We present new and previously unreported in-situ observations of Hertz frequency multi-harmonic mode field line resonances detected by the Electric Field and Waves (EFW) instrument on-board the NASA Van Allen Probes during low-L perigee passes. Spectral analysis of the spin plane electric field data reveals the waves in numerous perigee passes, in sequential passes of Probes A and B, and with harmonic frequency structures from ~0.5 to 3.5 Hz which vary with L-shell, altitude, and from day to day. Comparing the observations to wave models using plasma mass density values along the field line given by empirical power laws and from the International Reference Ionosphere model (IRI), we conclude the waves are standing Alfvén field line resonances, and that only odd-mode harmonics are excited. The model eigenfrequencies are strongly controlled by the density close to the apex of the field line, suggesting a new diagnostic for equatorial ionospheric density dynamics.

Various indices have been defined to characterize the phase and amplitude of the Madden-Julian oscillation (MJO). One widely used index is the Outgoing Longwave Radiation (OLR) based MJO index (OMI), which is calculated using the spatial pattern of 30-96-day eastward-filtered OLR. The EOFs used to calculate the OMI in observations are prone to degeneracy and exhibit oscillations on the order of 10-20 days, despite initial filtering of the OLR. We propose a simple modification to the OMI that involves aligning the EOFs between neighboring days while retaining the spatial pattern described by the EOFs. This rotation method is implemented as a postprocessing step of the current OMI calculation and cleanly removes the spurious oscillations and degeneracy issues seen in the standard method. A similar rotation procedure can be implemented in calculations of other MJO indices.

We introduce a minimal stochastic lattice model for the column relative humidity ($R$) in the tropics, which incorporates convective moistening, lateral mixing and subsidence drying. The probability of convection occurring in a location increases with $R$, based on TRMM observations, providing a positive feedback that could lead to aggregation. We show that the simple model reproduces many aspects of full-physics cloud resolving model experiments. Depending on model parameter settings and domain size and resolution choices, it can produce both random or aggregated equilibrium states. Clustering occurs more readily with larger domains and coarser resolutions, in agreement with full physics models. Using dimensional arguments and fits from empirical data, we derive a dimensionless parameter we call the aggregation number, $N_{ag}$, that predicts whether a specific model and experiment setup will result in an aggregated or random state. The parameter includes the moistening feedback strength, the diffusion, the subsidence timescale, the domain size and spatial resolution. Using large ensembles of experiments, we show that the transition between random and aggregated states occurs at a critical value of $N_{ag}$. We argue that $N_{ag}$ could help to understand the differences in aggregation states between full physics, cloud resolving models.

We use a Landscape Evolution Model (FastScape S2S) to explore the impact of inherited topography in the foreland domain of a rising mountain range on its stratigraphic architecture and sediment accumulation history, inspired by the northern Pyrenean foreland. We simulate an uplifting half mountain range, its foreland basin and forebulge, and beyond, an open marine domain. We ran models with 4 different initial reliefs in the foreland domain: an initially flat foreland domain at sea-level, an elevated flat continental foreland (+300 m), a pre-existing 1 km-deep and 100 km-wide bathymetry at the location of the future foreland basin associated with a forebulge domain either at sea-level or elevated at +300m. All models show a prograding mega-sequence associated with building of mountain topography and development of the flexural foreland basin and forebulge, coalescence of alluvial fans at the foot of the range, progressive continentalization of the foreland domain, and burial of the forebulge. An initially elevated foreland domain ultimately produces a thinner foreland basin while an initially deep foreland basin produces a thicker one. After 10-13 Myr, the initial relief of foreland domain is smoothed out and the landscape does not exhibit a record of pre-existing relief. In contrast, the stratigraphic architecture of the foreland basin allows to trace inherited relief with deep marine sediments in the initially deep foreland basin, marine sediments onlapping and then burying the forebulge initially at sea-level, and continental sediments onlapping and burying the initially elevated foreland domain. We compare these interpretations to the Pyrenean retro-foreland.

The significant impact of soil structure on soil hydraulic properties and then on the associated water and solute transport is well recognized. However, existing soil hydraulic models that account for the effect of soil structure are often at the cost of overparameterization, hindering further application on large scales. In this study, we developed a new model that considers the effect of soil structure in hydraulic conductivity prediction when introducing no new free parameters. Testing with 152 soil samples that include different soil types shows that the new model considerably improves the prediction of conductivity, with an average root-mean-square-value (RMSE) of 0.65 cm d−1. When applying the new model in fitting observations, the model showed excellent performance, with an RMSE of only 0.30 cm d−1 remaining. This new soil hydraulic model provides a simple and practical way to incorporate the influence of soil structure in water and solute transport simulations.

In this paper, we introduce a testbed for evaluating and comparing climate modeling systems at cloud resolving scales using hindcasts of the June 2012 North American derecho. The testbed is applied to two models: the regionally-refined Simple Cloud-Resolving E3SM Atmosphere Model (SCREAM) at horizontal resolutions ranging from 6.5 to 1.625 km and the Weather Research and Forecasting (WRF) model with 4 km grid spacing. We find the simulation results to be highly sensitive to the initial conditions, initialization time, and model configurations, with initial conditions from the Rapid Refresh (RAP) producing the best simulation. Significant improvement is identified in the SCREAM simulations as horizontal grid spacing is refined. While a propagation delay of approximately 2 hours is found in both models, SCREAM at 1.625 km simulates the observed bow echo structure of the derecho well and predicts strong surface gusts that exceed 30 m/s. In comparison, WRF hardly produces surface wind over 25 m/s, and the derecho wind gust in WRF is 42-46% lower than in SCREAM. Moreover, WRF has a lower bias in simulating cold clouds but overestimates the precipitation intensity. Both models well reproduce the observed outgoing longwave radiation spatial patterns (Pearson correlation > 0.88) while they simulate larger areas of composite radar reflectivity > 40 dBZ by up to 4 times and underestimate the precipitating area by ~ 70\% in WRF and 47\% in SCREAM compared to observations.

Megathrust earthquakes with a wide along-dip rupture extent show clearly depth-dependent variations in rupture characteristics such as rupture velocity, frequency contents of seismic radiation and slip distribution. Some recent studies propose that heterogeneous upper-plate rigidity determines this phenomenon, though along-dip variations in fault friction have long been thought to play a dominant role. In this study, we use dynamic rupture modeling to explore and compare roles of these two factors in depth-dependent rupture characteristics of megathrust earthquakes along a shallow-dipping subduction plane that is governed by the rate- and state- dependent friction. We find that an updip transition from velocity-weakening behavior downdip to velocity-strengthening behavior near the trench suppresses rupture propagation toward the trench and a thicker transition zone results in a more confined slip at depth. The updip transition in velocity-dependent frictional property also dominates high-frequency depletion in seismic radiation at shallow depth. With an addition of a conditionally stable zone at shallow depth, rupture velocity significantly decreases, resulting in longer rupture duration as the thickness of the conditionally stable zone increases. The low-velocity layers in the upper plate at shallow depth lead to a more compliant prism and thus significantly higher total slip near the trench. Although they place some limits to rupture velocity at shallow depth, they enhance high-frequency radiation and thus do not contribute to high-frequency depletion observed in recent megathrust earthquakes. We conclude that fault friction plays more important roles than upper-plate rigidity in determining depth-dependent rupture characteristics of megathrust earthquakes.

Extratropical cyclones are major contributors to consequential weather in the mid-latitudes and tend to develop in regions of enhanced cyclogenesis and progress along climatological storm tracks. Numerous studies have noted the influence that terrestrial snow cover exerts on atmospheric baroclinicity which is critical to the formation and trajectories of such cyclones. Fewer studies have examined the explicit role which continental snow cover extent has in determining cyclones intensities, trajectories, and precipitation characteristics. While several examinations of climate model projections have generally shown a poleward shift in storm tracks by the late 21st century, none have determined the degree to which the coincident poleward shift in snow extent is responsible. A method of imposing 10th , 50th , and 90th percentile values of snow retreat between the late 20th and 21st centuries as projected by 14 Coupled Model Intercomparison Project Phase Five (CMIP5) models is used to alter 20 historical cold season cyclones which tracked over or adjacent to the North American Great Plains. Simulations by the Advanced Research version of the Weather Research and Forecast Model (WRF-ARW) are initialized at 0 to 4 days prior to cyclogenesis. Cyclone trajectories and their central sea level pressure did not change substantially, but followed consistent spatial trends. Near-surface wind speed generally increased, as did precipitation with preferred phase change from solid to liquid state. Cyclone-associated precipitation often shifted poleward as snow was removed. Variable responses were dependent on the month in which cyclones occurred, with stronger responses in the midwinter than the shoulder months.

Tsunami earthquakes are a type of shallow subduction zone events that rupture slowly (<1.5 km/s) with exceptionally long duration and depleted high frequency radiation, resulting in a large discrepancy of Mw and Ms magnitudes and abnormally large tsunami along coastal areas. Heterogeneous fault frictional properties at shallow depth have been thought to dominate tsunami earthquake generation. Some recent studies propose heterogeneous upper-plate material properties determine rupture behavior of megathrust earthquakes, including characteristics of tsunami earthquakes. In this study, we use a recently developed dynamic earthquake simulator to explore tsunami earthquake generation and systematically examine roles of upper-plate material properties and fault frictional properties in tsunami earthquake characteristics in a physics-based framework. For heterogeneous fault friction, we consider isolated asperities with strongly velocity-weakening properties embedded in a conditionally stable zone with weakly velocity-weakening properties. For heterogeneous upper-plate properties, we consider a generic depth profile of seismic velocity and rigidity constrained from seismic surveys. We design a set of models to explore their effects on tsunami earthquake generation and characteristics. We find that the conditionally stable zone can significantly slow down rupture speeds of earthquakes that nucleate on asperities to be < 1.5 km/s over a large depth range (1-20 km), while heterogeneous upper-plate properties can only reduce rupture speeds to be ~1.5-2.0 km/s over a narrow depth range (1-3km). Nevertheless, heterogeneous upper-plate properties promote cascading rupture over multiple isolated asperities on the shallow subduction plane, contributing to large tsunami earthquake generation. We also find that heterogeneous friction dominates normalized duration and high-frequency depletion in tsunami earthquakes. In addition, the effective normal stress on the subduction plane, which affects fault frictional strength, significantly influences the characteristics of tsunami earthquakes, including long normalized duration and low stress drop.

In late June 2021, western North America, and in particular the Pacific Northwest experienced temperatures usually associated with hot desert climates. Using a blend of reanalysis data and Earth System Model (ESM) simulations, we disentangle the physical drivers underlying this exceptional event. A recent investigation has revealed the aggravating effect of human-induced climate change, while another study examined the dynamics behind the strong ‘Omega Block’. Nevertheless, both drivers cannot fully explain how the extreme heat was reached. Our analysis highlights the role of the anticyclonic circulation aloft, which converted previously gained potential energy — some of which by intense latent heating thousands of kilometers upwind over the North Pacific — back into hot air through subsidence. We demonstrate that this upwind latent heat release not only resulted in a hot troposphere above the heatwave region, but also contributed directly to escalating near-surface temperatures. Facilitated by the mountainous terrain and dry soils in the region, deep atmospheric boundary layers were established over the course of several days, connecting the air close to Earth’s surface to a massive heat reservoir many kilometers above. Overall, we consider this mega-heatwave the outcome of an intricate interplay between dynamic and thermodynamic processes. Nevertheless, our ESM experiments suggest that the same large-scale atmospheric circulation — fueled by thermodynamic drivers such as more available moisture for condensation upwind — could enable even more extreme near-surface temperatures. We identify regions prone to experience events with similar characteristics, and discuss the implications of our findings with increasing global warming.

The accurate simulation of oceanic turbulence is crucial to understanding global ocean circulation, impacting accurate prediction of global warming effects and national security problems. Nonetheless, high fidelity simulations are difficult due to the ocean’s complex physics and broad range of spatial and temporal scales. These range from the smallest dissipative scales with typical lengths of millimeters, to the largest energetic mesoscale eddies with characteristic wavelengths exceeding tens of kilometers. Whereas the accurate parameterization of all flow scales with the Reynolds-averaged Navier-Stokes equations (RANS) is nearly impossible, resolving all scales of turbulence through a direct numerical simulation (DNS) is beyond the capabilities of the most powerful supercomputers in the foreseeable future. Hence, efficient parameterizations are needed. In this work, we extend the bridging partially-averaged Navier-Stokes equations (PANS) model to ocean flows. This parameterization operates between RANS and DNS, and aims only to resolve the scales not amenable to modeling in order to increase the efficiency of ocean computations. We also propose a PANS scale-aware closure to model the unresolved scales. The initial validation tests of the new PANS model include two representative ocean channel flows. The results confirm the potential of the new parameterization to predict oceanographically relevant turbulence efficiently.

The article describes a new practical model for the infrared absorption of chlorofluorocarbons and other gases with dense spectra, based on HITRAN absorption cross-sections. The model is very simple, consisting of frequency-dependent polynomial coefficients describing the pressure and temperature dependence of absorption. Currently it is implemented for the halocarbon species required by the Radiative Forcing Model Intercomparison Project (RFMIP). This approach offers practical advantages compared to previously available options, and is traceable, since the polynomial coefficients follow directly from the laboratory spectra. The model is applied to the problem of computing instantaneous clear-sky halocarbon radiative efficiencies and present day radiative forcing. Results are in reasonable agreement with earlier assessments that were carried out with the less explicit Pinnock method, and thus broadly validate that method. Overall, halocarbons are responsible for a substantial share of the present-day forcing, 0.573 Watt/squaremeter (instantaneous clear-sky at the TOA), corresponding to approximately 20% of the total anthropogenic forcing, or 44% compared to anthropogenic CO2 forcing alone.

We present a new magnetic map integrating continental and oceanic features at Santos Basin, in order to investigate the connection between onshore and offshore tectonics and magmatism at the early stages of rifting. The magnetic and seismic data evidence the offshore continuity of the Florianópolis and Serra do Mar dyke swarms while the Ponta Grossa Dyke Swarm was discontinued by the marginal structures. This brings to evidence a different timing of formation of the latter relative to the early opening of the margin. A very high amplitude magnetic lineament - the Santos Marginal Anomaly (SAMA) reaches up to 1500 nT and trend NE-SW, with local E-W inflections forming the Cananéia and Guaratiba Arcuate Anomalies. Our modeling suggests that SAMA can be associated with dykes and syn-rift extrusives linked at depth to larger magmatic bodies at the proximal and necking domains. Based on the increasing width and amplitude of the magnetic anomalies oceanwards, we propose a mechanism where the igneous accretion augmented as rifting evolved from continent to ocean, creating a volcanic rift axis. This rift was segmented by crustal discontinuities that probably nucleated on preexisting rheological contrasts, forming tectono-magmatic segments during the early extensional stages. The onshore-offshore relationships highlight that where there was an agreement between basement fabric and dyke orientation, inheritance may have influenced their emplacement, although it was not a limiting factor for their formation. The regional geodynamics of the Gondwana opening was the first order control, with deviations of the magmatically influenced rift by inheritance.

Density staircases are observed in an idealised model of a deep western boundary current upon crossing the equator. We propose that the staircases are generated by the excitement of symmetric instability as the current crosses the equator. The latitude at which symmetric instability is excited can be predicted using simple scaling arguments. Symmetric instability generates overturning cells which, in turn, cause the inhomogenous mixing of waters with different densities. The mixing barriers and well mixed regions in density profiles coincide, respectively, with the boundaries and centres of the overturning cells generated by the symmetric instability. This new mechanism for producing density staircases may require us to re-evaluate the origins of some of the density staircases observed in the Tropical Atlantic.

Intensity-Duration-Frequency curves are useful in water resources engineering for the planning and design of hydrological structures. As opposed to the common use of only extreme data to build IDF curves, here, we use all the non-zero rainfall intensities, thereby making efficient use of the available information. As a parametric model, we use the Extended Generalized Pareto Distribution (EGPD) for the non-zero intensities. We consider three commonly used approaches to build the IDF curves. The first approach is based on the scale-invariance property of rainfall, the second relies on the general IDF formulation of Koutsoyiannis et al. (1998) while the last approach is purely data-driven (Overeem et al., 2008). Using these three approaches, and some extensions around them, we build a total of 10 models for the IDF curves and then we compare them in a split-sampling cross-validation framework. We consider a total of 81 stations at 10 min resolution in Switzerland. The results reveal the model based on the data-driven approach as the best model. It is able to correctly model the observed intensities across duration while being reliable and robust. It is also able to reproduce the space and time variability of extreme rainfall across Switzerland.

The objective of this study is to evaluate the potential for History Matching (HM) to tune a climate system with multi-scale dynamics. By considering a toy climate model, namely, the two-scale Lorenz96 model and producing experiments in perfect-model setting, we explore in detail how several built-in choices need to be carefully tested. We also demonstrate the importance of introducing physical expertise in the range of parameters, a priori to running HM. Finally we revisit a classical procedure in climate model tuning, that consists of tuning the slow and fast components separately. By doing so in the Lorenz96 model, we illustrate the non-uniqueness of plausible parameters and highlight the specificity of metrics emerging from the coupling. This paper contributes also to bridging the communities of uncertainty quantification, machine learning and climate modeling, by making connections between the terms used by each community for the same concept and presenting promising collaboration avenues that would benefit climate modeling research.

We investigate sharp changes in magnetic field that can produce Geomagnetically Induced Currents (GICs) which damage pipelines and power grids. We use one-minute cadence SuperMAG observations to find the occurrence distribution of magnetic field “spikes”. Recent studies have determined recurrence statistics for extreme events and charted the local time distribution of spikes; however, their relation to solar activity and conditions in the solar wind is poorly understood. We study spike occurrence during solar cycles 23 and 24, roughly 1995 to 2020. We find three local time hotspots in occurrence: the pre-midnight region associated with substorm onsets, the dawn sector associated with omega band activity, and the pre-noon sector associated with the Kelvin-Helmholtz instability occurring at the magnetopause. Magnetic field perturbations are mainly North-South for substorms and KHI, and East-West for omega bands. Substorm spikes occur at all phases of the solar cycle, but maximise in the declining phase. Omega-band and KHI spikes are confined to solar maximum and the declining phase. Substorm spikes occur during moderate solar wind driving, omega band spikes during strong driving, and KHI spikes during quiet conditions but with high solar wind speed. We show that the shapes of these distributions do not depend on the magnitude of the spikes, so it appears that our results can be extrapolated to extreme events.

This study aims to create a 21-year, high spatiotemporal resolution Global Satellite Mapping of Precipitation (GSMaP) rainfall product adjusted by rain gauge measurements over the Indian mainland. The targeted resolutions of the GSMaP are hourly and 0.1°× 0.1°. The National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC) daily gauge analysis (0.5° × 0.5°) and Indian Meteorological Department (IMD) daily gridded rainfall product (0.25° × 0.25°) were utilized to generate two long-term rainfall products, GSMaP_CPC and GSMaP_IMD rainfall, respectively. After preliminary verification of the GSMaP_CPC and GSMaP_IMD rainfalls with IMD gauges, these rainfall products are evaluated for the Indian Summer Monsoon (ISM) periods of 2000–2020 with comparisons of other merged rainfall products such as the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (IMERG). The results suggest GSMaP_IMD has a smaller root-mean-square difference (RMSD) and higher correlation than GSMaP_CPC, evaluated against independent rainfall products. In the three-hour mean analysis with spaceborne precipitation radar data, it is found that the value of RMSD decreases in GSMaP_IMD with respect to GSMaP_CPC throughout the day. The statistics against the hourly dense rain gauge network in Karnataka suggests that the GSMaP_IMD is more effective in capturing large spatiotemporal rainfall variation over India. Thus, validation results with the independent sources suggest that GSMaP_IMD rainfall generally improved over GSMaP_CPC rainfall. These improvements are significant in orographic regions with high rainfall amounts, mainly the western Ghats and northeastern parts of India.