changes related to cold air outbreaks from polar regions. While our results demonstrate that the SR phenomenon is a powerful tool to investigate global lightning, it is also clear that currently available technology limits the detailed quantitative evaluation of lightning activity on continental scales.

Lightning arc attachments during swept strokes are key information in the lightning protection design of fast-moving aircraft, wind turbines, rockets, etc. However, numerical modeling has not achieved success to predict the movement of lightning sweeping arcs due to the limited understanding of the complex multi-physics convolution in the arc fluid at present. This work builds a dynamic magneto-hydrodynamic (MHD) arc model based on the setup in the laboratory simulation of the lightning continuing current and couples the electric–magnetic–thermal–force processes to get insights into the lightning arc dynamics. The MHD theory and Newton’s second law of motion are also incorporated to describe the movement of arc segments in the conditions of aerodynamic flows with different intensities. Results show that, at the center region of the arc, the electromagnetic force, thermal buoyancy, and aerodynamic force are competitive, and all are determiners in predicting the arc displacement. In contrast, at the root region of the arc, the electromagnetic force dominates the arc movement with a flow speed under 10 m/s, while aerodynamic force takes the dominant role when the flow speed exceeds 50 m/s. The arc sweeping distance expands from 0.02 to 1.01 m as the aerodynamic flow increases from 5 to 200 m/s. Meanwhile, when increasing the pitch angle of the arc-connected surface, the arc root becomes more attached to the surface and the sweeping distance is predicted to get reduced. The conclusions offer references to construct a numerical model and predict the complex arc movement during lightning sweeping strokes.