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Earthquakes nucleate when large amounts of elastic energy, stored in the earth's crust, are suddenly released due to abrupt sliding over a fault. Fluid injections can reactivate existing seismogenic faults and induce/trigger earthquakes by increasing fluid pressure. Here we develop an analogous experimental system of simultaneously loaded and wetted absorbent porous paper to quantify theoretically the process of wetting‐induced earthquakes. This strategy allows us to gradually release the stored energy by provoking low intensity tremors. We identify the key parameters that control the outcome of the applied injection strategy, which include the initial stress state, fault segmentation, and segment‐activation rate. Subsequent injections, initiated at high stress levels, can drive the system faster toward its instability point, nucleating a large earthquake. Starting at low stress levels, however, they can reduce the magnitude of the natural event by at least one unit. Plain Language Summary: Understanding natural and anthropogenic seismicity is a major scientific challenge. Here we present a novel analogue fault model using absorbent porous paper, which gives new insights on earthquake mitigation. When scaled to in‐situ conditions, the porous paper model represents a natural seismic rupture of magnitude Mw = 5.9. By progressively wetting it, we simulate fluid injections in the earth's crust and draw analogies to large‐scale industrial projects. In our experiments, each injection is accompanied by tremors, which progressively release energy and modify the energy budget of the system. Without precise knowledge of the fault properties, we risk driving the system faster toward an unexpected large seismic event. However, provided that the model's key parameters–fault segmentation, segment‐activation rate, and stress state–are well known or controlled, the natural rupture can be mitigated by at least one unit. We expect that these results will facilitate risk reduction in current fluid injection projects and inspire earthquake mitigation strategies for real tectonic faults. Key Points: From an energetic point of view, absorbent porous paper can be an ideal, low‐cost surrogate rock material for studying induced seismicitySegmentation of faults and sequential fluid injection in each segment can mitigate potential earthquakes by at least one order of magnitudeWe show that fault segmentation, segment‐activation rate and stress state predominantly control the result of applied injection strategies [ABSTRACT FROM AUTHOR]

In recent years, it has been found that magnetic dip caused by diamagnetic motion of injected plasma can provide an appropriate environment for excitation of electromagnetic ion cyclotron (EMIC) waves. These findings have been widely reported in the Earth's magnetic environment. However, it has rarely been reported in Jupiter's magnetic environment. This paper reports the characteristics of EMIC waves observed by Juno in the magnetic dip of Jupiter. Multiple‐band EMIC waves are observed in frequency range from 10−3 Hz to several Hz. The theoretical analysis shows that in this event both He+ band and O+ band EMIC waves can be constrained in the magnetic dip, which is consistent with the wave emissions observed inside the magnetic dip. Our result provides the first evidence that EMIC wave can be ducted inside a magnetic dip in Jupiter's magnetosphere. Plain Language Summary: The relationship between magnetic structures and electromagnetic waves in the Earth's magnetosphere has been widely reported. Recent studies have theoretically displayed the effects of the magnetic dip on the EMIC wave propagation in Earth's magnetic field. In this paper, we use the event observed by Juno in the magnetic environment of Jupiter to analyze the effect of the magnetic dip on the propagation of EMIC waves. As a result, the observed EMIC waves can be ducted inside the magnetic dip of Jupiter's magnetosphere. Since these ducted waves would influence the dynamics of energetic protons through longtime scattering effects, the magnetic dip is expected to play an important role in the dynamics of Jupiter's magnetosphere. Key Points: Electromagnetic ion cyclotron (EMIC) waves are observed during the magnetic dip by JunoTheoretical analysis about the effects of magnetic dips on the EMIC wave propagation in Jupiter's magnetosphere is performedBoth He+ and O+ band EMIC waves can be ducted inside the magnetic dip in Jupiter's magnetosphere [ABSTRACT FROM AUTHOR]

Lightning channel morphology depends on the thunderstorm cloud charge structure, which in turn is influenced by the thunderstorm dynamics. In this paper, based on three‐dimensional radiation source localization data from the Lightning Mapping Array and radar‐based data, our analysis shows that the overall morphology and detailed morphology of the lightning channel correspond to different eddy dissipation rate (EDR) characteristics. Lightning with complex channel morphology occurs in regions with large EDRs. In single lightning events, channels that extend directly within a certain height range without significant bifurcation and turning tend to propagate in the direction of decreasing EDRs, while channel bifurcations and turns usually occur in regions with large radial velocity gradients and large EDRs. This study shows the relationship between channel morphology and thunderstorm dynamics and provides a new method for the direct application of channel‐level localization data to understand thunderstorm dynamics characteristics. Plain Language Summary: Turbulence in thunderstorms affects the charge distribution, which in turn affects the lightning channel morphology. Thus, the lightning channel morphology can reflect the characteristics of turbulence. The current understanding of the correlation between the two is still limited to the relationship between macroscopic thunderstorm dynamic characteristics and lightning activity. In this paper, the relationship between the morphology of the lightning channel and turbulence characteristics is investigated based on the Lightning Mapping Array (LMA) localization data and the cube root of the eddy dissipation rate (EDR) estimated from KABX radar‐based data. The turbulence strength affects the overall morphology of lightning, and lightning with complex morphology tends to occur in regions with large EDRs. In single lightning events, channels extending directly within a certain altitude range tend to propagate in the direction of decreasing EDRs, and lightning channel bifurcation or turning tends to occur in regions with large EDRs. This paper establishes the relationship between thunderstorm electricity and thunderstorm dynamics by using lightning channel morphology as a bridge and provides a new method for the direct application of channel‐level localization data to understand thunderstorm dynamics characteristics. Key Points: The morphology of lightning channels can reflect the distribution of turbulence intensity, and a certain correlation between the two existDirectly extended lightning channels tend to propagate in the direction of decreasing eddy dissipation ratesThe location of the channel bifurcations or turns often corresponds to regions with high eddy dissipation rates [ABSTRACT FROM AUTHOR]

Changes in precipitation patterns with climate change could have important impacts on human and natural systems. Zhang et al. (2021, https://doi.org/10.1029/2020gl092293) report trends in daily precipitation patterns over the last five decades in the western United States, focusing on meteorological drought. They report that dry intervals (calculated at the annual or seasonal level) have increased across much of the southwestern U.S., with statistical assessment suggesting the results are statistically robust. However, Zhang et al. (2021, https://doi.org/10.1029/2020gl092293) preprocess their annual (or seasonal) averages to compute 5‐year moving window averages before using established statistical techniques for trend analysis that assume independence about some fixed trend. Here we show that the moving window preprocessing violates that independence assumption and inflates the statistical significance of their trend estimates. This raises questions about the robustness of their results. We conclude by discussing the difficulty of adjusting for spatial structure when assessing time trends in a regional context. Plain Language Summary: A recent paper reports trends in drought in the western United States, in particular increases in drought in the southwestern United States, based on changes in the lengths of time intervals without precipitation. In this "comment" we note that the preprocessing approach used in the paper artificially increases the apparent statistical signal in the data and caution that the evidence for the trends reported is not as strong as presented in the paper. We conclude by discussing the difficulty of estimating trends in a statistically rigorous fashion across multiple weather stations. Key Points: Statistical uncertainty about trends in droughts in the southwestern US is larger than reported in a recent GRL letter [ABSTRACT FROM AUTHOR]

Paciorek and Wehner raise important questions around our use of the Mann‐Kendall nonparametric trend test on smoothed data for analyzing long‐term hydrometeorological trends in Zhang et al. (2021, https://doi.org/10.1029/2020gl092293). We thank them for initiating this important conversation and their gracious cooperation in exploring the issues addressed in their comment. In this reply we confirm the inflation of significant p‐values by our choice to smooth, illustrate the relatively minor impacts on the main conclusions of our paper, and add our voices to those of Paciorek and Wehner in highlighting the lack of methodology for hypothesis testing across multiple stations that have spatial structure (i.e., testing for regionally consistent trends). Plain Language Summary: Our colleagues Drs. Paciorek and Wehner have raised concerns about our paper (Zhang et al., 2021, https://doi.org/10.1029/2020gl092293), which showed widespread increases in the duration of drought events over the last five decades in the western United States. They point out that our decision to smooth the data using a moving average inflated the number of weather stations at which the trends toward longer droughts were deemed significant by a statistical test. We agree with them on this point, and we have recomputed all our results using unsmoothed data to determine the impacts. We find that for most stations and regions, trend magnitudes remained largely unchanged, with many stations nearby one another often suggesting similar trends. Finally, we agree with Paciorek and Wehner that there is a lack of statistical methods to test such coherent regional patterns, and we caution that over‐reliance on p‐values limits the power of regional data to identify important climate trends. Key Points: We agree that smoothing to 5‐year moving windows introduced serial correlation into time series of annual statistics of daily rainfall data, inflating the number of weather stations individually showing significant trends (p < 0.05) with the Mann‐Kendall testRecomputation with unsmoothed values produced substantially the same dry intervals trend magnitude and direction at most stations individually and had only minimal impacts on dry interval trends computed for National Ecological Observatory Network domains using the Regional Kendall testNo perfect statistical approach leverages the capacity of coherent regional patterns among spatially correlated weather stations, and an over‐reliance on p‐values as a binary (significant vs. insignificant) determinant of trends limits the power of regional data [ABSTRACT FROM AUTHOR]

The significant risk associated with fault reactivation often necessitates slip tendency analyses for effective risk assessment. However, such analyses are challenging, particularly in large areas with limited or absent reliable stress measurements and where the cost of extensive geomechanical analyses or simulations is prohibitive. In this paper, we propose a novel approach using a physics‐informed neural network that integrates stress orientation and satellite displacement observations in a top‐down multi‐scale framework to estimate two‐dimensional slip tendency analyses even in regions lacking comprehensive stress data. Our study demonstrates that velocities derived from a continental scale analysis, combined with reliable stress orientation averages, can effectively guide models at smaller scales to generate qualitative slip tendency maps. By offering customizable data selection and stress resolution options, this method presents a robust solution to address data scarcity issues, as exemplified through a case study of the South Australian Eyre Peninsula. Plain Language Summary: Fault reactivation poses significant risks, often requiring slip tendency analyses for thorough risk assessment. Yet, such analyses face challenges, especially in large areas lacking reliable stress measurements or where extensive geomechanical analyses are too costly. Our paper suggests a new method using a physics‐based neural network. This approach combines compressive direction and satellite displacement observations to estimate slip tendencies in two dimensions, even where stress data is lacking. Our study shows that by using displacements from a continental scale analysis and reliable averages of compressive directions, we can guide models to create smaller‐scale maps indicating where faults are more likely to reactivate. This method allows for customizable data selection and stress resolution, offering a strong solution to data scarcity issues. We demonstrate its effectiveness through a case study of South Australia's Eyre Peninsula. Key Points: Physics‐based neural networks allow two‐dimensional slip tendency analyses without prior full‐stress informationA multi‐scale approach provides required displacement constraints when inferring full stresses from global navigation satellite system (GNSS) and stress orientation dataWe present a new application for GNSS data that would welcome more stations, even in seismically stable areas [ABSTRACT FROM AUTHOR]

Understanding the impact of various climate forcing agents, such as aerosols, on extreme precipitation is socially and scientifically vital. While anthropogenic absorbing aerosols influence Earth's energy balance and atmospheric convection, their role in extreme events remains unclear. This paper uses convective‐resolving radiative‐convective‐equilibrium simulations, with fixed solar radiation, to investigate the influence of absorbing aerosols on extreme precipitation comprehensively. Our findings reveal an underappreciated mechanism through which absorbing aerosols can, under certain conditions, strongly intensify extreme precipitation. Notably, we demonstrate that a mechanism previously reported for much warmer (hothouse) climates, where intense rainfall alternates with multi‐day dry spells, can operate under current realistic conditions due to absorbing aerosol influence. This mechanism operates when an aerosol perturbation shifts the lower tropospheric radiative heating rate to positive values, generating a strong inhibition layer. Our work highlights an additional potential effect of absorbing aerosols, with implications for climate change mitigation and disaster risk management. Plain Language Summary: Aerosols, particles suspended in the atmosphere, can interact with the incoming solar radiation by scattering or absorbing it. Aerosol species that absorb solar radiation generate local warming of the atmosphere. This local warming changes the vertical profile of temperature and by that affects cloud and precipitation development. In this paper we use idealized computer simulations to investigate the effect of absorbing aerosols on precipitation, and specifically on extreme precipitation events in the tropics. We demonstrate that under certain conditions, absorbing aerosols can strongly enhance extreme precipitation even despite reducing the mean. We show that this trend can be explained by a mechanism previously reported for much warmer climate conditions than currently found on Earth, involving heating by radiation of the lower part of the troposphere. These results have implications for climate change mitigation and disaster risk management. Key Points: The effect of absorbing aerosol on extreme precipitation is examined in idealized convective‐resolving radiative‐convective‐equilibrium simulationsAerosol perturbation that shifts the lower tropospheric radiative heating rate to positive values strongly enhances extreme precipitationThis trend is explained by a mechanism reported before for hothouse climate conditions involving a shift into an "episodic deluge" regime [ABSTRACT FROM AUTHOR]

Fluid flow through fractured media is typically governed by the distribution of fracture apertures, which are in turn governed by stress. Consequently, understanding subsurface stress is critical for understanding and predicting subsurface fluid flow. Although laboratory‐scale studies have established a sensitive relationship between effective stress and bulk electrical conductivity in crystalline rock, that relationship has not been extensively leveraged to monitor stress evolution at the field scale using electrical or electromagnetic geophysical monitoring approaches. In this paper we demonstrate the use time‐lapse 3‐dimensional (4D) electrical resistivity tomography to image perturbations in the stress field generated by pressurized borehole packers deployed during shear‐stimulation attempts in a 1.25 km deep metamorphic crystalline rock formation. Plain Language Summary: Time‐lapse electrical geophysical sensing is used to image 3D changes in rock stress generated by an isolated and pressurized interval of a borehole in a deep, dense, fractured rock formation. Key Points: Remotely monitoring stress is challenging but important for relating geomechanical behavior to flow pathways during energy productionBulk electrical conductivity is sensitive to stress in crystalline rockTime‐lapse electrical resistivity tomography can be used to remotely monitor 3D changes in effective stress [ABSTRACT FROM AUTHOR]

Extracting balanced geostrophic motions (BM) from sea surface height (SSH) observations obtained by wide‐swath altimetry holds great significance in enhancing our understanding of oceanic dynamic processes at submesoscale wavelength. However, SSH observations derived from wide‐swath altimetry are characterized by high spatial resolution while relatively low temporal resolution, thereby posing challenges to extract the BM from a single SSH snapshot. To address this issue, this paper proposes a deep learning model called the BM‐UBM Network, which takes an instantaneous SSH snapshot as input and outputs the projection corresponding to the BM. Training experiments are conducted both in the Gulf Stream and South China Sea, and three metrics are considered to diagnose model's outputs. The favorable results highlight the potential capability of the BM‐UBM Network to process SSH measurements obtained by wide‐swath altimetry. Plain Language Summary: Oceanic dynamic processes can be classified into two categories: balanced geostrophic motions (BM), including large‐scale circulation, mesoscale and submesoscale eddy turbulence, and unbalanced wave motions (UBM), including barotropic tides, and inertia–gravity waves (IGWs). Both types of motions coexist and have respective contributions to the sea surface height (SSH). How to extract the BM from the total SSH observations obtained by satellite altimetry is the crucial problem to be solved in this paper. To tackle this issue, we propose a deep learning model named the BM‐UBM Network to establish the relationship between the total SSH and the BM component. The BM‐UBM Network can generate SSH estimations for the BM when provided with a well‐resolved SSH snapshot. Key Points: A Deep learning model is developed to extract balanced motions from sea surface height snapshot based on a realistic simulationDiagnostics of three metrics reveal the effectiveness of the model in extracting balanced motionsThe model exhibits remarkable advantages over the Gaussian filter (baseline) in capturing the gradient and Laplacian information [ABSTRACT FROM AUTHOR]

The fundamental features of one kind of rarely known stratocumulus, which was termed as "Millipede Cloud," occurred over the Eastern Pacific Ocean in 2017 were first documented by using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery. These clouds had long and meandering "central axes" extending from several hundreds to thousands kilometers, and a number of "radical cloud arms" extending several tens of kilometers in its two sides. Total 59 "Millipede Clouds," 4 and 55 of them, were formed over the Northern and the Southern Hemispheres, respectively. Their environmental backgrounds were analyzed by using ERA5 reanalysis data and MODIS sensor Level‐2 data. The cloud top pressures of these "Millipede Clouds" were between 850 and 800 hPa, and their top heights were about 1–2 km. There existed "inversion layer" of air temperature near the cloud tops at 800 hPa, which strongly suggested that these clouds were lower stratocumulus in essence. Plain Language Summary: "Millipede Cloud," one kind of rarely known stratocumulus which looks like "Millipede" shape, is termed for the first time in this paper. It has an obvious "central axis" and a number of well‐organized "radial cloud arms" in two sides of the "central axis" extending in several tens of kilometers length. This paper introduces the fundamental features of "Millipede Clouds" occurred over the Eastern Pacific Ocean in 2017 from the perspective of satellite image. Totally, 59 "Millipede Clouds" were found to occur over the Eastern Pacific Ocean. Their geographic distribution, cloud top features and vertical structure of one typical case on 16 July 2017 were documented. Key Points: The fundamental features of "Millipede Clouds" over the Eastern Pacific Ocean in 2017 were documented by using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imageryThe environmental backgrounds of these "Millipede Clouds" were analyzed by using ERA5 reanalysis data and MODIS dataThe cloud top pressures of these "Millipede Clouds" are between 850 and 800 hPa, and their top heights are about 1–2 km [ABSTRACT FROM AUTHOR]

This paper studies a new concept of using global navigation satellite system (GNSS) signals coherently reflected over relatively smooth ocean and ice surfaces from very low elevation angles (below ∼8°) and received by low Earth orbit (LEO) satellites to retrieve the tropospheric information. This approach can provide horizontal profiles of tropospheric zenith delay and total column water vapor (TCWV) with centimeter‐level high precision and spatial resolutions of tens of km by ∼1 km, depending on the elevation angle, with a sampling spacing of ∼100 m. This approach can potentially be applied to most sea ice and calm ocean areas and provide tropospheric sensing data, which can complement and augment existing observation systems. A few case studies are conducted in this paper using the Spire grazing‐angle GNSS‐R data. The retrieved TCWV is compared to ERA5 products and the Sentinel‐3 Ocean and Land Color Instrument measurements and shows promising performances. The errors associated with the GNSS‐R tropospheric measurements are also discussed. Plain Language Summary: The atmospheric water vapor is an important component for the weather and climate systems and is difficult to measure, especially over ocean and ice surfaces. This paper studies a new approach to measuring atmospheric water vapor using global navigation satellite system (GNSS) signals reflected off ocean and ice surfaces. If the reflection is from a low elevation angle (below ∼8°) and the reflected signal is coherent (all signal rays are reflected in the same direction), this approach can provide very high precision observation of the horizontal gradients of the tropospheric delay and the vertically integrated atmospheric water vapor with good spatial resolutions. This paper presents the methodology of the proposed approach and a few case studies to demonstrate the feasibility and performance by comparing the GNSS‐R retrieved water vapor measurements with models and the Sentinel‐3 satellite radiometry measurements. The errors associated with the GNSS Reflectometry (GNSS‐R) tropospheric measurements are also discussed. Key Points: A new tropospheric sensing concept is studied that relies on coherent‐reflection global navigation satellite system (GNSS) signals off ocean and ice surfacesAlgorithms are developed and demonstrated using Spire grazing‐angle GNSS‐R data to retrieve tropospheric delay and water vaporThe presented approach provides high‐precision tropospheric delay and total column water vapor horizontal profiles, as validated using the Sentinel‐3 Ocean and Land Color Instrument data [ABSTRACT FROM AUTHOR]

On 7 January 2014, a solar storm erupted, which eventually compressed the Earth's magnetosphere leading to the generation of chorus waves. These waves enhanced local wave‐particle interactions and led to the precipitation of electrons from 10 s eV to 100 s keV. This paper shows observations of a low energy cutoff in the precipitation spectrum from Van Allen Probe B Helium Oxygen Proton Electron measurements. This low energy cutoff is well replicated by the predicted loss calculated from pitch angle diffusion coefficients from wave and plasma observations on Probe B. To our knowledge, this is the first time a single spacecraft has been used to demonstrate an accurate theoretical prediction for chorus wave‐induced precipitation and its low energy cutoff. The specific properties of the precipitating soft electron spectrum have implications for ionospheric activity, with the lowest energies mainly contributing to thermospheric and ionospheric upwelling, which influences satellite drag and ionospheric outflow. Plain Language Summary: On 7 January 2014, a large storm erupted from the Sun. This storm encountered the Earth and compressed the magnetosphere a few days later. The compression of the magnetosphere led to the creation of chorus waves, a wave‐type known to interact only with electrons with specific energies. In this case, the waves interacted with electrons in the magnetosphere's outer radiation belt. They caused the loss of electrons from 10 s eV to 100 s keV into the ionosphere and upper atmosphere. This paper uses theory to determine which energies we expect will interact with the observed chorus wave. We use the Helium Oxygen Proton Electron instrument from the Van Allen Probes to see if our predictions are correct. We care about these processes because the loss of these electrons can affect ionospheric activity. Key Points: Upper band chorus waves can have a minimum resonant energy in the 10 s eV energy rangeChanges in the minimum resonant energy can change the cut off for what lower energy particles will be lostThe lower energy cut off can be observed in the Van Allen Probes Helium Oxygen Proton Electron data [ABSTRACT FROM AUTHOR]

Scafetta (2022, https://doi.org/10.1029/2022gl097716) purports to test Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models through a comparison of temperature changes over three decades. Unfortunately, the paper contains numerous conceptual and statistical errors that undermine all of the conclusions. First, no uncertainty is given for the observational temperature difference, making it impossible to assess compatibility with any model result. Second, the CMIP6 data are the ensemble means for each model, but the metric being tested is sensitive to the internal variability and so the full ensemble for each model must be used. When this is corrected, the conclusion that "all models with ECS > 3.0°C overestimate the observed global surface warming" is not sustained. Third, the statistical test in Section 2 would reject all models even in a perfect model setup given sufficient ensemble members, thus the second conclusion "that spatial t‐statistics rejects the data‐model agreement" is also not sustainable. Plain Language Summary: Comparisons of models and observations need to account from multiple sources of uncertainty in both the observations and due to the chaotic dynamics of the weather. The analyses in Scafetta (2022, https://doi.org/10.1029/2022gl097716) do not take either of these issues into account and thus the conclusions in that paper are not supportable. Key Points: Scafetta (2022) contains errors in both of the statistical tests used that make the conclusions unsupportable [ABSTRACT FROM AUTHOR]

Air‐entraining whitecaps provide an important source of bubbles over the global oceans, yet the rate at which the associated air is entrained is not well known. This lack of understanding limits the ability to accurately parameterize bubble‐mediated gas exchange and sea spray aerosol flux. In this paper I present a model to predict the total volume of air entrained by individual whitecaps and extend it to estimate the rate at which air is entrained per unit sea surface area. The model agrees well with existing models and measurements and can be forced by the rate at which energy is dissipated by the wavefield which can be routinely provided by spectral wave models. I then use the model to present the first distributions of the estimated total volume of air entrained by individual whitecaps, as well as their rate of air entrainment and air degassing. Plain Language Summary: The amount of air in the oceans in the form of bubbles at any given time is not well known because of the difficulty associated with making in‐situ measurements. This lack of knowledge inhibits how well ocean‐atmosphere exchange processes that are driven by air and bubbles can be represented in climate models. In this paper, I present a new model to estimate the volume of air entrained by individual breaking waves called whitecaps, as well as how quickly the air is entrained into the oceans and how quickly it leaves the oceans when bubbles rise to the surface and burst. Key Points: A model for the entrainment of air by oceanic whitecaps is presented which agrees well with existing models and measurementsDistributions of the estimated volume of air entrained by individual oceanic whitecaps are presented for the first timeKey uncertainties in the air fraction and entrainment velocities of individual whitecaps remain due to a lack of measurements [ABSTRACT FROM AUTHOR]

Heavy cold ions at Mars are gravitationally bound to the planet unless some process provides energy to them. Observations show that cold (<20 eV) and dense (∼>1 cm−3) O+/O2+ ions with bulk velocities equal to energies ∼1 keV can reach deep into the nightside Martian magnetosheath. These ions are co‐located with a change of the sign of the sunward component of the magnetic field. This magnetic field topology implies the persistence of a localized planetary ions escape channel associated with draped magnetic field lines that are convecting tailward. The observed ion populations propagate approximately in the same direction as surrounding magnetosheath flow and are likely to be almost unheated ionospheric ions from low altitudes. The paper discusses planetary ion energization via Hall electric field originated from ions and electron separation associated with magnetic field curvature. Plain Language Summary: In‐situ observations above the nightside of Mars show the presence of localized dense planetary ion fluxes at altitudes exceeding 2,000 km and escaping from the planet at high energies, comparable to that of the solar wind. These fluxes are accompanied by the reversal of sunward component of magnetic field. Unlike most atmospheric escape channels, the reported phenomenon is characterized by an increase in heavy to light ions density ratio with the distance from the planet at the observed altitudes up to nearly 5,000 km, as well as an increase in overall plasma number density inside this escape channel relative to the ambient sheath environment. This behavior is consistent with acceleration process initiated by a bent magnetic flux tube. Key Points: Ions of different species gain similar energies in the Martian magnetosheath by Hall electric fields associated with magnetic curvatureA high concentration of ionospheric ions correlates with a near void of shocked solar wind protons and a magnetic field reversalNumber density at the reversal increases with distance from Mars in comparison to the surrounding sheath at least till two Martian radii [ABSTRACT FROM AUTHOR]

The variability of Mars exosphere over monthly to solar‐cycle scales at 251 and 412 km altitude is quantified by analysis of 41‐Ls mean densities derived from precise orbit determination of the Mars Reconnaissance Orbiter (MRO) and Mars Odyssey (MO) satellites, respectively. The data encompass 2006–2020 (MRO) and 2002–2020 (MO). At both altitudes, most of the variance is captured by cos(Ls–ϕ), where ϕ ≈ 258°. This term represents the effects of solar heating changes due to the eccentricity of Mars orbit around the Sun, and climatological changes in heating due to lower‐atmosphere dust loading, which does not play a significant role. The remaining variability is connected with the "irregular" variability of solar flux over monthly time scales. For MO, the presence of Helium disrupts a clean correlation with these sources. Plain Language Summary: The force of continuous bombardment of atmospheric particles on a satellite slowly changes its orbit. By tracking the satellite, changes in its orbit can in turn be used to infer atmospheric particle densities. In this paper, we use tracking from NASA's Deep Space Network, an international array of giant antennas, to measure minute orbital changes of the Mars Reconnaissance Orbiter and Mars Odyssey satellite orbits (and thereby atmospheric densities) around Mars during the 2002–2020 time frame. These data are used to study how the outer regions of Mars atmosphere respond to changes in heating due to solar radation absorption by various chemical species, and by dust that is elevated into the atmosphere by strong winds near the surface. The major finding of this paper is that the variability of Mars outer atmosphere is mainly controlled by solar heating changes due to the eccentricity of Mars orbit around the Sun. Key Points: Densities from precise orbit determination of Mars Reconnaissance Orbiter and Mars Odyssey are used to quantify long‐term variability of Mars' exosphereThe annual term is the greatest contributor to total variance, followed by the irregular component of EUV/UV fluxDust‐induced inter‐annual variability is small, but the analysis method does suppress potentially important regional‐scale events [ABSTRACT FROM AUTHOR]

An array of atmospheric profile observations consists of three‐dimensional vectors representing pressure, temperature, and humidity, with each profile forming a continuous curve in this three‐dimensional space. In this paper, the Signature method, which can quantify a profile's curve, was adopted for the atmospheric profiles, and the accuracy of profile representations was investigated. The description of profiles by the signature was confirmed with adequate accuracy. The machine‐learning‐based model, developed using the signature, exhibited a high level of annual accuracy with minimal absolute mean differences in temperature and water vapor mixing ratio (<2.0 K or g kg−1). Notably, the model successfully captured the vertical structure and atmospheric instability, encompassing drastic variations in water vapor and temperature, even during intense rainfall. These results indicate the Signature method can comprehensively describe the vertical profile with information on how ordered values are correlated. This concept would potentially improve the representation of the atmospheric vertical structure. Plain Language Summary: The atmospheric profile can be visualized as a three‐dimensional curve representing pressure, temperature, and humidity. By utilizing the Signature method, we can measure and quantify the profile's curve, allowing for comprehensive modeling of the atmosphere. In this paper, we confirmed the accuracy of atmospheric profile representation using signatures and introduced the characteristics of signatures revealed through machine‐learning models. The description of profiles by the signature was confirmed with adequate accuracy. Moreover, the model demonstrated robust annual accuracy, with minimal temperature and water vapor discrepancies. It effectively captured the vertical structure and instability of the atmosphere, even during heavy rainfall, characterized by significant temperature and water vapor content fluctuations. Key Points: The utilization possibility of the Signature method for atmospheric profiles was confirmedBy utilizing the Signature method, we can measure and quantify the profile's curve, allowing for comprehensive modeling of the atmosphereThe machine‐learning model developed with the signature can predict the profiles with high annual accuracy [ABSTRACT FROM AUTHOR]

In this study, a method for assimilating FY4A advanced geostationary radiance imager (AGRI) cirrus‐effected radiances (CER) is investigated, and the impact of this method on water vapor analysis and rainstorm forecasting is examined through observing system simulation experiments and actual case experiments. The high proportion of inverted humidity profiles in the cirrus‐effected pixels is the main reason for the negative effect of assimilation in the mid‐to‐lower troposphere. To address this, relevant constraint conditions are incorporated into the cost function. The statistical results reveal that the addition of a CER assimilation improves the analysis increment of water vapor, with pattern correlation coefficients of 0.33, 0.35, and 0.20 at 200, 300, and 400 hPa, respectively, which are greater than those of a clear‐sky radiance assimilation (0.28, 0.33, and 0.17, respectively). Moreover, the inclusion of a CER assimilation greatly improves data utilization, and has a neutral to positive effect on precipitation forecasting. Plain Language Summary: Infrared all‐sky radiance assimilation is an attractive but challenging problem in satellite data assimilation. Is there an alternative approach to achieve the assimilation of infrared radiance for a certain type of cloud? It is found that cirrus clouds, which are loose ice clouds composed of ice crystals, can partially transmit infrared radiance and are large in number. Considering these advantages, a new direct assimilation method for FY4A advanced geostationary radiance imager (AGRI) cirrus‐effected radiances is proposed in this paper. The method addresses the uncertainty in the assimilation of cirrus‐effected radiances by adding weak constraints of inverse humidity. This research highlights the substantial increase in AGRI data usage when incorporating cirrus‐effected radiance data, as well as the neutral‐to‐positive impact on water vapor analysis and precipitation forecasting. This study also suggests that future endeavors could combine infrared channels with lower‐level microwave channels, which may have a more significant contribution to infrared radiance assimilation. Key Points: A high percentage of inverse humidity exists in the atmospheric profile corresponding to cirrus cloud fields of view over landThe assimilation of cirrus cloud pixels greatly improves the utilization rateAssimilating advanced geostationary radiance imager cirrus‐effected radiances has a neutral to positive effect on water vapor analysis and precipitation forecasting [ABSTRACT FROM AUTHOR]

Earthquake swarms are most commonly composed of small‐magnitude earthquakes. However, a recent study by Yoshida, Uchida, et al. (2023, https://doi.org/10.1029/2023GL106023) analyzed a swarm beneath the Noto Peninsula in Japan that, after more than two years of moderate‐magnitude seismicity, triggered the moment magnitude (Mw) 6.2 Suzu mainshock in May 2023. Based on high‐precision earthquake locations and a slip inversion of the mainshock, these authors found that the Mw 6.2 Suzu earthquake occurred on the updip extension of a fault that was active during the swarm, likely driven by increased fluid pressure. After publication of that paper, a much larger and more destructive Mw 7.5 event occurred nearby on 1 January 2024. These events underscore the potential for swarms to be precursors to large, damaging earthquakes. Forecasting the eventual evolution of swarms is currently very challenging but could be aided in the future by new observations and models. Plain Language Summary: Although earthquake swarms often only contain small earthquakes, occasionally they can trigger large, damaging earthquakes. Yoshida, Uchida, et al. (2023, https://doi.org/10.1029/2023GL106023) examined the triggering of a magnitude 6.2 earthquake in Japan, which caused extensive damage, following two years of smaller earthquakes. This sequence, like many swarms, may have been driven by elevated fluid pressure migrating upwards within the crust. After publication of that paper, a much larger and more destructive Mw 7.5 event occurred nearby. Forecasting the eventual evolution of ongoing swarms is currently very challenging but could be aided in the future by new observations and models. Key Points: Earthquake swarms can be precursors to damaging earthquakes, including the 1 January 2024, Mw 7.5 Noto Peninsula, Japan eventFluid pressure changes often drive swarm activity, and their degree of confinement may influence swarm evolutionDirect observations of fluids in fault zones combined with numerical modeling may aid swarm forecasting in the future [ABSTRACT FROM AUTHOR]

Coral reef islands are amongst the most vulnerable environments to sea‐level rise (SLR). Recent physical and numerical modeling studies have demonstrated that overwash processes may enable reef islands to keep up with SLR through island accretion. Sediment supply to these islands from the surrounding reef system is critical in understanding their morphodynamic adjustments, but is poorly constrained due to insufficient knowledge about sediment delivery rates. This paper provides the first estimation of sediment delivery rates to coral reef islands. Analysis of topographic and geochronological data from 28 coral reef islands indicates an average rate of sediment delivery of c. 0.1 m3 m−1 yr−1, but with substantial inter‐island variability. Comparison with carbonate sediment production rates from census‐based studies suggests that this represents one quarter of the amount of sediment produced on the reef platform. Results of this study are useful in future modeling studies for predicting morphodynamic adjustments of coral reef islands to SLR. Plain Language Summary: Low‐lying coral reef islands are under threat of sea‐level rise (SLR). However, when these islands are flooded, ocean waves can bring in sediment that can increase the island elevation. This would enable coral reef islands to better withstand flooding in the future. Knowing how much sediment is brought in will help in our understanding of future changes to these islands due to SLR. In this paper, we use data from 28 Indo‐Pacific coral reef islands to compute sediment supply to the islands. We find that on average 0.1 m3 of sediment (roughly 100 kg) is delivered each year for every meter of island shoreline. We further suggest that this implies that only one quarter of the sediments produced by the coral reef system is delivered to the island shoreline. Most of the sediment produced remains on the reef flat or is exported to the ocean or the lagoon. Our results will help future studies to predict more accurately how coral reef islands will adjust to SLR. Key Points: We provide the first estimates of carbonate sediment delivery rates to 28 coral reef islands using all data available from the literatureSediment delivery to the reef islands occurs at a rate of c. 0.1 m3 m−1 yr−1, but with substantial inter‐island variabilityWhere island building has been continuous through island history, long‐term delivery rates provide valuable estimates for contemporary rates [ABSTRACT FROM AUTHOR]

Magnetic reconnection is a fundamental process known to play a crucial role in electron acceleration and heating, however, the mechanism of electron energization during reconnection is still not fully understood. This study introduces a novel electron acceleration mechanism in which electrons can be accelerated by secondary reconnection in the separatrix region. The secondary reconnection occurs in a thin current sheet resulted from the shear of the out‐of‐plane Hall magnetic fields of the primary magnetopause reconnection. It results in the intense electron energy fluxes toward the primary X‐line. This mechanism will likely be an important piece in the puzzle of particle acceleration by reconnection. Plain Language Summary: Magnetic reconnection is a fundamental process that plays a critical role in electron acceleration and heating. The separatrix region of magnetic reconnection, which distinguishes the inflow and outflow regions, is a crucial area in the particle acceleration of the reconnection process. However, previous studies mainly focused on the electron acceleration mechanism in the separatrix region under two‐dimensional reconnection, without considering the influence of the three‐dimensional structures. This paper reports, for the first time, the secondary reconnection occurring in the opposite Hall magnetic field in the three‐dimensional separatrix region of magnetopause primary reconnection. This secondary reconnection can accelerate and heat the inflow electrons of the primary reconnection, which provides important clues on how the particles are accelerated and heated by reconnection. Key Points: Secondary reconnection occurs between Hall magnetic fields in the separatrix region of the primary magnetopause reconnectionIntense electron enthalpy fluxes are injected toward the primary X‐line along the separatrixElectrons are accelerated by the secondary reconnection before being injected into the primary X‐line for further energization [ABSTRACT FROM AUTHOR]

Terrestrial Gamma‐ray Flashes (TGFs) are ten‐to‐hundreds of microsecond bursts of gamma‐rays produced when electrons in strong electric fields in thunderclouds are accelerated to relativistic energies. Space instruments have observed TGFs with source photon brightness down to ∼1017–1016. Based on space and aircraft observations, TGFs have been considered rare phenomena produced in association with very few lightning discharges. Space observations associated with lightning ground observations in the radio band have indicated that there exists a population of dimmer TGFs. Here we show observations of TGFs from aircraft altitude that were not detected by a space instrument viewing the same area. The TGFs were found through Monte Carlo modeling to be associated with 1015–1012 photons at source, which is several orders of magnitude below what can be seen from space. Our results suggest that there exists a significant population of TGFs that are too weak to be observed from space. Plain Language Summary: Terrestrial Gamma‐ray Flashes (TGFs) are short bursts of gamma‐rays produced in the strong electric fields in thunderclouds. Based on space and aircraft observations, TGFs have been considered a rare phenomena. In this paper, we present observations of TGFs from an aircraft campaign that were not detected by a space instrument viewing the same area. Our results reveal that these TGFs were too weak to be observed from space, indicating a significant population of TGFs that are undetectable by space instruments. Key Points: The ALOFT flight campaign detected six TGFs within a few minutes during an ISS overpass, and none were detected by the ASIM instrumentWe show that there must be a population of TGFs that are too weak to be observed from spaceThe fluence at 15 km of this population is 2–5 orders of magnitude lower than the TGFs observed from space [ABSTRACT FROM AUTHOR]

A crucial factor influencing the mass balance of the West Antarctic Ice Sheet is the Amundsen Sea Low (ASL), a climatological low‐pressure region situated off the West Antarctic coast. However, albeit the deepening of the ASL since the 1950s has been attributed to anthropogenic forcing, the multi‐decadal variability of the ASL remains poorly understood, because of a lack of long observations. Here, we apply a newly developed data assimilation method to reconstruct the ASL over 1870–2000. We study the forced and internal variability of the ASL using our new reconstruction in concert with existing large ensembles of climate model simulations. Our findings robustly demonstrate that an atmospheric teleconnection originating from the tropical Indo‐Pacific is the main driver of ASL variability at the multi‐decadal time scale, with resemblance to the Interdecadal Pacific Oscillation. Since the mid‐20th century, anthropogenic forcing has emerged as a dominant contributor to the strengthening of the ASL. Plain Language Summary: Changes in the West Antarctic Ice Sheet mass balance (i.e., the difference between the gain and loss of ice mass) are partly influenced by large‐scale winds, and in particular, a climatological low‐pressure feature located off the West Antarctic coast called the Amundsen Sea Low (ASL). Yet, although the long‐term strengthening of the ASL since the mid‐20th century has been demonstrated to be related to anthropogenic forcing, our understanding of the variability of the ASL on time‐scales of decades is poorly known. In this paper, we therefore investigate the origins of this variability since 1870, and quantify the relative contributions of human‐caused climate changes and natural variability of the climate system. For this purpose, we use several ensembles of model simulations as well as new climate reconstructions that combine paleoclimate records with model simulations using a statistical method. Our results indicate that the multi‐decadal variability of the ASL is strongly driven by tropical variability in the Indo‐Pacific through atmospheric connections between this region and the Amundsen Sea. Our reconstruction, when compared with a large ensemble of model simulations, indicates that since 1950, human‐induced climate forcing has become a dominant driver of long‐term ASL variability, contributing equally to tropical variability. Key Points: The large‐scale atmospheric circulation in West Antarctica exhibits a strong multi‐decadal variability superimposed by a 20th‐century trendThe multi‐decadal variability of this large‐scale atmospheric circulation is strongly governed by the Indo‐Pacific tropical variabilitySince 1950, anthropogenic forcing has emerged as a key driver of the long‐term change of this atmospheric circulation [ABSTRACT FROM AUTHOR]

Altitude‐triggered lightning provides favorable conditions for the research of bidirectional leader system. In the summer of 2023, altitude‐triggered lightning experiment was conducted on the Field Experiment Base on Lightning Sciences, China Meteorological Administration. The multifrequency radiation characteristics of bidirectional leaders and the interactions of both ends during the propagation are analyzed. Specifically, the discharge processes that produce LF‐MF magnetic radiations from bidirectional leaders are revealed by high‐speed images, and these LF‐MF radiations correspond to the VHF radiations generated by bidirectional leaders well. Unlike the strong correlation between LF‐MF radiation strengths and discharge intensities, the VHF radiation strengths exhibit significant variation even among similar‐intensity discharge events, as VHF radiations correspond to the random and microscopic discharge processes associated with streamers. Furthermore, the changes in leader speed and channel brightness before and after the initiation of bidirectional leaders indicate that the development of the two ends of bidirectional leaders is mutually reinforcing. Plain Language Summary: With the advancement of lightning detection technology, more and more observations show that the common development form of leaders is bidirectional. As the basic scientific problems in lightning physics, the characteristics of radiation and development of bidirectional leaders need to be further analyzed. Altitude‐triggered lightning is conducted by a small rocket dragging an electrically floating wire, where the distance between the bottom of the wire and the ground is tens to hundreds of meters. For a negative altitude‐triggered lightning flash, the discharge between the distance and the discharge along the upper end of steel wire can reproduce the development processes of downward negative leader and upward positive leader of natural lightning, respectively. Hence, altitude‐triggered lightning provides favorable conditions for the research of bidirectional leader system. The paper examines the comprehensive observations of the altitude‐triggered lightning, including the LF‐MF radiations recorded by the high‐sensitivity magnetic antennas, VHF radiations mapped by the broadband interferometer, and the development images captured by high‐speed camera. Based on these synchronous data, the multifrequency radiation characteristics of bidirectional leaders and the interactions of both ends during the propagation are analyzed in detail. The related results further promote the understanding of bidirectional leaders. Key Points: Optical images reveal the discharge processes relevant to LF‐MF magnetic radiations of bidirectional leaders in altitude‐triggered lightningVHF radiation strengths of bidirectional leaders vary greatly even for similar discharge events due to the randomness of breakdown directionChanges in leader speed and channel brightness indicate that the development of two ends of bidirectional leaders is mutually reinforcing [ABSTRACT FROM AUTHOR]

We report an Arase‐all sky imager (ASI) conjugate event in which the pulsating aurora (PsA) has a one‐to‐one correspondence with chorus bursts. Wavelet analysis displayed three peaks at ∼0.3 Hz, 4 Hz, and >10 Hz, corresponding to the main pulsation, internal modulation, and fast modulation, respectively. These correspond to the old terms of ∼5–15 s pulsations, chorus risers/elements and subelements/subpackets, respectively. Electron "microbursts" correspond to the 4‐Hz peak. The internal and fast modulations are further verified by the analysis based on fast Fourier transform analyses. Moreover, the spatial distributions of the Fourier spectral amplitude show that the internal and fast modulations are well‐structured within auroral patches. The above results indicate a paradigm shift away from quasilinear theory which implicitly assumes diffuse wave generation. The three time‐scale modulations are consistent with coherent chorus which has been theoretically argued to lead to pitch angle transport three orders of magnitude faster. Plain Language Summary: Pulsating aurora exhibit irregular patches of brightness with quasiperiodic on‐off transitions (∼2–20 s). More rapid modulations, such as internal modulation (∼3–4 Hz) or fast modulation (>10 Hz), have been detected within the pulsation "on" time. However, due to the measurement limitations, the simultaneous observation of three time‐scale modulations has never been reported. In this study, we analyze the conjugate observations of the pulsating aurora (PsA) and chorus recorded by the ground‐based Arase‐all sky imager and the Arase satellite, which demonstrates the coexistence of three time‐scale modulations in the PsA. The spatial distributions of Fourier spectral amplitude show that the internal and fast modulations are well‐structured within the aurora patches. The three time scales of chorus modulation have been previously called chorus 5–15 s pulsations, risers/elements and subelements/subpackets corresponding to the three aurora peaks. This study verifies the existence of internal and fast modulations in PsA, implying an extremely rapid electron loss mechanism. Quasilinear theory cannot explain any of the three time‐scale modulations. The discovery that chorus is coherent and will lead to pitch angle transport 1,000 times faster than diffuse waves is consistent with the fast modulations shown in this paper. A new theory of chorus generation is needed to update that of quasilinear theory. Key Points: We report a conjugate event in which the pulsating aurora (PsA) has a one‐to‐one correspondence with chorus burstsThe frequency spectra of auroral intensities obtained by wavelet analysis and fast Fourier transform (FFT) show the coexistence of ∼0.3 Hz, 4 Hz, and >10 Hz modulationsSpatial distributions show that the internal and fast modulations are well‐structured within aurora patches [ABSTRACT FROM AUTHOR]

The North Atlantic Oscillation (NAO) and North Atlantic tripole sea surface temperature (SST_tri) are important modes in the atmosphere and ocean over the North Atlantic, respectively. The link between the two is well‐known. However, this link weakened during 1980–2001, which is particularly pronounced in late winter and was not detected in early winter. This phenomenon has not been well revealed. The role of NAO in the above correlation changes was discussed. In late winter, a significant eastward shift (up to 20° longitude) of NAO south center during 1980–2001 was observed in both observation and CMIP6, accompanied by the eastward expansion of NAO north center. Spatial shift of the NAO forced the region of strong air‐sea interactions to shift and resulting in the collapse of NAO‐related SST_tri. These findings deepen our understanding of the NAO on the subseasonal scale. Plain Language Summary: A significant correlation between the North Atlantic Oscillation (NAO) and the North Atlantic tripole sea surface temperature (SST_tri) is widely recognized. However, this study found that the correlation between the two was significantly weakened in late winter during 1980–2001 while no such change was detected in early winter. Therefore, this paper will focus on the late winter and discuss the role of NAO spatial structure in the above correlation changes. When the NAO south center is located over the North Atlantic, both observations and the climate models indicate that the corresponding North Atlantic SST shows a significant tripole pattern. However, during 1980–2001, the NAO south center shift significantly eastward from the North Atlantic toward Western Europe (up to 20° longitude). At the same time, the NAO north center expanded eastward significantly. The eastward shift of the NAO results in the significant eastward shift of the turbulent heat flux and wind stress anomalies. Shift in the region of strong air‐sea interaction led to the collapse of NAO‐related SST_tri. These findings deepen our understanding of the NAO on the subseasonal scale and also provide implications for subseasonal‐seasonal predictions of Eurasian climate extremes. Key Points: The link between NAO and North Atlantic tripole SST weakened obviously during 1980–2001 late winter, which was not detected in early winterThe eastward shift of ∼20° longitude in NAO south center forced the strong air‐sea interactions region to shift in observations and CMIP6NAO's spatial shift in late winter caused the changes in the link between NAO and North Atlantic tripole SST [ABSTRACT FROM AUTHOR]

Powerful lightning strikes generate broadband electromagnetic signals. At Extremely Low Frequencies (ELF), the signal partly leaks into the ionosphere and produces whistlers that can be detected by satellites. Indeed, the satellites of the European Space Agency (ESA) Swarm Earth Explorer mission can detect those signals during 250 Hz burst‐mode acquisition campaigns of their Absolute Scalar Magnetometers (ASM). The dispersion of these whistlers depends on their propagation path and the distribution of ionization in the ionosphere crossed along that path. In this paper, we introduce a technique to derive a new measure of ionosphere electron content, the Total square‐Root Electron Content (TREC), using the arrival times of two frequencies of the whistler signal. We validate this approach by using data from ionosondes and from in situ measurements of the electron density at Swarm location. This technique brings new opportunities for sounding the ionosphere in regions poorly observed by other techniques. Plain Language Summary: A lightning strike generates an electromagnetic impulse that propagates within Earth's atmosphere and eventually leaks out into the ionosphere. As it propagates through the ionosphere toward low‐Earth orbiting (LEO) satellites, it gets converted into a so‐called whistler, with high frequencies arriving earlier than low frequencies. This frequency dispersion depends on the state of the ionosphere. Here, we analyse such whistler waves detected by magnetometers onboard the European Space Agency Swarm satellites to recover information about the state of the ionosphere below the satellites. We first introduce a new metric, the Total Root Electron Content (TREC), which quantifies the cumulative value of the square root of electron density along the path of the whistler. We next propose a method to recover the TREC from the analysis of the whistler dispersion. We finally validate this method by using independently derived ionospheric electron density profiles to infer expected TREC values. Our results show that whistlers detected by LEO satellites can be used to locally improve the widely used empirical International Reference Ionosphere model. Such whistler inferred TREC values could be used to sound the ionosphere above places difficult to sample with conventional measuring techniques, and help better model and understand the highly dynamic ionosphere. Key Points: Total square‐Root Electron Content (TREC) is a new measure of the ionospheric electron content for electromagnetic signals in the ELF bandA method to retrieve TREC from fractional‐hop whistlers in the ELF detected by the ESA Swarm mission is proposedThe method is validated using TREC computed with independently constrained electron density profiles close to the Swarm whistler locations [ABSTRACT FROM AUTHOR]

This study investigates the effects of sea spray on extreme precipitation forecast in Beijing of China between 28 July and 2 August 2023 as a case test. In this case, fully coupled model increased upward moisture in the Bohai and Yellow Seas and increased accumulated rainfall by 21% in North China. For the extreme precipitation events with the 5‐day accumulated precipitation exceeding 500 mm, the atmosphere‐only model did not forecast the events; the coupled model without sea spray performed well with the 0.29 threat score (TS) and 88 mm root mean square error (RMSE); in the fully coupled model, the effects of sea spray increased atmospheric instability, which increased the precipitation around Beijing and yielded a more accurate forecast with the 0.37 TS and 65 mm RMSE. This paper suggests a scientific clue to improve numerical simulation for extreme rainfall events, however, more cases are still needed for statistical evaluation. Plain Language Summary: Although meteorological forecasting ability has been improved considerably during the past decades, precipitation during extreme events remains typically underestimated. Improving model accuracy for heavy rainfall events is, indeed, a ground challenge. In this study, we conducted three numerical experiments under different conditions to evaluate the effects of sea spray on extreme precipitation forecasts for the Beijing extreme rainfall event between 28 July and 2 August 2023. Our results indicated that for this 5‐day forecasting case, including sea spray in the simulations enhanced precipitation in North China by transporting more moisture from the Bohai and Yellow Seas upward into the atmosphere. The moister and warmer air caused by sea spray effects was transported to Beijing through northwestward wind and lifted by the local terrain, then caused a more unstable atmosphere and higher intensity of precipitation around Beijing. By using statistical indicators and setting different precipitation thresholds for this extreme rainfall case, we determined that in this case, the fully coupled model including sea spray yielded more accurate forecast. To statistically evaluate the effects of sea spray in extreme precipitation forecasts, more cases are still needed. Key Points: Fully coupled model led more upward moisture in the Bohai and Yellow Seas and increased 5‐day accumulated rainfall by 21% in North ChinaFully coupled model improved the forecasting threat score from 0 to 0.37 with the 5‐day accumulated precipitation exceeding 500 mmSea spray led to more unstable atmosphere for the Beijing extreme rainfall event, yielding a more accurate forecast [ABSTRACT FROM AUTHOR]

Subsurface gas storage is crucial for achieving a sustainable energy future, as it helps to reduce CO2 emissions and facilitates the provision of renewable energy sources. The confinement effect of the nanopores in caprock induces distinctive thermophysical properties and fluid dynamics. In this paper, we present a multi‐scale study to characterize the subsurface transport of CO2, CH4, and H2. A nanoscale‐extended volume‐translated Cubic‐Plus‐Association equation of state was developed and incorporated in a field‐scale numerical simulation, based on a full reservoir‐caprock suite model. Results suggest that in the transition from nanoscale to bulk‐scale, gas solubility in water decreases while phase density and interfacial tension increase. For the first time, a power law relationship was identified between the capillary pressure within nanopores and the pore size. Controlled by buoyancy, viscous force and capillary pressure, gases transport vertically and horizontally in reservoir and caprock. H2 has the maximum potential to move upward and the lowest areal sweep efficiency; in short term, CH4 is more prone to upward migration compared to CO2, while in long term, CH4 and CO2 perform comparably. Thicker caprock and larger caprock pore size generally bring greater upward inclination. Gases penetrate the caprock when CH4 is stored with a caprock thickness smaller than 28 m or H2 is stored with a caprock pore size of 2–10 nm or larger than 100 nm. This study sheds light on the fluid properties and dynamics in nanoconfined environment and is expected to contribute to the safe implementation of gigatonne scale subsurface gas storage. Plain Language Summary: Aiming at the ambitious targets of net‐zero emission and global energy transition, effective technologies need to be developed and deployed with huge efforts. Subsurface gas storage is expected to play a critical role in reducing CO2 emission and providing large‐scale renewable energy. However, unforeseen gas leakage can cause potential environmental risks and energy efficiency concerns. Despite its importance, the mechanisms governing gas transport and distribution within caprocks, particularly under nanoconfinement conditions, are poorly understood. From nano‐scale and bulk‐scale thermodynamic properties predictions to field‐scale numerical simulations, this work carried out a comprehensive multi‐scale analysis to evaluate the storage security of CO2, CH4, and H2. Relationships between VLE, phase density, interfacial tension, capillary pressure and pore size, temperature, pressure and wettability were discussed, offering novel perspectives on the mechanisms governing gas leakage. Furthermore, loss potentials of different gases were compared and evaluated and the impacts of caprock thickness and pore size were examined, providing valuable insights for screening eligible storage sites. Overall, this work contributes to the development of secure and reliable gas storage techniques, supporting the goals of climate change mitigation and facilitating the transition to a net‐zero energy future. Key Points: A multi‐scale analysis is carried out to assess the loss potential of CO2, CH4, and H2 during subsurface storageMigration and distribution characteristics of gases in the caprock are investigatedShort‐term and long‐term gas storage security are analyzed and compared [ABSTRACT FROM AUTHOR]

Granular flows, such as rockslide debris flows, can reach high velocities and travel vast distances, but mechanisms behind this high mobility remain elusive. Using the Discrete Element Method, we implement confined shear flow with fractal particle size distribution to study the physical mechanisms inside the highly localized basal zones. Our results show that the shear strength of confined flow decreases at large fractal dimensions, and a notable transition in shear behavior also occurs with increasing fractal dimension: large particles exhibit low granular temperature and act as solid phase within a semi‐diluted suspension, and small particles enter the rolling regime. The small particles behave as interstitial fluids and reduce the shear strength of confined flow. We suggest that the reduction of shear strength stemming from the fractal particle size distribution may provide insights into the mechanisms within the highly localized basal zones, ultimately contributing to the high mobility of granular flows. Plain Language Summary: Some geophysical flows are known for their high velocities and long travel distances. Many theories have been proposed to explain the long travel distances. There remains a puzzle among them, that is, whether the particle size distribution is one of the influencing factors. In this paper, we implement a series of confined shear flows to assess the effect of fractal particle size distribution on the shear behavior of the basal zone within granular flows. Our results show that, at a large fractal dimension, the large and small particles behave like solids and the fluid in suspension, respectively. Small particles with high angular velocity reduce the shear strength of the confined shear flows. We suggest that the reduction of shear strength might be one of the reasons for the long travel distances of some geophysical flows. Key Points: We demonstrate the relevance of two invariant quantities to the fractal dimension of confined shear flowWe show the suspension‐like transition of confined shear flow at a large fractal dimensionRolling regime of small particles results in "granular lubrication" phenomenon and reduces the shear strength of confined shear flow [ABSTRACT FROM AUTHOR]

Over‐estimation of summer precipitation over the Tibetan Plateau (TP) is a well‐known and persistent problem in most climate models. This study demonstrates the impact of a Gaussian Probability Density Function cloud fraction scheme on rainfall simulations using the Weather Research and Forecasting model. It is found that this scheme in both 0.1° and 0.05° resolutions significantly reduces the wet bias through both local feedbacks and large‐scale dynamic process. Specifically, increased cloud water/ice content with this scheme reduces surface shortwave radiation, and consequently surface heat fluxes and evapotranspiration. This, in turn, dampens the large‐scale thermal effect of the TP and weakens the exaggerated monsoon circulation and low‐level moisture convergence. It is this large‐scale dynamic process that contributes the most (∼70%) to the wet bias reduction. Although this paper presents a modeling study, it highlights the cloud radiative feedback to the large‐scale dynamics and precipitation over the TP. Plain Language Summary: Despite numerous attempts to correct the overestimation of summer precipitation over the Tibetan Plateau (TP) in current global and regional climate models, the issue persists. This study applies the Gaussian Probability Density Function (GPDF) cloud fraction scheme in the Weather Research and Forecasting model at two different resolutions (0.1° and 0.05°) during a summer over the TP. The results show that the GPDF scheme significantly mitigates the precipitation overestimation, particularly in the high‐resolution modeling. We explored the physical processes, both local and remote, that contribute to this improvement. Specifically, an increase in cloudiness reduces the amount of radiation reaching the land surface. This decrease in surface radiative heating not only reduces local evaporation but also weakens the thermal effect of the TP. The latter is a major driver of the South Asian monsoon that conveys moisture to the TP, and its weakening reduces moisture convergence over the TP. Both the decreases in local evaporation and remote moisture convergence contribute to the alleviation of the precipitation overestimation, and the latter plays a dominant role. These findings provide a unique perspective for reducing the wet bias over the TP, focusing on the surface available energy and associated remote moisture processes. Key Points: The use of the Gaussian Probability Density Function cloud fraction scheme in high resolution greatly reduces wet bias over the Tibetan Plateau (TP) during summerMore cloud water/ice with the scheme lessens TP's thermal effect, causing a weaker South Asian monsoon and moisture convergenceWet bias reduction is mainly governed by the decrease in remote moisture rather than local evapotranspiration [ABSTRACT FROM AUTHOR]

The present paper develops a new framework to predict the mean flow through an array of cylinders in which the flow around the array (array‐scale) and the flow around individual cylinders (element‐scale) are modeled separately using actuator disc theory and empirical drag models respectively, and then coupled through the net drag force. Applying this framework only requires knowledge of the array geometry and incident flow. The framework is validated using high‐fidelity direct numerical simulations for arrays of between 7 and 109 cylinders having different arrangements (staggered, concentric, random) and bounding shapes (circular, square) in both two‐ and three‐dimensional flows. In general, the framework outperforms existing models which require calibration and are only valid for part of the practical parameter space. The demonstrated scale separation suggests different combinations of element‐scale and array‐scale models/theories may be used for other arrangements of bluff bodies. Plain Language Summary: The temporally and spatially averaged velocity, or mean flow, within an array of cylinders is important for many practical applications including, for example, estimating the rates of sediment transport and nutrient uptake within a patch of aquatic vegetation, the hydrodynamic forces on offshore structures, and the efficiency of power generation with a turbine farm. However, a general predictive tool for mean flow is currently lacking. To develop this predictive capacity, this study uses the idea of scale separation to integrate models from different research fields, including the actuator disc theory originally developed for design of turbines and propellers, and element‐scale drag models initially proposed for aquatic vegetated flows, to develop a unifying, predictive, analytical framework to estimate the mean flow. The framework is validated using high‐fidelity direct numerical simulations of arrays of cylinders with different arrangements (staggered, concentric, random) and array shapes (square, circular). With only knowledge of the array geometry and incident flow, the framework predicts the mean flow exceptionally well across a wide range of practical flow conditions. This leads to a step‐change in our ability to predict the fraction of incoming flow passing through a porous array, hydrodynamic load on offshore structures, and sediment flux within aquatic vegetation systems when combined with bed load transport models. The framework could be an important guide for coastal and riverine restoration projects using aquatic vegetation and the design of offshore structures and turbine farms. Key Points: A predictive framework based on an idea of scale separation is proposed for the mean velocity of the flow passing through an array of cylindersThe framework is versatile, being applicable for a wide range of practically meaningful cylinder arrangements, array shapes and array flow blockagesHigh‐fidelity, three‐dimensional direct numerical simulations are employed to investigate flow through the array of cylinders and validate the new predictive framework [ABSTRACT FROM AUTHOR]

Time series of radiative cooling of the upper mesosphere and lower thermosphere (UMLT) by carbon dioxide (CO2) are examined for evidence of trends over 20 years. Radiative cooling rates in K day−1 provided by the SABER instrument are converted to time series of infrared power radiated from three distinct layers between 0.1 hPa and 0.0001 hPa (65–105 km). Linear regression against time and a predictor for solar variability provides estimates of the trend in exiting longwave radiation (ELR) from these layers. Trends in ELR are not significantly different from zero at 95% or 99% confidence in each layer. These results demonstrate energy conservation in the UMLT on decadal time scales and show that the UMLT continues to radiate the same amount of energy it receives despite cooling and contracting over two decades. These results are enabled by the long‐term stability of the SABER instrument calibration. Plain Language Summary: The Earth's upper mesosphere and lower thermosphere (UMLT) is the region between approximately 65 and 105 km in altitude. Infrared radiation emitted by CO2 is a fundamental component of the energy budget of the UMLT. Carbon dioxide (CO2) is increasing in this region. The amount of infrared energy emitted by CO2, over time, must balance the amount of energy from sunlight that is absorbed in the UMLT. Observations from orbiting satellites over the past two decades have shown that the temperature in the UMLT is decreasing due to the increasing CO2. A decreasing temperature implies a decrease in infrared energy emitted by an object. Energy conservation, however, requires that the amount of infrared energy radiated from the UMLT balance the amount of energy received, regardless of the temperature. In this paper we show that there is no significant long‐term change with time (i.e., zero trend) in the infrared energy emitted from the UMLT even though the UMLT temperature has been decreasing for the last 20 years (and longer) due to increasing CO2. This result confirms that energy is conserved in a cooling and contracting UMLT. Key Points: Time series of infrared power (W) radiated by CO2 from the upper mesosphere & lower thermosphere (UMLT) are developedTrends in radiated power are not different from zero at 95% or 99% confidence in the three layers examined between 0.1 and 0.0001 hPaIncreasing CO2 reduces the UMLT temperature while radiating the same amount of energy over time, thus conserving energy [ABSTRACT FROM AUTHOR]

We examine the low‐frequency (LF) magnetic pulses recorded for the positive leader in two natural lightning flashes as recorded on the high‐speed video camera. The captured positive leader in both cases propagated in a stepwise manner at least over several milliseconds according to the concurrent sequence of LF magnetic pulses. By comparing with previous studies on positive leaders under different situations, the average inter‐pulse intervals (11.7 and 20.3 μs, respectively) obtained for our natural cases are shorter than that of upward positive leaders in rocket‐triggered (typically >25 μs) and tower‐initiated lightning (62 μs in one case). Hence, the stepwise propagation of positive leaders in natural lightning is likely more frequent than that in object‐induced lightning, reflecting a possible influence of ambient electric field and other meteorological factors on the stepwise feature of positive leader. The stepwise extension might be common for positive leaders developing in various scenarios. Plain Language Summary: Many observations have been made with respect to upward positive leaders in rocket‐triggered lightning and tower‐initiated lightning. From the previous observations, it is obvious that the upward positive leader sometimes propagates in a stepwise manner. When it comes to natural lightning, many reliable conjectures have been put forward about the stepwise propagation of positive leaders, but there is still a lack of solid observations. Whether the positive leader in natural lightning also propagates in a stepwise manner remains an open question. In this paper, we analyze the propagation of positive leaders in two natural lightning flashes using high‐speed imaging and concurrent measurements of the associated radio frequency magnetic fields. It aims to understand the variability in the propagation manner of positive leaders in natural lightning and revealing the differences and similarities of leader propagation between natural and object‐initiated lightning. From the low‐frequency magnetic field measurements, the positive leaders in natural lightning could also propagate in a stepwise manner. Our analysis shows that the inter‐pulse intervals of positive leaders in observed natural lightning are shorter than that previously observed in tower‐initiated and rocket‐triggered lightning. Key Points: First coordinated observations of stepwise propagation for the positive leader of natural lightning flashesThe average inter‐pulse interval of positive leader stepping in two cases is estimated to be around 12 and 20 μs, respectivelyThe stepping of positive leaders in natural lightning appears to be more frequent than that of the object‐initiated upward positive leaders [ABSTRACT FROM AUTHOR]

Prediction of the rapid intensification (RI) of tropical cyclones (TCs) remains challenging. In this paper, by using information from the early period following TC formation, the rapid intensification warning index (RIWI) is developed based on the analog method. A 10‐year cross‐validation and data from Hurricane Ida (2021) are used to verify its potential application. Results show that the RIWI can efficiently discriminate between RI and non‐RI storms and has a significant positive correlation with the lifetime maximum intensity (LMI) of the TCs. By using this index, an early warning can be issued ∼30 hr before the onset of RI, which is much earlier than the predictions made using the probabilistic Statistical Hurricane Intensity Prediction System RI index. In addition, by using the RIWI as a predictor, the prediction of LMI provides an early estimate of TC severity. Plain Language Summary: Variations in the intensity of tropical cyclones (TCs) are difficult to forecast, especially the occasional TCs that develop quickly over a period of couple of days; that is, rapid intensification (RI). Recent studies have demonstrated that it is easier to forecast RI likelihood than RI timing. Thus, instead of attempting to forecast the exact timing of RI, it may be more useful to predict whether RI will occur during the lifetime of a TC. In this paper, by using information from the early stage of TCs, we use the analog method to develop the RI warning index (RIWI). Then, using an appropriate threshold, it can be used to predict whether RI will occur during the lifetime of a TC. Furthermore, an important TC parameter, the lifetime maximum intensity (LMI) of the TC, has a significant positive correlation with its RIWI. The application of RIWI to Hurricane Ida (2021) is also presented in this paper and shows that an early warning can be issued ∼30 hr before the onset of RI. Key Points: By using the information of tropical cyclones (TCs) early stage, a rapid intensification warning index (RIWI) is constructed based on the analog methodThe RIWI can distinguish between rapid intensification (RI) and non‐RI storms during the early stages of TCsThere is a significant positive correlation between a TC's RIWI and its lifetime maximum intensity [ABSTRACT FROM AUTHOR]

Large‐scale seepage faces occur on tidal flats with gentle slope, which are widely distributed worldwide. Evaporation on these seepage faces, leading to salt retention and accumulation, may significantly impact the density‐dependent groundwater flow beneath the tidal flats. However, due to nonlinear complexities of the groundwater flow and solute transport on seepage faces, explicit boundary conditions and numerical models to quantify these processes are lacking. In this study, we present both mathematical and numerical models to quantify these processes. Compared to the results of our previous study, this paper shows that seepage‐face evaporation can (a) significantly increase the groundwater salinity in the upper intertidal zone, and form multiple groundwater circulation cells in the intertidal zone, (b) cause the disappearance of multiple seepage‐faces and reduce the spatial extent of seepage faces notably, (c) and intensify the groundwater and salt exchange as well as the seawater‐groundwater circulation through the intertidal zone. Plain Language Summary: Tidal flats with very gentle slopes are ubiquitously distributed around the world. Due to these minimal slopes, the intertidal zone can extend for several kilometers, leading to large‐scale seepage faces during ebb and low tides. Previous studies have shown that evaporation without seepage face causes salt retention and accumulation, which not only increases groundwater salinity significantly, but also alters the groundwater density and its subsurface pathways. However, explicit mathematical expression and associated numerical models to quantify the process of evaporation on seepage faces are lacking, due to the mathematically nonlinear complexities to deal with the density‐dependent groundwater flow and solute transport. Here, a new boundary condition considering evaporation on seepage faces as well as salt retention and accumulation was proposed. Compared with the seepage‐face boundary conditions of our previous study, we found that multiple seepage‐faces disappear and multiple groundwater circulation cells develop due to evaporation. Seepage‐face evaporation significantly increases the salinity in the upper intertidal zone, reduces the scale and existence of seepage faces, and promotes groundwater/salt exchange. This study systematically reveals the effects of seepage‐face with evaporation on the density‐dependent groundwater flows and salinity distributions in coastal aquifers, which have strong implications on biogeochemical processes within the intertidal zone. Key Points: A new evaporation boundary condition was proposed on seepage face with salt retained and accumulatedMultiple seepage‐faces disappear and multiple groundwater circulation cells develop due to evaporation in the intertidal zoneSeepage‐face evaporation significantly promotes groundwater and salt exchange within the intertidal zone [ABSTRACT FROM AUTHOR]

Large constellations of small satellites will significantly increase the number of objects orbiting the Earth. Satellites burn up at the end of service life during reentry, generating aluminum oxides as the main byproduct. These are known catalysts for chlorine activation that depletes ozone in the stratosphere. We present the first atomic‐scale molecular dynamics simulation study to resolve the oxidation process of the satellite's aluminum structure during mesospheric reentry, and investigate the ozone depletion potential from aluminum oxides. We find that the demise of a typical 250‐kg satellite can generate around 30 kg of aluminum oxide nanoparticles, which may endure for decades in the atmosphere. Aluminum oxide compounds generated by the entire population of satellites reentering the atmosphere in 2022 are estimated at around 17 metric tons. Reentry scenarios involving mega‐constellations point to over 360 metric tons of aluminum oxide compounds per year, which can lead to significant ozone depletion. Plain Language Summary: With ongoing plans for many constellations of small satellites, the number of objects orbiting the Earth is expected to continue increasing in the foreseeable future. At the end of service life, satellites are disposed into the atmosphere, burning up during the process and generating aluminum oxides, which are known to accelerate ozone depletion. The environmental impacts from the reentry of satellites are currently poorly understood. This paper investigates the oxidation process of the satellite's aluminum content during atmospheric reentry utilizing atomic‐scale molecular dynamics simulations. We find that the population of reentering satellites in 2022 caused a 29.5% increase of aluminum in the atmosphere above the natural level, resulting in around 17 metric tons of aluminum oxides injected into the mesosphere. The byproducts generated by the reentry of satellites in a future scenario where mega‐constellations come to fruition can reach over 360 metric tons per year. As aluminum oxide nanoparticles may remain in the atmosphere for decades, they can cause significant ozone depletion. Key Points: We present the first atomic‐scale molecular dynamics simulation of high‐temperature aluminum ablation during reentry from low‐Earth orbitThe amount of aluminum oxide nanoparticles generated is quantified and the accumulation in the atmosphere is estimatedThe long‐term accumulation of aluminum oxides from reentering satellites can cause significant ozone depletion [ABSTRACT FROM AUTHOR]

Ground magnetic field variations have been used to investigate ionospheric dynamics for more than a century. They are usually explained in terms of an electric circuit in the ionosphere driven by an electric field, but this is insufficient to explain how magnetic field disturbances are dynamically established. Here we explain and simulate how the ionosphere dynamically responds to magnetospheric forcing and how it leads to magnetic field deformation via Faraday's law. Our approach underscores the causal relationships, treating the magnetic field and velocity as primary variables (the B, v paradigm), whereas the electric field and current are derived, in contrast to the E, j paradigm commonly used in ionospheric physics. The simulation approach presented here could be used as an alternative to existing circuit‐based numerical models of magnetosphere‐ionosphere coupling. Plain Language Summary: Ground magnetic field variations have been used to investigate ionospheric dynamics for more than a century. They are usually explained in terms of an electric circuit in the ionosphere driven by an electric field; similar to how we would explain the magnetic field of a current‐carrying wire. However, this approach oversimplifies the complex reality in space plasmas. Here, the relationship between currents and magnetic fields is reversed: currents should be understood as a consequence of the magnetic field, and not the other way around. In this paper we explain and simulate how the ionosphere dynamically responds to forcing from above, and how it leads to magnetic field deformation that corresponds to an electrical current. Key Points: We present a model and simulation of how the magnetic field in the ionosphere dynamically changes in response to external forcingOur model emphasizes the causal relationships, treating B and v as primary instead of E and j, as is commonly done in ionospheric physicsSimulation results highlighting the self‐consistent ionospheric response in the B, v paradigm are presented [ABSTRACT FROM AUTHOR]

Whistler‐mode waves are commonly observed within the lunar environment, while their variations during Interplanetary (IP) shocks are not fully understood yet. In this paper, we analyze two IP shock events observed by Acceleration, Reconnection, Turbulence and Electrodynamics of the Moons Interaction with the Sun (ARTEMIS) satellites while the Moon was exposed to the solar wind. In the first event, ARTEMIS detected whistler‐mode wave intensification, accompanied by sharply increased hot electron flux and anisotropy across the shock ramp. The potential reflection or backscattering of electrons by the lunar crustal magnetic field is found to be favorable for whistler‐mode wave intensification. In the second event, a magnetic field line rotation around the shock region was observed and correlated with whistler‐mode wave intensification. The wave growth rates calculated using linear theory agree well with the observed wave spectra. Our study highlights the significance of magnetic field variations and anisotropic hot electron distributions in generating whistler‐mode waves in the lunar plasma environment following IP shock arrivals. Plain Language Summary: The surface of the Earth's Moon is frequently exposed to the incoming solar wind flow and IP shocks due to its lack of internal magnetic fields that can deflect the solar wind particles. Within the lunar environment, whistler‐mode waves, characterized by electromagnetic fluctuations with frequencies below the electron gyrofrequency, are commonly present. Interplanetary shocks that are often associated with significant disturbances in electron flux and magnetic field can potentially lead to anisotropic distributions of electrons, which are known to provide free energy source for whistler‐mode wave generation. To assess the whistler wave generation under shock conditions, we conduct an in‐depth analysis of two IP shock events. These events provide clear evidence of shock‐induced enhancements in electron pitch angle anisotropy and flux, as well as a potential rotation of magnetic field around the shock region, resulting in the intensification of whistler‐mode waves downstream of the shock. We calculated a timeseries of linear wave growth rate for the entire duration of shock events, which remarkably accounted for the observed whistler‐mode wave spectra both before and after the shock arrival. Our findings are important for understanding the associated physical process of whistler‐mode wave generation in the lunar plasma environment during IP shock events. Key Points: Two Interplanetary shock events in the lunar environment are analyzed to unveil whistler wave generation around shock regionLinear wave growth calculations show that whistler‐mode waves are generated locally due to enhanced electron anisotropy and fluxMagnetic field line connection to lunar surface is found to be important for enhancing whistler wave intensity [ABSTRACT FROM AUTHOR]

The cloud feedback may result in amplification or damping of Arctic warming. Two common techniques used to diagnose the top‐of‐the‐atmosphere cloud feedback are the Adjusted Cloud Radiative Effect (AdjCRE) method and the Cloud Radiative Kernel (CRK) method. We apply both to CMIP5 and CMIP6 model data, finding that the AdjCRE calculated Arctic shortwave cloud feedback is twice as correlated with sea ice loss in CMIP5, and four times in CMIP6, as the CRK method. We find that the CRK method produces Arctic all‐sky residual percentages exceeding 20% in 15 of 18 models. We use the CRK method to decompose the feedback in CMIP5 and CMIP6 finding that its median value changed from negative to positive driven by a less‐negative cloud optical depth feedback. Despite its lack of closure, we conclude that the CRK method is better suited for Arctic SW feedbacks as it is less impacted by surface albedo changes. Plain Language Summary: The cloud feedback is the process by which cloud property changes in a warming climate can either further enhance warming or damp it. The Arctic is warming faster than the rest of the globe, and one of the largest sources of uncertainty in its climate projections is the cloud feedback. There are two popular methods to calculate the cloud feedback: the Adjusted Cloud Radiative Effect technique, and the Cloud Radiative Kernel technique. In this paper we compare the two methods in a suite of climate models by considering the extent to which changes in Arctic sea ice impact the cloud feedbacks. From this analysis we conclude that the Cloud Radiative Kernel method is less affected by sea ice loss. We then apply the Cloud Radiative Kernel technique to data from the two most recent generations of global climate models to investigate how polar day Arctic cloud feedbacks have changed between these generations. We find that the median value of these Arctic feedbacks is slightly positive in the newest generation of models, a change from slightly negative in the previous generation that is largely fueled by a weakening of the feedback associated with changes in cloud optical depth. Key Points: The Cloud Radiative Kernel method is less sensitive to surface albedo changes than the Adjusted Cloud Radiative Effect techniqueThe Cloud Radiative Kernel method provides poor radiative closure in a suite of global climate modelsThe median shortwave Arctic cloud feedback in recent climate models is slightly positive due to a weakened cloud optical depth feedback [ABSTRACT FROM AUTHOR]

Increasing snow accumulation over the Antarctic Ice Sheet may mitigate future sea level rise. However, current estimates of mitigation potential are poorly constrained due to limited records of past variability. We present an annually resolved reconstruction of Antarctic snow accumulation from 1801 to 2000 CE, employing a paleoclimate data assimilation methodology to integrate ice core records with a multi‐model ensemble of climate simulations. Our reconstruction correlates well with instrumental reanalysis, and we find that Antarctic accumulation rates increased over the 20th‐century, resulting in a modest amount (∼1 mm) of sea level mitigation. Mitigation is primarily driven by an accelerating trend since around 1970. Our results contrast with previous mitigation estimates of ∼10–12 mm; this discrepancy is due to poorly constrained baseline estimates of 19th‐century accumulation in East Antarctica. Our reconstruction suggests that the uncertainty of future sea level mitigation from increasing Antarctic accumulation has been underestimated. Plain Language Summary: Ice loss from Antarctica causes sea level to rise, but Antarctica can also mitigate sea level rise if snowfall on the continent increases faster than the ice loss. There is not currently a consensus on whether Antarctica will contribute to or mitigate sea level rise in the coming century, due to a lack of Antarctic snowfall records. In this paper, we merge information from ice core records with climate models to reconstruct annual Antarctic snowfall from 1801 to 2000. We find that snowfall on Antarctica increased during the 20th‐century, but only had a modest counteracting effect of 1 mm on sea level rise in the 20th‐century, much less than a previous estimate of 10 mm. The large discrepancy is due to uncertainty in East Antarctic accumulation in the 19th‐century. The potential for Antarctica to mitigate sea level is uncertain, affecting projections for future sea level rise. Key Points: Antarctic accumulation reconstruction using paleoclimate data assimilation finds modest (∼1 mm) 20th‐century sea level mitigationWe find similar 20th‐century trends to previous work; lower sea level mitigation (1 vs. 10 mm) is due to the 19th‐century baselineUncertainty in past East Antarctic accumulation limits confidence in future projections of Antarctic sea level mitigation [ABSTRACT FROM AUTHOR]

Herein, fast fracture initiation in glacier ice is modeled using a Material Point Method and a simplified constitutive law describing tensile strain softening. Relying on a simple configuration where ice flows over a vertical step, crevasse patterns emerge and are consistent with previous observations reported in the literature. The model's few parameters allows identification of a single dimensionless number controlling fracture spacing and depth. This scaling law delineates two regimes. In the first one, ice thickness does not play a role and only ice tensile strength controls the spacing, giving rise to numerous surface crevasses, as observed in crevasse fields. In this regime, scaling can recover classical values for ice tensile strength from macroscopic field observations. The second regime, governed by ice bending, produces large‐scale, deep fractures resembling serac falls or calving events. Plain Language Summary: In ice sheets and alpine glaciers, fast‐flowing sections are often characterized by crevasse fields that play a significant role in the cryo‐hydrologic system by facilitating meltwater flow, enhancing basal sliding, weakening the ice, and impacting glacier thermodynamics. Modeling these fractures at the glacier scale remains challenging and often necessitates integrating diverse models which hinders the straightforward consideration of physical issues associated with crevasse fields on a large scale. Here, a new numerical framework allows us to conduct field‐scale experiments and paves the way for a scaling law to elucidate the macroscopic factors influencing fracture fields and to easily incorporate crevasse depth and spacing into large‐scale models. A newly discovered scaling law highlights the transition between a mechanical behavior where the regular crevasse spacing is unaffected by geometry to a regime where geometry plays a significant role, particularly in large‐scale fracture processes like glacier calving. While the numerical experiments in this paper focus on glaciers, the model and conceptual framework is versatile and can address the mechanical behavior of fractures in broader geophysical contexts such as snow, rock or ice avalanches, tectonics and landslides. Key Points: Fractures in glacier flow are modeled using material point method with elastoplasticity and tensile strain softeningA dimensional analysis reveals a key dimensionless number characterizing two different regimes of fast fractureOne regime predicts acknowledged ice tensile strength from field observations and characterizes the regular crevasse spacing [ABSTRACT FROM AUTHOR]

Two ∼2‐week Ultra‐Fast Kelvin Wave (UFKW) events centered on days 158(203) during 2021 are investigated using winds, temperatures, plasma drifts and electron densities (Ne) measured by the Ionospheric CONnections (ICON) mission. Eastward‐propagating longitudinal wave‐1 (s = −1) structures with periods 2.5–4.0d, thought to mainly reflect Ultra‐Fast Kelvin waves (UFKWs), reveal ±45 ms−1 zonal winds (U) at 100 km for both events. Height‐latitude structures of the 3.0(3.5)d‐period UFKWs are obtained for the first time for both temperature (T, 94–120 km) and U (94–280 km) between 12°S and 39°N latitude. Maximum values of 36(29) ms−1 for U and 12(15)K for T occur at 102(106) km altitude and within ±3° latitude. The U‐T peak height displacement remains unexplained. Vertical wavelengths are in the range 36–43 km for both U and T during both events. Concurrent with the E‐region dynamo winds, topside (580 km) F‐region field‐aligned (±20–40 ms−1), meridional (±5–10 ms−1) and vertical (±5–10 ms−1) drift and Ne (±20–40%) 2.5–4.0d s = −1 variations are also measured. These key elements of atmosphere‐ionosphere (A‐I) coupling, contemporaneously measured for the first time, are relevant to testing the internal consistency of A‐I models. The mean wind propagation environment of the UFKWs is also quantified, showing no appreciable effects on the UFKW structures, consistent with modeling and theory. Plain Language Summary: Atmospheric Ultra‐Fast Kelvin waves (UFKWs) are eastward‐ and vertically propagating oscillations with periods less than 1 week that are centered on the equator and confined to low latitudes. They are part of a whole spectrum of waves forced by the spatial‐temporal variability of the heat of condensation ("latent heating") that is released when rising moist air forms rain droplets, mainly in the tropics. At high altitudes UFKW reach large amplitudes due to the decrease in ambient density with height. Their growth is curtailed by the viscosity of the atmosphere near 102 km altitude where they reach maximum amplitudes. Here they interact with ionized particles (the ionosphere) and generate electric fields that ultimately drive ionospheric variability at higher altitudes (>200 km), thus presenting an element of "space weather" to navigation and communications systems. In this paper, UFKWs measured in connection with the ICON mission are used to reveal, for the first time, the height and latitudinal structures of temperatures and winds associated with UFKWs in this dissipative regime, and their corresponding effect on the ionosphere. These results can thus serve to assess the internal consistency of models that seek to simulate atmosphere‐ionosphere coupling and related space weather effects induced by the wave spectrum excited in the lower atmosphere. Key Points: First coincident UFKW winds and temperature structures covering 94‐120 km,12°S‐39°N latitude are reportedFirst UFKW wind profiles in the 100‐280 km altitude dissipative regime are reportedFirst contemporaneous measurements of ionosphere response and driving UFKW E‐region dynamo winds are reported [ABSTRACT FROM AUTHOR]

We present the first direct evidence of an in situ excitation of drift‐compressional waves driven by drift resonance with ring current protons in the magnetosphere. Compressional Pc4–5 waves with frequencies of 4–12 mHz were observed by the Arase satellite near the magnetic equator at L ∼ 6 in the evening sector on 19 November 2018. Estimated azimuthal wave numbers (m) ranged from −100 to −130. The observed frequency was consistent with that calculated using the drift‐compressional mode theory, whereas the plasma anisotropy was too small to excite the drift‐mirror mode. We discovered that the energy source of the wave was a drift resonance instability, which was generated by the negative radial gradient in a proton phase space density at 20–25 keV. This proton distribution is attributed to a temporal variation of the electric field, which formed the observed multiple‐nose structures of ring current protons. Plain Language Summary: During magnetic storms and substorms, energetic ions are sporadically injected into the geospace, which distorts the stable population and velocity distributions of ions in space. At these moments, various plasma instabilities lead to ultra‐low frequency (ULF) wave excitations. The lowest‐frequency waves in the ULF range have a wavelength comparable to the size of the Earth and are typically analyzed using magnetohydrodynamic principles. This approach considers the plasma environment using macroscale parameters such as pressure and density. In this paper, we report a spacecraft observation of a broadband compressional ULF wave that cannot be interpreted using magnetohydrodynamics. Such waves have rarely been reported and analyzed; however, their interaction with energetic ions is important to understand magnetospheric energy dynamics. The plasma conditions were described using the kinetic theory, which involves particle velocity distributions. We observed that a drift resonance occurred between the energetic protons and waves, while the gradient instability condition was satisfied for a part of time. Therefore, we concluded that the wave was in a drift‐compressional mode excited through drift resonance and gradient instability. The interpretation of compressional waves via satellite observations of energetic ions has been receiving increasing attention to understand their excitation mechanism. Key Points: Pc4–5 compressional ultra‐low frequency waves with an azimuthal wave number of −130 were observed in the nose structure on dusksideTheoretically predicted values of drift‐compressional mode frequency match the observed wave frequencyBoth radial ion temperature gradient and drift resonance of 20–25 keV protons serve as energy sources of the wave [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs) significantly impact the hydrological cycle and associated extremes in western continental regions. Recent studies suggest ARs also influence water resources and extremes in continental interiors. AR detection tools indicate that AR conditions are relatively frequent in areas east of the Rocky Mountains. The origin of these ARs, whether from synoptic‐scale waves or mesoscale processes, is unclear. This study uses meteorological composite maps and transects of AR conditions during the four seasons. The analysis reveals that ARs east of the Rockies are associated with long‐wave, baroclinic Rossby waves. This result demonstrates that eastern North American ARs are dynamically similar to their western coastal counterparts, though mechanisms for vertical moisture flux differ between the two. These findings provide a foundation for understanding future climate change and ARs in this region and offer new methods for evaluating climate model simulations. Plain Language Summary: Atmospheric rivers (ARs) are a weather pattern that brings high amounts of atmospheric water and winds in a relatively narrow region. ARs are typically considered a "west coast" phenomenon, largely because the majority of the scientific research on ARs has focused on ARs in western coastal regions: particularly the western United States (US). ARs occur in continental interiors, but there has been some debate about whether these ARs represent the same type of weather as those in western coastal regions. This paper uses two objective methods for identifying ARs and finds times when ARs are present in three locations in the eastern half of the US: Norman, OK, Bloomington, IN, and Washington, DC. Examination of weather conditions during these AR times shows remarkable similarity to conditions associated with west coast ARs. This gives strong evidence that ARs do occur in the eastern half of the US. This result is important because it suggests that ARs may be important for water resources and extreme weather in the eastern half of the US, just as they are in the western US. This result also suggests that ARs may be important for water resources and extremes in other continental interiors. Key Points: Atmospheric rivers (ARs) east of the Rockies are associated with baroclinic wavesWestern coastal ARs and eastern/midwest ARs are dynamically similarSynoptic‐scale uplift, combined with convective instability, provide efficient mechanisms for generating precipitation [ABSTRACT FROM AUTHOR]

In the California Current System (CCS), changes in the phenology (i.e., seasonal timing) of coastal upwelling alter the functioning of this productive marine ecosystem. Recently developed coastal upwelling indices that account for upwelling strength and nutrient flux to the surface provide a more complete understanding of bottom‐up forcing in the region. Using these indices, we describe CCS upwelling phenological variability in vertical transport and nutrient flux. Physical and biogeochemical spring transitions generally co‐occur in winter or spring, followed by increased upwelling and nutrient flux. In the latter half of the year, upwelling continues but nutrient flux wanes as declining source nutrient concentrations limit the biological efficacy of coastal upwelling. Earlier spring transitions and higher season‐integrated upwelling intensity occur during strong La Niña events at all latitudes, driven largely by stronger alongshore wind stress. Understanding phenological changes in coastal upwelling is critical, as they could have significant ecosystem consequences. Plain Language Summary: In the California Current System (CCS), coastal upwelling carries nutrient‐rich waters to the surface, supporting primary production and driving the coastal ecosystem. This upwelling varies on a seasonal and interannual basis, as reflected in recently developed indices that account for the amount of water upwelled to the surface as well as the amount of nutrients carried in that water. Generally upwelling and nutrient transport are high in the first half of the year. Upwelling persists into the second half of the year, but nutrient transport decreases as the deep‐water sources of these nutrients are depleted. Upwelling in the CCS is also affected by the El Niño Southern Oscillation. During La Niña conditions, strong trade winds enhance upwelling and nutrient transport on the California coast. This paper presents regional, seasonal and interannual patterns of upwelling and nutrient delivery in the CCS, which are important drivers of change to this coastal ecosystem. Key Points: We define new upwelling phenology indices for the California Current System that include nutrient transportWe identify spatial, seasonal, and interannual patterns of upwelling and nutrient deliveryWe relate the physical mechanisms of coastal upwelling with its biological efficacy [ABSTRACT FROM AUTHOR]

This study addresses a critical gap in understanding anthropogenic influences on tropical climate dynamics by investigating the impact of aerosol‐cloud interactions on large‐scale circulation. Despite extensive research on greenhouse gas‐induced warming and its effects on tropical circulation, the impact of aerosols, particularly their interactions with clouds, on large‐scale circulation remains understudied. Utilizing large‐domain radiative convective equilibrium cloud‐resolving simulations, this research demonstrates that increasing aerosol concentration intensifies tropical overturning circulation, evaluated at the mid‐troposphere (I) $(\mathcal{I})$, strongly correlating with domain mean cloud and radiative properties. Employing a weak temperature gradient approximation, I attribute variations in I $\mathcal{I}$ to changes in clear‐sky radiative cooling rather than stability. These radiative cooling changes are linked to humidity changes driven by warm rain suppression by aerosols. This study's findings underscore the need to take into account microphysical changes, particularly aerosol concentrations, when studying anthropogenic effects on tropical circulation. Plain Language Summary: Clouds intricately couple with the large‐scale overturning circulation in the tropics. Aerosols, small particles suspended in the atmosphere, interact with clouds. Thus, changes in aerosols concentration (Na) have the potential to affect cloud properties and by that the large‐scale circulation, with important implications for tropical climate and climate change. Despite this crucial relationship, the subject has been insufficiently explored. In this paper, I use a simplified, yet realistic, representation of the tropical atmosphere—radiative convective equilibrium large‐domain cloud‐resolving simulations—to investigate the effect of aerosol‐cloud interaction on overturning tropical circulation. By varying Na under diverse sea surface temperatures, I demonstrate that the strength of the overturning circulation intensifies with increasing Na. Utilizing a semi‐analytical expression for subsidence strength in clear‐sky tropical regions, I show that the intensified overturning circulation with increased Na can be attributed to heightened humidity in the mid‐troposphere, leading to intense radiative cooling. This mid‐tropospheric humidification can be explained by warm rain suppression by aerosols, which fosters stronger humidity mixing between clouds and their surroundings. These findings underscore the need to consider aerosol concentration changes when examining anthropogenic impacts on tropical circulation. Key Points: The overturning tropical circulation intensifies with aerosol concentration in a radiative convective equilibrium modelChanges in circulation strength can be explained by changes in diabatic subsidence driven by radiative changesRadiative changes can be explained by increased humidity at the mid‐troposphere due to warm rain suppression [ABSTRACT FROM AUTHOR]

How and when sedimentary rocks record Earth's magnetic field is complex. Most studies assume a time‐progressive lock‐in mechanism during sediment deposition called depositional remanent magnetization (DRM). However, magnetic minerals can also form in situ, recording a chemical remanent magnetization (CRM) that is discontinuous in time. Disentangling the two mechanisms represents a major hurdle, and differences in their recording efficiencies remain unexplored. Here, our theoretical solutions demonstrate that CRM intensities exceed DRM by a factor of six when acquired in the same magnetic field. Novel experiments growing greigite (Fe3S4) in sediments and subsequent redeposition under identical magnetic field conditions confirm the predicted difference in recording efficiency. Thus, if left unrecognized, CRM leads to overestimated paleointensity and deserves more attention when interpreting Earth's magnetic history from sedimentary records. Recognition of fundamental differences between CRM and DRM characteristics provide a way forward to distinguish the recording mechanisms through routine laboratory protocols. Plain Language Summary: Remanent magnetizations preserved in sedimentary rocks serve as a continuous record of Earth's magnetic field history and play a fundamental role in understanding the Earth system. It is commonly assumed that magnetic minerals align with the magnetic field as a particle settles through the water column, known as a depositional remanent magnetization (DRM). However, diagenesis can lead to chemical growth of magnetic minerals, known as a chemical remanent magnetization (CRM). CRM lacks stratigraphic continuity and can obscure or completely overprint the original magnetization any time after sediment deposition, leading to a magnetic record that is uncorrelated with the age of the rock. Yet, CRMs go largely unrecognized. Theory and experiments in our paper document that CRMs record the magnetic field six times more efficiently than DRMs. Our work provides a way to distinguish the two through routine laboratory protocols. Key Points: Recording efficiency of chemical remanent magnetization (CRM) is six times higher than depositional remanent magnetization (DRM)Undetected chemical remanences lead to overestimated relative paleointensity estimatesComparison of natural and laboratory magnetization and demagnetization behavior help identify chemical remanent magnetizations in sediments [ABSTRACT FROM AUTHOR]

As the world's glaciers recede in response to a warming atmosphere, a change in the magnitude and frequency of related hazards is expected. Among the most destructive hazards are glacier lake outburst floods (GLOFs), and their future evolution is concerning for local populations and sustainable development policy. Central to this is a better understanding of triggers. There is a long‐standing assumption that earthquakes are a major GLOF trigger, and seismic activity is consistently included as a key hazard assessment criterion. Here, we provide the first empirical evidence that this assumption is largely incorrect. Focusing on the Tropical Andes, we show that, of 59 earthquakes (1900–2021) the effects of which intersect with known glacier lakes, only one has triggered GLOFs. We argue that, to help develop climate resilient protocols, the focus for future assessments should be on understanding other key GLOF drivers, such as thawing permafrost and underlying structural geology. Plain Language Summary: Climate change is increasing glacier melt in high mountains and increasing the size and number of glacial lakes. Over time, these lakes may drain catastrophically to generate glacier lake outburst floods (GLOFs), which can devastate downstream communities and destroy valuable infrastructure such as roads, bridges, and hydroelectric power facilities. As a result, many risk assessments have been carried out to understand what the triggers of GLOFs might be, in order to better predict their occurrence. One of the main triggers has been assumed to be earthquakes, but this association has not been properly tested. In this paper, we use earthquake and GLOF data from the Peruvian and Bolivian Andes to test this and find that there is little association between earthquakes and GLOFs. We conclude by arguing that focus needs to be on other GLOF triggers, and earthquake activity should be used as a secondary – not primary – indicator in GLOF susceptibility studies. Key Points: Glacier lake outburst floods (GLOFs) are a major risk concern, with earthquakes being commonly considered effective triggers in hazard assessmentsOf 59 earthquakes (1900–2021), the effects of which intersect lakes in the Tropical Andes, only one triggered GLOFsWe suggest that earthquake activity should be used as a secondary – not primary – susceptibility indicator for GLOFs [ABSTRACT FROM AUTHOR]