ALBERT

Description: Abstract The Ultralong‐Wavelength (ULW) regime of longer than 10 m (corresponding to frequencies below 30 MHz) remains as the last virtually unexplored window in radio astronomy and is presently attracting considerable attention as an area of potentially rewarding studies. However, the opaqueness of the Earth's ionosphere makes the ULW celestial radio emission very difficult to detect with ground‐based instrumentation. The impact of the ionosphere on ULW radio emission depends on the solar cycle activities and varies with time. In addition, the ULW spectrum region is densely populated by intensive artificial radio frequency interference. An obvious solution of these problems is to place an ULW radio telescope in space. However, this solution is expensive and poses nonnegligible technological challenges. An alternative approach is triggered by recent studies showing that the period of post 2020 will be most suitable for exploratory ground‐based ULW radio observations due to the expected “calm” state of the ionosphere; the ionospheric cutoff frequency could be well below 10 MHz, even in the day time. In anticipation of this upcoming opportunity, we propose and present in this paper a concept of an experimental ULW radio array, with the intention of setting it up in Inner Mongolia, China. This ULW facility will use the infrastructure of the currently operational Mingantu spectral radio heliograph. The proposed ULW array covers the frequency range from 1 to 72 MHz. This experimental array will be used for exploratory studies of celestial radio emission in the ULW range of the spectrum.

Description: Abstract Dust devils are convective vortices with a vertical axis of rotation made visible by lifted soil particles. Currently, there is great uncertainty about the extent to which dust devils contribute to the atmospheric aerosol input and thereby influence Earth's radiation budget. Past efforts to quantify the aerosol transport and study their formation, maintenance, and statistics using large‐eddy simulation (LES) have been of limited success. Therefore, some important features of dust devil‐like vortices simulated with LES still do not compare well with those of observed ones. One major difference is the simulated value of the core pressure drop, which is almost 1 order of magnitude smaller compared to the observed range of 250 to 450 Pa. However, most of the existing numerical simulations are based on highly idealized setups and coarse grid spacings. In this study, we investigate the effects of various factors on the simulated vortex strength with high‐resolution LES. For the fist time, we are able to reproduce observed core pressures by using a high spatial resolution of 2 m, a model setup with moderate background wind and a spatially heterogeneous surface heat flux. It is found that vortices mainly appear at the lines of horizontal flow convergence above the centers of the strongly heated patches, which is in contrast to some older observations in which vortices seemed to be created along the patch edges.

Description: Abstract Location information from super‐pressure balloons flown by Project Loon provide an unprecedented opportunity to analyze wind fields in the mid‐latitude stratosphere. Horizontal velocity spectra from the balloons' quasi‐intrinsic frame of reference show clear evidence of a persistent peak in the intrinsic wind spectrum around the inertial frequency. In the Southern Hemisphere mid‐latitudes, peak‐to‐peak amplitudes of horizontal velocity perturbations (on the order of 20 ms‐1) are larger than those seen in previous super‐pressure balloon campaigns in polar regions and similar to those observed in vertical soundings in the mid‐latitudes. A rotary spectral analysis shows that near‐circular anti‐cyclonic rotation of horizontal wind perturbations around the inertial frequency dominate at most times and locations. The strongest anti‐cyclonic rotation is more common in balloon flight segments with weak zonal winds and during the austral summer. Flight segments with strong eastward zonal velocities during austral winter and spring are more likely to have mixed cyclonic and anti‐cyclonic power around the inertial frequency. These results confirm previous model and radiosonde observations of the peak in horizontal kinetic energy at the inertial frequency, and demonstrate they are associated with increased anti‐cyclonic wave power indicative of near‐inertial oscillations or inertia‐gravity waves. Flight segments with mixed cyclonic and anti‐cyclonic power around the inertial frequency display a continuum of wave power from planetary to gravity‐wave scales. These results help explain the divergence of actual and modelled balloon trajectories in previous studies and provide a baseline against which reanalysis or meteorological model realizations of the intrinsic velocity field can be assessed.

Description: Abstract From 17–22 August 2017 simultaneous enhancements of ammonia (NH3), carbon monoxide (CO), hydrogen cyanide (HCN), and ethane (C2H6) were detected from ground‐based solar absorption Fourier transform infrared (FTIR) spectroscopic measurements at two high‐Arctic sites: Eureka (80.05°N, 86.42°W) Nunavut, Canada, and Thule (76.53°N, 68.74°W), Greenland. These enhancements were attributed to wildfires in British Columbia and the Northwest Territories of Canada using FLEXPART back‐trajectories and fire locations from Moderate Resolution Imaging Spectroradiometer (MODIS) and found to be the greatest observed enhancements in more than a decade of measurements at Eureka (2006–2017) and Thule (1999–2017). Observations of gas‐phase NH3 from these wildfires illustrate that boreal wildfires may be a considerable episodic source of NH3 in the summertime high Arctic. Comparisons of GEOS‐Chem model simulations using the Global Fire Assimilation System (GFASv1.2) biomass burning emissions to FTIR measurements and Infrared Atmospheric Sounding Interferometer (IASI) measurements showed that the transport of wildfire emissions to the Arctic was underestimated in GEOS‐Chem. However, GEOS‐Chem simulations showed that these wildfires contributed to surface layer NH3 and NH enhancements of 0.01–0.11 ppbv and 0.05–1.07 ppbv, respectively, over the Canadian Archipelago from 15–23 August 2017.

Description: Abstract Accurate estimates of NOx and SO2 emissions are important for air quality modeling and management. To incorporate chemical interactions of the two species in emission estimates, we develop a joint hybrid inversion framework to estimate their emissions in China and India (2005–2012). Pseudo observation tests and posterior evaluation with surface measurements demonstrate that joint assimilation of SO2 and NO2 can provide more accurate constraints on emissions than single‐species inversions. This occurs through synergistic change of O3 and OH concentrations, particularly in conditions where satellite retrievals of the species being optimized have large uncertainties. The percentage changes of joint posterior emissions from the single‐species posterior emissions go up to 242% at grid scales, although the national average of monthly emissions, seasonality, and interannual variations are similar. In China and India, the annual budget of joint posterior SO2 emissions is lower, but joint NOx posterior emissions are higher, because NOx emissions increase to increase SO2 concentration and better match Ozone Monitoring Instrument SO2 observations in high‐NOx regions. Joint SO2 posterior emissions decrease by 16.5% from 2008 to 2012, while NOx posterior emissions increase by 24.9% from 2005 to 2011 in China—trends which are consistent with the MEIC inventory. Joint NOx and SO2 posterior emissions in India increase by 15.9% and 19.2% from 2005 to 2012, smaller than the 59.9% and 76.2% growth rate using anthropogenic emissions from EDGARv4.3.2. This work shows the benefit and limitation of joint assimilation in emission estimates and provides an efficient framework to perform the inversion.

Description: Abstract By using 90 radiosonde stations with high vertical resolution data during the period 1998–2011, the latitudinal variation of the tropopause inversion layer (TIL) in different seasons and the interactions with the inertial gravity wave (IGW) activities in the region covering the Northern Hemispheric latitudes from 5° to 75° are studied. For the midlatitudes, the TIL features show obviously seasonal variations. In the Arctic region, TIL is strong and thick. The averaged Arctic TIL intensity peaks in summer. The intense interaction between the TIL and IGW is found in the region of 5°N to 75°N. The TIL could inhibit the upward propagation of IGWs from ~2 km below the tropopause in a larger region (40–75°N). It is found that for the middle‐latitude regions, the enhanced wind shear layer just above the tropopause could lead to instability and finally result in IGW breaking and intensive turbulence, which then leads to strong wave energy dissipation and a downward heat flux. The IGW‐induced cooling around the tropopause, which resulted from the downward heat flux, then makes a colder and sharper tropopause and finally form the TIL. The IGW‐associated strong downward heat flux is also found around the Arctic tropopause. However, there is no corresponding wind shear enhancement above the tropopause. This indicates that this strong heat flux may result from some other processes and then form the strong TIL in the Arctic.

Description: Abstract Proxy system models (PSMs) are an important bridge between climate simulations and climate records prior to the period where instrumental observations are available. PSMs help to interpret what proxies show and how they record climate. Although previous studies have evaluated PSMs for individual sites, their systematic evaluation on a global scale has not yet been conducted. This study evaluated the performance of PSMs for stable water isotopes in ice cores, corals, and tree‐ring cellulose for the period 1950–2007. Spatial distributions of the mean state were well simulated for all proxy types, albeit with a bias for tree‐ring cellulose. Interannual variability was well simulated for corals and tree‐ring cellulose. These results indicate that the models represent key mechanisms for the proxies. In contrast, the reproducibility of interannual variability in ice cores was markedly lower than that for the other proxies. Although the reproducibility was limited by the atmospheric forcing used to drive the model, the results suggest that the PSM may be missing post‐depositional processes, such as sublimation for ice cores on the interannual timescale.

Description: Abstract Three years of nighttime Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation data was used in synergy with CloudSat measurements to quantify how strongly aerosol type and aerosol load affect the cloud phase in low‐level clouds over the Arctic. Supercooled liquid layers were present in the majority of observed low‐level clouds (0.75 ≤ z ≤ 3.5 km) between −10 and −25 °C. Furthermore, based on the subset (6%) of data with high quality assurance for aerosol typing, ice formation is more common in the presence of dust or continental aerosols as opposed to marine or elevated smoke aerosols. With the first aerosol group, glaciated clouds were found at cloud top temperatures of 2 to 4 °C warmer than with the latter aerosol types. Further association of the aerosol concentration with the cloud phase showed that the aerosol concentration outweighs the aerosol type effect. Depending on the aerosol load, the temperature at which a cloud completely glaciates can vary by up to 6–10 °C. However, this behavior was most pronounced in stable atmospheric conditions and absent over open ocean with lower tropospheric stability values and probably less stratified clouds. Finally, more mixed‐phase clouds were associated with high aerosol load, suggesting that mixed‐phase clouds have an extended lifetime in the Arctic under high cloud condensation nuclei concentrations. This implies that in a pristine environment, with few or no local aerosol sources, and under the investigated conditions the amount of aerosol particles affects the cloud phase more than the aerosol type does.

Description: Abstract A series of five realistic, nested, hydrostatic numerical ocean model simulations are used to study semidiurnal internal tide generation and propagation from the continental slope, through the shelf break and to the midshelf adjacent to Point Sal, CA. The statistics of modeled temperature and horizontal velocity fluctuations are compared to midshelf observations (30‐ to 50‐m water depth). Time‐ and frequency‐domain methods are used to decompose internal tides into components that are coherent and incoherent with the barotropic tide, and the incoherence fraction is 0.5–0.7 at the midshelf locations in both the realistic model and observations. In contrast, the incoherence fraction is at the most 0.45 for a simulation with idealized stratification, and neither atmospheric forcing nor mesoscale currents. Negligible conversion from barotropic to baroclinic energy occurs at the local shelf break. Instead, the dominant internal tide energy sources are regions of small‐scale near‐critical to supercritical bathymetry on the Santa Lucia escarpment (1,000–3,000 m), 70–80 km from the continental shelf. Near the generation region, semidiurnal baroclinic energy is primarily coherent and rapidly decays adjacent to the shelf break. In the realistically forced model, incoherent energy is less than 10% in the generation region, with a steady increase in incoherence fraction from the continental slope to the midshelf. Backward ray tracing from the midshelf to the Santa Lucia escarpment identifies multiple energy pathways potentially leading to spatial interference. As internal tides shoal on the predominantly subcritical slope/shelf system, temporally variable stratification and Doppler shifting from mesoscale and submesoscale features appear equally important in leading to the loss of coherence.

Description: Abstract Aerosol pH is a useful diagnostic of aerosol chemistry for formation of secondary aerosol and has been hypothesized to be a key factor in specific chemical reaction routes producing sulfate and nitrate (Yue et al., 2009; Zhang et al., 2012; Hu et al., 2014). In this study, we measured hourly concentrations of water soluble ions (WS‐ions) in PM2.5, along with gaseous pollutants in Tianjin, China, from 4th January to 31st January 2015. The following source contributions to WS ions were estimated by PMF (Positive Matrix Factorization): secondary sulfate (13%), secondary nitrate (44%), coal (14%), vehicle (16%), and dust (13%). ISORROPIA‐II was used to investigate the complex relationships among aerosol pH, ammonia, and secondary aerosol formation. The estimated hourly aerosol pH varied from ‐0.3 to 7.7, with an average of 4.9 (±0.78); the median value was 4.89, and the interquartile range (IQR) was 0.72. During less polluted conditions, aerosol pH ranged from less than 0 to about 7; during heavily polluted conditions, pH was close to 5 (3.9‐7.9) despite large amounts of sulfate. Sufficient ammonia/ammonium were present to balance high sulfate and nitrate formation. NH4+/NH3 (g) helped stabilize pH while nonvolatile cations contributed less to decreasing aerosol acidity. High acidy (pH〈3), light pollution (Total water soluble ions (TWI)〈30 μg m‐3), and low water content (less than 5μg m‐3) were more correlated with higher rates of sulfate formation than nitrate formation in the winter.

Description: Abstract Understanding the mechanisms by which earthquake cycles produce folding and accommodate shortening is essential to quantify the seismic potential of active faults and integrate aseismic slip within our understanding of the physical mechanisms of the long‐term deformation. However, measuring such small deformation signals in mountainous areas is challenging with current space‐geodesy techniques, due to the low rates of motion relative to the amplitude of the noise. Here we successfully carry out a multitemporal Interferometric Synthetic Aperture Radar analysis over the North Qaidam fold‐thrust system in NE Tibet, where eight Mw〉 5.2 earthquakes occurred between 2003 and 2009. We report various cases of aseismic slip uplifting the thickened crust at short wavelengths. We provide a rare example of a steep, shallow, 13‐km‐long and 6‐km‐wide afterslip signal that coincides spatially with an anticline and that continues into 2011 in response to a Mw 6.3 event in 2003. We suggest that a buried seismic slip during the 2003 earthquake has triggered both plastic an‐elastic folding and aseismic slip on the shallow thrusts. We produce a first‐order two‐dimensional model of the postseismic surface displacements due to the 2003 earthquake and highlight a segmented slip on three fault patches that steepen approaching the surface. This study emphasizes the fundamental role of shallow aseismic slip in the long‐term and permanent deformation of thrusts and folds and the potential of Interferometric Synthetic Aperture Radar for detecting and characterizing the spatiotemporal behavior of aseismic slip over large mountainous regions.

Description: Abstract Structural details of the crust play an important role in controlling the distribution of volcanic activity in arc systems. In southwest Washington, several different regional structures associated with accretion and magmatism have been invoked to explain the broad distribution of Cascade volcanism in this region. In order to image these regional structures in the upper crust, Pg and Sg travel times from the imaging Magma Under St. Helens (iMUSH) active‐source seismic experiment are inverted for Vp, Vs, and Vp/Vs models in the region surrounding Mount St. Helens. Several features of these models provide new insights into the regional structure of the upper crust. A large section of the Southern Washington Cascades Conductor is imaged as a low Vp/Vs anomaly that is inferred to represent a broad sedimentary/metasedimentary sequence that composes the upper crust in this region. The accreted terrane Siletzia is imaged west of Mount St. Helens as north/south trending high Vp and Vp/Vs bodies. The Vp/Vs model shows relatively high Vp/Vs regions near Mount St. Helens and the Indian Heaven Volcanic Field, which could be related to the presence of magmatic fluids. Separating these two volcanic regions below 6‐km depth is a northeast trending series of high Vp and Vs bodies. These bodies have the same orientation as several volcanic/magmatic features at the surface, including Mount St. Helens and Mount Rainier, and it is argued that these high‐velocity features are a regional‐scale group of intrusive bodies associated with a crustal weak zone that focuses magma ascent.

Description: Abstract SO2 column densities from OMI provide important information on emission trends and missing sources, but there are discrepancies between different retrieval products. We employ three OMI SO2 retrieval products (NASA standard (SP), NASA prototype, and BIRA) to study the magnitude and trend of SO2 emissions. SO2 column densities from these retrievals are most consistent when viewing angles and solar zenith angles are small, suggesting more robust emission estimates in summer and at low latitudes. We then apply a hybrid 4D‐Var/mass balance emission inversion to derive monthly SO2 emissions from the NASA SP and BIRA products. Compared to HTAPv2 emissions in 2010, both posterior emission estimates are lower in US, India and Southeast China, but show different changes of emissions in North China Plain. The discrepancies between monthly NASA and BIRA posterior emissions in 2010 are less than or equal to 17% in China and 34% in India. SO2 emissions increase from 2005 to 2016 by 35% (NASA) ‐ 48% (BIRA) in India, but decrease in China by 23% (NASA) ‐ 33% (BIRA) since 2008. Compared to in‐situ measurements, the posterior GEOS‐Chem surface SO2 concentrations have reduced NMB in China, the US, and India but not in South Korea in 2010. BIRA posteriors have better consistency with the annual growth rate of surface SO2 measurement in China and spatial variability of SO2 concentration in China, South Korea and India, whereas NASA SP posteriors have better seasonality. These evaluations demonstrate the capability to recover SO2 emissions using OMI observations.

Description: Abstract Seismicity of several intraplate seismic zones in the North American midcontinent is believed to be related to reactivation of ancient faults in Precambrian continental rifts by the contemporary stress field. Existence of such a rift system beneath the Wabash Valley Seismic Zone (WVSZ) is not clear. Here we obtained a crustal structural image along a 300‐km‐long profile across WVSZ using a dense linear seismic array. We first calculated teleseismic receiver functions of stations and applied the Common‐Conversion‐Point stacking method to image crustal interfaces and the Moho. We then used ambient noise cross correlation to obtain phase and group velocities of Rayleigh and Love waves. Finally, we jointly inverted the receiver function and surface wave dispersion data to determine shear wave velocity structure along the profile. The results show a thick (50‐ to 60‐km) crust with a typical Proterozoic crustal layering: a 1‐ to 2‐km thick Phanerozoic sedimentary layer, an upper crust ∼15 km thick, and a 30‐ to 40‐km‐thick lower crust. The unprecedented high‐resolution image also reveals a 50‐km‐wide high‐velocity body above an uplifted Moho and several velocity anomalies in the upper and middle crust beneath the La Salle Deformation Belt. We interpreted them as features produced by magmatic intrusions in a failed, immature continental rift during the end of Precambrian. Current seismicity in WVSZ is likely due to reactivation of ancient faults of the rift system by a combination of stress fields from the far‐field plate motion and prominent crustal and upper mantle heterogeneities in the region.

Description: Abstract The Charlevoix Seismic Zone (CSZ) is located along the early Paleozoic St. Lawrence rift zone in southeastern Quebec at the location of a major Devonian impact structure. The impact structure superimposed major, steeply dipping basement faults trending approximately N35°E. Approximately 250 earthquakes are recorded each year and are concentrated within and beneath the impact structure. Most M4+ earthquakes associated with the rift faults occurred outside the impact structure. Apart from the unique distribution of earthquakes, stress inversion of focal mechanisms shows stress rotations within the CSZ, and in the CSZ relative to the stress orientation determined from borehole breakouts. The primary goal of this research is to investigate the combined effects of the preexisting structures and regional stresses on earthquake activity and stress rotations in the CSZ. We approach this using PyLith, a finite‐element code for simulations of crustal deformation. Adopting the results from recent hypocenter relocation and 3‐D tomography studies, we modify the locations and dips of the rift faults and assess the effect of the new fault geometries on stress distributions. We also discuss the effects of resolved velocity anomalies. We find that the observed stress rotation is due to the combined effect of the rift faults and the impact structure. One‐dimensional velocity models of the CSZ with an embedded impact structure and a combination of 65°‐40°‐40° and constant 70° fault dip models with a very low friction coefficient of 0.3 and cohesion of 0 MPa can explain the observed seismicity and more than 50% of the stress rotations.

Description: Abstract Seismic anisotropy provides important information on the structure and geodynamics of the Earth. The forearc mantle wedge in subduction zones mainly exhibits trench‐parallel azimuthal anisotropy globally, which is inconsistent with the model of olivine a axis aligning with the slab‐driven corner flow. Its formation mechanism is currently unclear. Here we present high‐resolution 3‐D P wave anisotropic tomography of the Tohoku subduction zone. We suggest that ductile deformation of the forearc lithospheric mantle of the overriding plate induces the trench‐parallel azimuthal anisotropy and positive radial anisotropy (i.e., horizontal velocity 〉 vertical velocity) in Tohoku. Our results provide the first seismic anisotropic evidence for the slab‐mantle decoupling at a common depth of ~70 km. On the basis of the high‐resolution seismic images, we propose a geodynamic model suggesting that the forearc mantle wedge anisotropy is produced via ductile deformation of dry olivine or hydrous antigorite lithospheric mantle, which accords well with the trench‐parallel shear wave splitting measurements dominant in subduction zones globally.

Description: Abstract We investigate 3‐D seismic structures (Vp, Vs, and Poisson's ratio) and Vp azimuthal anisotropy in the source area of the 2018 Eastern Iburi earthquake (M 6.7) in Hokkaido, Japan. Its mainshock occurred at the edge of a high‐Vp (2–4%) seismogenic zone. Significant low‐Vs (−1% to −3%) and high Poisson's ratio (2–7%) anomalies are imaged in and below the source zone and extend to the upper surface of the subducting Pacific slab, most likely reflecting ascending fluids released by the slab dehydration. A high consistency between the fault plane and the low‐Vs and high Poisson's ratio anomalies indicates that the fluids may have entered the fault and affected the rupture nucleation. A high‐V (1–3%) anomaly is revealed in the fore‐arc mantle wedge and connects with the high‐V seismogenic zone, probably reflecting a lithospheric fragment and contributing to cool down the mantle wedge. Complex seismic anisotropy is revealed in the crust in and around the source area, which may reflect complicated stress regime and strong structural heterogeneities there.

Description: Abstract Deep convective clouds similar to those arising in the TC eyewall are simulated using a parcel model and 2D slab symmetric cloud model with spectral bin microphysics (the Hebrew University Cloud Model, HUCM). The size distribution of sea spray particles (SSP) at cloud base is calculated using the Lagrangian‐Eulerian bin‐microphysics model (LEM). The model describes the SSP production, advection and formation of the size distribution of SSP in the hurricane atmospheric boundary layer at different strong wind speeds. The SSP distributions calculated by the LEM are used in the parcel model and the HUCM to investigate the microphysical and dynamical effects of SSP on clouds. The SSP ascending in cloud updrafts dramatically increase the number concentration of cloud drops within a wide range of drop sizes. As a result, sea spray creates clouds with unique property combinations of both maritime and continental types. These clouds have droplet size distributions characterized by a high drop concentration and a low effective radius, as in continental clouds. At the same time, the presence of SSP of a few hundred microns in radii triggers intense rain just above the cloud base, which is typical of extreme maritime clouds. In the presence of large sea spray drops, the smallest cloud condensational nuclei, including the smallest SSP, are activated, giving rise to the permanent in‐cloud nucleation of small droplets which produce a high concentration of small ice crystals above the level of homogeneous freezing. We showed that the SSP substantially increased the maximum vertical velocity, cloud water content and mass contents of ice particles. The results are compared with available observed data.

Description: Abstract Terrestrial gamma ray flashes (TGFs) are very short bursts of gamma radiation associated to thunderstorm activity and are the manifestation of the highest‐energy natural particle acceleration phenomena occurring on Earth. Photon energies up to several tens of megaelectronvolts are expected, but the actual upper limit and high‐energy spectral shape are still open questions. Results published in 2011 by the AGILE team proposed a high‐energy component in TGF spectra extended up to ≈100 MeV, which is difficult to reconcile with the predictions from the Relativistic Runaway Electron Avalanche (RREA) mechanism at the basis of many TGF production models. Here we present a new set of TGFs detected by the AGILE satellite and associated to lightning measurements capable to solve this controversy. Detailed end‐to‐end Monte Carlo simulations and an improved understanding of the instrument performance under high‐flux conditions show that it is possible to explain the observed high‐energy counts by a standard RREA spectrum at the source, provided that the TGF is sufficiently bright and short. We investigate the possibility that single high‐energy counts may be the signature of a fine‐pulsed time structure of TGFs on time scales ≈4 μs, but we find no clear evidence for this. The presented data set and modeling results allow also for explaining the observed TGF distribution in the (Fluence × duration) parameter space and suggest that the AGILE TGF detection rate can almost be doubled.

Description: Abstract Deltaic deposits mapped along the martian crustal dichotomy boundary scarp have been suggested to delineate an ancient ocean in the northern lowlands of Mars. Using recently acquired orbital data, we have expanded the dichotomy delta inventory and performed an updated analysis of delta front elevations, a proxy for paleo‐water levels. Our analysis focused near Gale crater, home of the Curiosity rover. We found that delta front elevations vary by ca. 2400 meters, but these elevation variations do not correspond to modeled deformation from true polar wander or Tharsis. Locally, delta front elevations vary by ≤60 meters, and using present‐day topography, they correspond to distinct enclosed basins. We infer that these deltas formed in paleo‐lakes up to ca. 13,000 km2 and ca. 0.4 kilometers deep, perhaps coeval with paleo‐lakes in Gale. Our results suggest that a northern ocean is not needed to explain the deltaic deposits in the Gale crater region.

Description: Abstract On 3 January 1975, the largest shallow moonquake (MW 4.1) occurred at Laue impact crater on the Moon. The fault responsible for the moonquake and origins of coseismic boulder avalanches are unknown. Our study reveals a set of previously unreported, seismically active, young lobate scarps near the epicenter. In addition, hundreds of boulder falls are observed on the interior walls of two impact craters on either side of the lobate scarps. The varying preservation levels and crater size‐frequency distributions of impact craters superimposed on the boulder falls indicate their episodic origins at 1.6 Ma and during the 1975 shallow moonquake. Our ground motion simulations confirm that the MW 4.1 moonquake along the lobate scarp at 1‐ to 5‐km focal depths produced strong ground shaking that triggered the boulder avalanches. Also, the fault slip along the Lorentz basin wall beneath the Laue crater floor produced the lobate scarps and the shallow moonquakes.

Description: Abstract High‐speed video and electric field change data have been used to examine the initiation and propagation of 21 recoil leaders, 7 of which evolved into dart (or dart‐stepped) leaders (DLs) initiating return strokes and 14 were attempted leaders (ALs), in a Canton‐Tower upward flash. Three DLs and two ALs clearly exhibited bidirectional extension. Each DL was preceded by one or more ALs and initiated near the extremity of the positive end of the preceding AL. The positive end of each bidirectional DL generally appeared to be inactive (stationary) or intermittently propagated along the positive part of the preceding AL channel and extended into the virgin air. A sequence of two floating channel segments were formed ahead of the approaching positive end of one DL, causing its abrupt elongation.

Description: Abstract On 24 December 2018, a violent eruption started at Mount Etna from a fissure on the southeastern flank. The intrusive phenomenon, accompanied by intense Strombolian and lava fountain activity, an ash‐rich plume, and lava flows, was marked by significant ground deformation and seismicity. In this work, we show how an integrated investigation combining high‐rate GPS data, volcano‐tectonic earthquakes, volcanic tremor, infrasound tremor, and infrasound events allows tracking the magma intrusion phenomenon spatially and temporally with unprecedented resolution. Moreover, it enabled showing how the central magma column lowered as a response to the flank eruption and to constrain the zone of interaction between the dike and the central plumbing system at a depth of 2–4 km below sea level. This is important for understanding flank and summit interaction, suggesting that explosive summit activity may in some cases be driven by lateral dike intrusions.

Description: Abstract Arctic amplification (AA) is typically associated with Planck, lapse rate, and ice albedo feedbacks. However, the relative importance of poleward energy transport on AA remains uncertain. Here, we analyze integrations from a Chemistry Climate Model to investigate the impact of the Montreal Protocol on forcing, feedback, and transport contributions to AA. Two ensembles of future integrations are considered—one projecting decreasing ozone‐depleting substance concentrations and stratospheric ozone recovery and another assuming that ozone‐depleting substances are not regulated (the “World Avoided”). We find similar degrees of AA in both ensembles, despite a negative radiative forcing over the Arctic in the “World Avoided” from massive ozone loss. That negative radiative forcing is primarily balanced from positive atmospheric energy flux convergence and long‐wave cloud feedbacks. Our results highlight the impact of inhomogeneous radiative forcing on regional differences in forcing and feedback strength and the importance of radiative forcing meridional structure on poleward energy transport.

Description: Abstract We examine oceanic drivers of widespread droughts over the contiguous United States (herein pan‐CONUS droughts) during the Common Era in what is one of the first analyses of the new Paleo Hydrodynamics Data Assimilation (PHYDA) product. The canonical understanding of oceanic influences on North American hydroclimate suggests that pan‐CONUS droughts are forced by a contemporaneous cold tropical Pacific Ocean and a warm tropical Atlantic Ocean. We test this hypothesis using the paleoclimate record. Composite analyses find a robust association between pan‐CONUS drought events and cold tropical Pacific conditions, but not with warm Atlantic conditions. Similarly, a self‐organizing map analysis shows that pan‐CONUS drought years are most commonly associated with a global sea surface temperature pattern displaying strong La Niña and cold Atlantic Multidecadal Oscillation (AMO) conditions. Our results confirm previous model‐based findings for the instrumental period and show that cold tropical Pacific Ocean conditions are the principal driver of pan‐CONUS droughts on annual timescales.

Description: Abstract The cavi unit at the north pole of Mars is a deposit of aeolian sand and water ice underlying the Late Amazonian north polar layered deposits. Its strata of Middle to Late Amazonian age record wind patterns and past climate. The Mars Reconnaissance Orbiter Shallow Radar (SHARAD) reveals extensive internal and basal layering within the cavi unit, allowing us to determine its general structure and relative permittivity. Assuming a basalt composition for the sand (ε′ = 8.8), results indicate that cavi contains an average ice fraction between 62% in Olympia Planum and 88% in its northern reaches beneath the north polar layered deposits and thus represents one of the largest water reservoirs on the planet. Internal reflectors indicate vertical variability in composition, likely in the form of alternating ice and sand layers. The ice layers may be remnants of former polar caps and thus represent a unique record of climate cycles predating the north polar layered deposits.

Description: Abstract In recent years, experimental results have consistently shown evidence of electromagnetic ion cyclotron (EMIC) wave‐driven electron precipitation down to energies as low as hundreds of keV. However, this is at odds with the limits expected from quasi‐linear theory. Recent analysis using nonlinear theory has suggested energy limits as low as hundreds of keV, consistent with the experimental results, although to date this has not been experimentally verified. In this study, we present concurrent observations from Polar‐orbiting Operational Environmental Satellite, Radiation Belt Storm Probes, Global Positioning System, and ground‐based instruments, showing concurrent EMIC waves and sub–MeV electron precipitation, and a global dropout in electron flux. We show through test particle simulation that the observed waves are capable of scattering electrons as low as hundreds of keV into the loss cone through nonlinear trapping, consistent with the experimentally observed electron precipitation.

Description: Abstract Modelling and observations have shown that energy diffusion by chorus waves is an important source of acceleration of electrons to relativistic energies. By performing long‐term simulations using the three‐dimensional Versatile Electron Radiation Belt code, in this study, we test how the latitudinal dependence of chorus waves can affect the dynamics of the radiation belt electrons. Results show that the variability of chorus waves at high latitudes is critical for modelling of MeV electrons. We show that, depending on the latitudinal distribution of chorus waves under different geomagnetic conditions, they cannot only produce a net acceleration but also a net loss of MeV electrons. Decrease in high‐latitude chorus waves can tip the balance between acceleration and loss towards acceleration, or alternatively, the increase in high‐latitude waves can result in a net loss of MeV electrons. Variations in high‐latitude chorus may account for some of the variability of MeV electrons.

Description: Abstract Wet scavenging of black carbon (BC) has been subject to large uncertainty, which importantly determines its atmospheric lifetime and indirect forcing impact on cloud microphysics. This study reveals the complex BC‐hydrometeor interactions in mixed‐phase clouds via single particle measurements in the real‐world environment, by capturing precipitation processes throughout cloud formation, cold rain/graupel, and subsequent snow events at a mountain site influenced by anthropogenic sources in wintertime. We found highly efficient BC wet scavenging during cloud formation, with large and thickly coated BC preferentially incorporated into droplets. During snow processes, BC core sizes in the interstitial phase steadily increased. A mechanism was proposed whereby the BC mass within each droplet was accumulated through droplet collision, leading to larger BC cores, which were then released back to the interstitial air through the Wegener‐Bergeron‐Findeisen processes when ice dominated. These results provide fundamental basis for constraining BC wet scavenging.

Description: This paper explores temperature variability over southern South America. Four states of temperature variability are revealed in both winter and summer seasons. Synoptic‐scale meteorological patterns help diagnose the temperature variability states with low‐level temperature and moisture advection are closely related to patterns of temperature variability. Large‐scale modes of climate variability show some connection to temperature variability states, no single mode appears to be a primary driver. Abstract Key spatiotemporal patterns of monthly scale temperature variability are characterized over southern South America using k‐means clustering. The resulting clusters reveal patterns of temperature variability, referred to as temperature variability states. Analysis is performed over summer and winter months separately using data covering the period 1980–2015. Results for both seasons show four primary temperature variability states. In both seasons, one state is primarily characterized by warm temperature anomalies across the domain while another is characterized by cold anomalies. The other two patterns tend to be characterized by a warm north–cold south and cold north–warm south feature. This suggests two primary modes of temperature variability over the region. Composites of synoptic‐scale meteorological patterns (wind, geopotential height, and moisture fields) are computed for months assigned to each cluster to diagnose the driving meteorology associated with these variability states. Results suggest that low‐level temperature advection promoted by anomalies in atmospheric circulation patterns is a key process for driving these variability states. Moisture‐related processes also are shown to play a role, especially in summer. The El Niño–Southern Oscillation and the Southern Annular Mode exhibit some relationship with temperature variability state frequency, with some states more common during amplified phases of these two modes than others. However, the climate modes are not a primary driver of the temperature variability states.

Description: Abstract Auroral kilometric radiation (AKR) can potentially produce serious damage to space‐borne systems by accelerating trapped radiation belt electrons to relativistic energies. Here we examine the global occurrences of AKR emissions in radiation belts based on Van Allen Probes observations from 1 October 2012 to 31 December 2016. The statistical results (1,848 events in total) show that AKR covers a broad region of L= 3–6.5 and 00–24 magnetic local time (MLT), with a higher occurrence on the nightside (20–24 MLT and 00–04 MLT) within L= 5–6.5. All the AKR events are observed to be accompanied with suprathermal (∼1 keV) electron flux enhancements. During active geomagnetic periods, both AKR occurrences and electron injections tend to be more distinct, and AKR emission extends to the dayside. The current study shows that AKR emissions from the remote sources are closely associated with electron injections.

Description: Abstract In a metal, as in Earth's core, the thermal and electrical conductivities are assumed to be correlated. In a planetary dynamo this implies a contradiction: that both electrical conductivity, which makes it easier to induce current and magnetic field, and conductive heat transport, which hinders thermal convection, should increase simultaneously. Here we show that this contradiction implies that the magnetic induction rate peaks at a particular value of electrical and thermal conductivity and derive the low‐ and high‐conductivity limits for thermal dynamo action. A dynamo regime diagram is derived as a function of electrical conductivity and temperature for Earth's core that identifies four distinct dynamo regimes: no dynamo, thermal dynamo, compositional dynamo, and thermocompositional dynamo. Estimates for the temperature‐dependent electrical conductivity of the core imply that the geodynamo may have come close to its high‐conductivity “no dynamo” limit prior to inner core nucleation, consistent with recent paleomagnetic observations.

Description: Abstract High‐intensity precipitation represents a threat for several regions of the world because of the related risk of natural disasters (e.g., floods and landslides). This work focuses on low‐level precipitation enhancement that occurs in the cloud warm layer and has been observed in relation to collision‐coalescence (CC) leading to flash floods and extreme rainfall events in tropical and temperate latitudes. Specifically, signatures of precipitation enhancement (referred to as CC‐dominant precipitation) are investigated in the observations from the Global Precipitation Measurement (GPM) core mission Dual‐frequency Precipitation Radar (DPR) over the central/eastern Contiguous United States (CONUS) during June 2014 – May 2018. A classification scheme for CC‐dominant precipitation, developed for dual‐polarization S‐band radar measurements and applied in a previous work to X‐band radar observations in complex terrain, is used as a benchmark. The scheme is here applied to the GPM ground validation dataset that matches ground‐based radar observations across CONUS to space‐borne DPR retrievals. The occurrence of CC‐dominant precipitation is documented and the corresponding signatures of CC‐dominant precipitation at Ku‐ and Ka‐band are studied. CC‐dominant profiles show distinguishing features when compared to profiles not dominated by CC, e.g., characteristic vertical slopes of reflectivity at Ku‐ and Ka‐band in the liquid layer, lower freezing level height, and shallower ice layer, which are linked to environmental conditions driving the peculiar CC microphysics. This work aims at improving satellite quantitative precipitation estimation, particularly GPM retrievals, by targeting CC development in precipitation columns. This article is protected by copyright. All rights reserved.

Description: Abstract The processes that control the isotopic composition of precipitation in the mid‐latitudes are complicated, but can provide valuable insights into precipitation‐generating processes and are critical for interpreting stable isotope‐based paleoclimate records. In this study, we investigated the controls on changes in the isotopic composition of rainwater in central Texas using a combination of existing monthly stable isotope data from the global network of isotopes in precipitation (GNIP) and 20 months of event‐based rainwater collection from Austin, TX. We find that the strongest control on the isotopic composition of precipitation is the varying proportion of convective and stratiform rainfall, with other factors such as precipitation amount, temperature, storm track playing a secondary role. Isotopic values are generally lower in the cold season than the warm season precipitation because cold season precipitation is predominantly stratiform often associated with a northerly storm track. However, the majority of the precipitation in the south‐central United States (US) occurs during the warm season in association with mesoscale convective systems (MCS) that are fed with moisture by the southerly winds. MCS are characterized by a combination of a leading edge of organized deep convection and trailing stratiform precipitation. Stronger MCS tend to contain higher proportions of stratiform rainfall and as a result, have more isotopically depleted values. Therefore, changes in the stable isotopic composition of rainfall may be interpreted as reflecting changes in the intensity of MCS.

Description: Abstract Atmospheric nitrate (NO3− = particulate NO3− + gas‐phase nitric acid [HNO3]) and sulfate (SO42−) are key molecules that play important roles in numerous atmospheric processes. Here, the seasonal cycles of NO3− and total suspended particulate sulfate (SO42−(TSP)) were evaluated at the South Pole from aerosol samples collected weekly for approximately 10 months (26 January to 25 October) in 2002 and analyzed for their concentration and isotopic compositions. Aerosol NO3− was largely affected by snowpack emissions in which [NO3−] and δ15N(NO3−) were highest (49.3 ± 21.4 ng/m3, n = 8) and lowest (−47.0 ± 11.7‰, n = 5), respectively, during periods of sunlight in the interior of Antarctica. The seasonal cycle of Δ17O(NO3−) reflected tropospheric chemistry year‐round with lower values observed during sunlight periods and higher values observed during dark periods, reflecting shifts from HOx‐ to O3‐dominated oxidation chemistry. SO42−(TSP) concentrations were highest during austral summer and fall (86.7 ± 73.7 ng/m3, n = 18) and are indicated to be derived from dimethyl sulfide (DMS) emissions, as δ34S(SO42−)(TSP) values (18.5 ± 1.0‰, n = 10) were similar to literature δ34S(DMS) values. The seasonal cycle of Δ17O(SO42−)(TSP) exhibited minima during austral summer (0.9 ± 0.1‰, n = 5) and maxima during austral fall (1.3 ± 0.3‰, n = 6) and austral spring (1.6 ± 0.1‰, n = 5), indicating a shift from HOx‐ to O3‐dominated chemistry in the atmospheric derived SO42− component. Overall, the budgets of NO3− and SO42−(TSP) at the South Pole were complex functions of transport, localized chemistry, biological activity, and meteorological conditions, and these results will be important for interpretations of oxyanions in ice core records in the interior of Antarctica.

Description: Abstract Variability of the flow across the Solomon Sea's southern entrance was examined using end point subsurface moorings and seafloor pressure sensors, reconstructed velocity profiles based on satellite‐derived surface velocity and bottom pressure‐derived subsurface velocity, and 1993–2017 proxy volume transport based on satellite altimetry. The reconstructed velocity correctly represents the fluctuating surface flow and subsurface core providing a high‐frequency continuous observing system for this sea. The mean equatorward volume transport over 0‐ to 500‐m depth layer is 15.2 Sv (1 Sv ≡ 106 m3/s) during July 2012 to May 2017. The measurements resolve the full spectrum of the volume transport including energetic subseasonal variability that fluctuates by as much as 25 Sv over one week. At low‐frequency timescales, the study finds that linear Rossby waves forced by Ekman pumping in the interior of the Pacific influence not only seasonal fluctuations as found by previous studies but also interannual variability. As found previously, the El Niño–Southern Oscillation highly influences interannual volume transport. During the 2015/2016 El Niño, observations show the seasonal cycle to be suppressed from the second half of 2014, prior to the mature phase of the El Niño, to September 2016 along with an increase in across‐transect transport. At subseasonal timescales, local Ekman pumping and remote wind stress curl are responsible for a third of the subseasonal variance. The study highlights the importance of high‐frequency observations at the southern entrance of the Solomon Sea and the ability of a linear Rossby model to represent the low‐frequency variability of the transport.

Description: Abstract A direct method is presented to obtain the meridional overturning and heat transport in oceanic basins from observations under the sole assumptions of geostrophy and hydrostatics,. The method is made possible because of the rising Argo float displacements data base which can provide a reference level at 1000 dbar for the time mean circulation at 1° × 1° resolution. To achieve the overturning and heat transport objectives, the absolute geostrophic time mean circulation must have non divergent barotropic transports and this requires the solutions of two Poisson equations with suitable boundary conditions, one for the geopotential at 1000 dbar and one for the barotropic streamfunction. Applied to the subpolar Atlantic for the period 2000‐2009, an overturning of 16‐18 Sv is found around 40o‐50oN, a meridional heat transport of 0.59 PW is found at 40oN (0.23 PW at 60oN) so that on average ~50 Wm‐2 are exported from ocean to atmosphere to feed the atmospheric storm track. The zonally averaged flow (the overturning) falls short of explaining the observed heat transport and the barotropic component of the circulation accounts for up to 50% of the heat transport poleward of 55oN. With the rising Argo float data base, the method offers high potential to reconstruct the World Ocean time mean circulation and its heat transport away from the equator at higher resolution. The drawback is that it requires in some critical places additional current observations on the shallow shelves which are not sampled by the Argo floats.

Description: Abstract The injection region's formation, scale size, and propagation direction have been debated throughout the years, with new questions arising with increased plasma sheet observations by missions like Cluster and THEMIS. How do temporally and spatially small‐scale injections relate to the larger injections historically observed at geosynchronous orbit? How to account for opposing propagation directions—earthward, tailward, and azimuthal—observed by different studies? To address these questions, we used a combination of multisatellite and ground‐based observations to knit together a cohesive story explaining injection formation, propagation, and differing spatial scales and timescales. We used a case study to put statistics into context. First, fast earthward flows with embedded small‐scale dipolarizing flux bundles transport both magnetic flux and energetic particles earthward, resulting in minutes‐long injection signatures. Next, a large‐scale injection propagates azimuthally and poleward/tailward, observed in situ as enhanced flux and on the ground in the riometer signal. The large‐scale dipolarization propagates in a similar direction and speed as the large‐scale electron injection. We suggest small‐scale injections result from earthward‐propagating, small‐scale dipolarizing flux bundles, which rapidly contribute to the large‐scale dipolarization. We suggest the large‐scale dipolarization is the source of the large‐scale electron injection region, such that as dipolarization expands, so does the injection. The 〉90‐keV ion flux increased and decreased with the plasma flow, which died at the satellites as global dipolarization engulfed them. We suggest the ion injection region at these energies in the plasma sheet is better organized by the plasma flow.

Description: ABSTRACT Blended acquisition along with efficient spatial sampling is capable of providing high‐quality seismic data in a cost‐effective and productive manner. While deblending and data reconstruction conventionally accompany this way of data acquisition, the recorded data can be processed directly to estimate subsurface properties. We establish a workflow to design survey parameters that account for the source blending as well as the spatial sampling of sources and detectors. The proposed method involves an iterative scheme to derive the survey design leading to optimum reflectivity and velocity estimation via joint migration inversion. In the workflow, we extend the standard implementation of joint migration inversion to cope with the data acquired in a blended fashion along with irregular detector and source geometries. This makes a direct estimation of reflectivity and velocity models feasible without the need of deblending or data reconstruction. During the iterations, the errors in reflectivity and velocity estimates are used to update the survey parameters by integrating a genetic algorithm and a convolutional neural network. Bio‐inspired operators enable the simultaneous update of the blending and sampling operators. To relate the choice of survey parameters to the performance of joint migration inversion, we utilize a convolutional neural network. The applied network architecture discards suboptimal solutions among newly generated ones. Conversely, it carries optimal ones to the subsequent step, which improves the efficiency of the proposed approach. The resultant acquisition scenario yields a notable enhancement in both reflectivity and velocity estimation attributable to the choice of survey parameters.

Description: Abstract As sea level rise and possible changes in storminess threaten coastal communities and infrastructure, the capacity for foredunes to provide protection depends on their geomorphology, which is determined by interactions between physical beach processes and vegetation. Here we use descriptive Bayesian network analyses to examine how sediment supply, beach characteristics, and two species of beachgrass (Ammophila arenaria and Ammophila breviligulata) alter foredune morphology and patterns of sand accretion on U.S. Pacific Northwest foredunes. We show that sediment supply and beach type primarily determine foredune morphology. Beachgrass density also influences foredune shape, but its effects differ among species: increasing density of A. arenaria was associated with steeper sloping dunes, whereas increasing density of A. breviligulata was associated with wider, more shallow sloping dunes. An examination of the change in foredune morphology over a 2‐year period found sand accretion was most strongly influenced by species‐specific patterns of vegetation growth and beach type. Specifically, A. breviligulata exhibited more lateral growth, resulting in greater sand accretion at the seaward margin of the foredune. In contrast, A. arenaria exhibited little lateral growth, resulting in comparatively more sand accretion near the foredune crest. Consequently, growth form‐generated sand accretion patterns resulted in steep, narrow A. arenaria‐dominated foredunes and shallow‐sloping, wider A. breviligulata‐dominated foredunes. These results illustrate that vegetation density and patterns of growth influence foredune morphology and its changes over time.

Description: Abstract Convergent orogens exhibit high elevations and relief, features characteristic of active rock uplift, the latter influencing normalized channel steepness (Ksn). In systems with significant horizontal displacement, Ksn values and interfluves are elevated over a region of tens of kilometers and gradually decline in the direction of rock advection. To evaluate potential relationships between elevated Ksn, a gradual decline in interfluve elevation (i.e., tapered topography) and lateral advection, we integrated kinematic models that simulate advection over a mid‐crustal ramp with a 2D surface processes model. Varying convergence rate, bedrock erodibility and ramp angle, we tracked topographic evolution over time. The process of advection through the region of active rock uplift above a mid‐crustal ramp is preserved in the geomorphic record through transient legacy landscapes characterized by (i) high‐relief, advection‐parallel interfluves, (ii) tapered topography, (iii) elevated and gradually declining Ksn values, and (iv) higher Ksn in trunk relative to tributary streams likely reflecting the influence of increased sediment flux, elevated interfluves, and changes in drainage area. The width of legacy landscapes provides a minimum constraint on the total lateral displacement, controlled by the duration of ramp activity and the rates of advection and erosion. The development of legacy landscapes is facilitated by spatial variations in flow convergence that occur in a 2D setting but are not captured in idealized 1D approaches. The presence of elevated Ksn and high relief, advection‐parallel interfluves beyond the region of active rock uplift likely reflects the horizontal advection component inherent to convergent orogenic systems.

Description: Abstract Understanding global soil moisture‐air temperature (θ‐Ta) coupling is needed to improve the representation of land‐atmosphere interactions in Earth system models. Most studies on θ‐Ta coupling have focused on hot extremes, where precipitation‐related indices and model‐derived soil moisture products are commonly used. In this study, global θ‐Ta coupling is examined based on monthly air temperature anomalies and the Gravity Recovery and Climate Experiment (GRACE) terrestrial water storage (TWS). A discrete wavelet decomposition is used to partition the TWS into different components. The results show that TWS is useful in revealing the spatial patterns of θ‐Ta coupling. Decomposed GRACE TWS shows improved skill compared to raw TWS in explaining temporal variability of monthly air temperature, which likely reflects different roles of soil moisture at different depths in the θ‐Ta coupling. The explanatory power improves further by using a combination of decomposed GRACE TWS and precipitation. Such improvement is observed particularly in places where vegetation tends to have a deeper rooting system, such as eastern region of South America, the southern tip of Africa, and north of the Tropic of Capricorn in Australia. The occurrence of θ‐Ta coupling is mainly constrained by the coupling of root zone moisture and land surface temperature. In addition to deeper rooting systems, clear wet and dry season alternation is another favorable factor for developing significant monthly θ‐Ta coupling.

Description: Abstract This study investigates heat wave variability over Korea during 1979‐2017. It is found that most of heat waves in Korea can be classified into two distinct types based on the spatial patterns of atmospheric circulation anomalies: the zonal wave (Z‐wave) type and the meridional wave (M‐wave) type. The Z‐wave type is accompanied by large‐scale atmospheric waves across the Eurasian continent, while the M‐wave type is associated with convective activities over the subtropical western North Pacific. The Z‐wave type occurs when the high‐pressure node of eastward propagating wave located around Korea and it seems that the associated wave energy could originate from North Atlantic Ocean. The M‐wave type, on the other hand, is driven by northward propagating wave train from subtropical western North Pacific to East Asia, which is triggered by anomalous convective activity over the subtropical western North Pacific. By analyzing thermodynamical as well as dynamical variables, detailed descriptions on the physical characteristics of two types of heat wave are presented in this study along with the possible implications for summer climate variability over Korea.

Description: Abstract Here we present an observation‐based study of the coupled land‐ocean regions of influence for the transformation of precipitation over land into coastal river plume structure in the Gulf of Mexico (GoM). First, we locate the regions on land for which precipitation and runoff generation have the strongest relationship with local river discharge. Then we map, on average, the apparent unique contribution of individual river discharge forcing to specific features of river plume structure across the GoM. To this end, we employ a spatial‐temporal lagged correlation analysis that relates satellite‐based precipitation, soil moisture, and sea surface salinity observations to in situ river discharge for the three primary freshwater input sources for the GoM. On land, we find a likely source region for the northeastern GoM in the southeastern Mississippi basin at 16‐day lead time, a likely source region for the northeastern GoM in the Mobile Bay basin at 3‐day lead time and a likely source region for the Central GoM from the Texas basin region at 4‐day lead time. In the ocean, we find statistically significant regions of distinct contribution for each of the three sources of freshwater on plume structure at lag times from weeks to several months. Though a statistical approach is limited in its interpretability, this result advances progress toward a predictive framework for mapping of the impacts of hydrological flood events from land into the ocean using observations alone.

Description: Abstract Recent laboratory evidence shows that compaction creep in porous rocks may develop through stages of acceleration, especially if the material is susceptible to strain localization. This paper provides a mechanical interpretation of compaction creep based on viscoplasticity and nonlinear dynamics. For this purpose, a constitutive operator describing the evolution of compaction creep is defined to evaluate the spontaneous accumulation of pore collapse within an active compaction band. This strategy enables the determination of eigenvalues associated with the stability of the response, i.e. able to differentiate decelerating from accelerating strain. This mathematical formalism was linked to a constitutive law able to simulate compaction localization. Material point simulations were then used to identify the region of the stress space where unstable compaction creep is expected, showing that accelerating strains correspond to pulses of inelastic strain rate. Such pulses were also found in full‐field numerical analyses of delayed compaction, revealing that they correspond to stages of inception and propagation of new bands across the volume of the simulated sample. These results illustrate the intimate relation between the spatial patterns of compaction and their temporal dynamics, showing that while homogeneous compaction develops with decaying rates of accumulation, localized compaction occurs through stages of accelerating deformation caused by the loss of strength taking place during the formation of a band. In addition, they provide a predictive modeling framework to simulate and explain the spatiotemporal dynamics of compaction in porous sedimentary formations.

Description: Abstract This paper evaluates the intraseasonal variability of sea surface temperature (SST) along the Sumatra‐Java southern coast using available satellite‐derived oceanic and atmospheric data combined with output from a numerical model. The result reveals that the intraseasonal variability of SST is greater during boreal summer–fall (June–October) than during boreal winter–spring (November–May). Composite analysis shows a correlation between positive/negative intraseasonal SST variabilities and coastal downwelling/upwelling, as well as onshore/offshore Ekman transport during summer–fall. During this period, with the significantly increasing role of oceanic advection, oceanic processes are evidently enhanced and dominate the intraseasonal variability of SST. Meanwhile, the contribution of atmospheric processes drops by 67%. During winter–spring, the intraseasonal SST is primarily contributed by atmospheric processes but has a nonsignificant relationship with sea level anomalies. Intraseasonal SST anomalies vary out of phase with surface wind anomalies. The result also shows a relatively small contribution by vertical processes throughout the year, with the maximum in April and the minimum during August–September. Further analysis reveals that the alternating dominance of atmospheric and oceanic processes on intraseasonal variability of SST is responsible for the seasonality along the Sumatra‐Java southern coast. Moreover, the result indicates that the seasonality in intraseasonal SST is different in the eastern Indonesian Seas, which tends to be relatively strong in boreal winter. Distinct dominance of atmospheric and oceanic processes in intraseasonal SST is the main reason for these differences in seasonal variation characteristics.

Description: Abstract Precise measurements of height changes (HCs) are important for improved estimates of mass balance of the Greenland Ice Sheet (GrIS). Here we determine 10 years of precise, high‐resolution HCs of the GrIS from Envisat radar altimeter using a subwaveform retracker and a modified repeat‐track method. The HCs show clear seasonal changes and monotonic declines over glaciers on the coasts such as Zachariae Isstrøm. We enhance mass‐change estimates from GRACE data using HCs and densities from interannual correlations between GRACE‐derived mass changes and HCs. We estimate the mass changes of eight drainage basins with our combined mass change. The largest mass change between 2002 and 2012 occurred in the northwest basin and the smallest in the northeast basin. We separate the ice and snow HC rates to derive a ratio (f) between them to characterize the relative importance of ice or snow to mass change. The snow HC rates are mostly positive over the GrIS, except on the margins of the west coast and Zachariae Isstrøm. The mean ice HC rate is ‐6.6±3.9 cm yr‐1 over 2002–2006, which accelerated to ‐13.9±2.4 cm yr‐1 over 2007–2012. The f factors show a clear post‐2006 ice dominance in the GrIS mass loss, particularly on the west coast, with a mean 91.4% ice contribution over 2002–2006, increasing to 94.5% over 2007–2012. This indicates increasing mass loss after 2006. A coincident radar altimeter and gravimetry mission is important for studying mass balance and separating snow and ice contributions over space and time.

Description: ABSTRACT Detailed P wave velocity and anisotropy structure of the uppermost mantle below the central United States is presented based on a tomographic inversion of Pn traveltimes for earthquakes in the range 2 to 14°. Dense raypath coverage throughout the northern Mississippi Embayment is obtained using the Northern Embayment Lithosphere Experiment and U.S. Transportable Array data sets. A detailed analysis of the trade‐off between velocity and anisotropy variations demonstrates that both are well resolved over most of the study area. Anomalously fast Pn velocities are identified below the northern Mississippi Embayment, centered on the New Madrid seismic zone. A prominent region of low velocity coincides with the southwestern margin of the Illinois basin. Pn anisotropy displays complex patterns and differs from absolute plate motion directions and SKS splitting directions. A circular pattern of fast anisotropy directions is centered on the New Madrid seismic zone and may be related to the presence of the mafic “rift pillow.”

Description: Abstract An analysis of the counter‐electrojet occurrence (CEJ) during 2008–2014 is presented for the African and American sectors based on local daytime (0700–1700 LT) observations from the Communications and Navigation Outage Forecasting System (C/NOFS) vertical ion plasma drift (equivalent to vertical E×B at an altitude of about 400 km) and ground‐based magnetometers. Using quiet time (Kp≤ 3) data, differences and/or similarities between the two data sets with reference to local time and seasonal dependence are established. For the first time, it is shown that C/NOFS satellite data are consistent with magnetometer observations in identifying CEJ occurrences during all seasons. However, C/NOFS satellite data show higher CEJ occurrence rate for almost all seasons. With respect to local time, C/NOFS satellite observes more CEJ events than magnetometer observations by average of about 20% and 40% over the American and African sectors, respectively, despite both data sets showing similar trends in CEJ identification. Therefore, when a space weather event occurs, it is important to first establish the original variability nature and/or magnitude of the eastward electric field in equatorial regions before attributing the resulting changes to solar wind‐magnetosphere and ionosphere coupling processes since CEJ events can be present even during quiet conditions.

Description: Abstract Sedimentary relative paleointensity (RPI) records are often carried by complex magnetic mineral mixtures, including detrital and biogenic magnetic minerals. Recent studies have demonstrated that magnetic inclusions within larger detrital silicate particles can make significant contributions to sedimentary paleomagnetic records. However, little is known about the role such inclusions play in sedimentary paleomagnetic signal recording. We analyzed paleomagnetic and mineral magnetic data for marine sediment core MD01‐2421 from the North Pacific Ocean, offshore of central Japan, to assess how magnetic inclusions and other detrital magnetic minerals record sedimentary paleomagnetic signals. Stratigraphic intervals in which abundant magnetic inclusions dominate the magnetic signal are compared with other intervals to assess quantitatively their contribution to sedimentary RPI signals. The normalized remanence record from core MD01‐2421 does not correlate clearly with global RPI stacks, which we attribute to a demonstrated lower paleomagnetic recording efficiency of magnetic inclusions compared to other detrital magnetic minerals. We also carried out the first laboratory redeposition experiments under controlled Earth‐like magnetic fields for particles with magnetic inclusions using material from core MD01‐2421. Our results confirm that such particles can be aligned by ambient magnetic fields but with a lower magnetic recording efficiency compared to other detrital magnetic minerals, which is consistent with normalized remanence data from core MD01‐2421. Our demonstration of the role of sedimentary magnetic inclusions should have wide applicability for understanding sedimentary paleomagnetic recording.

Description: Abstract In this study, we estimate atmospheric turbulence in the free atmosphere in terms of the Thorpe scale (LT) and eddy dissipation rate (ε) using U.S. high vertical‐resolution radiosonde data over 4 years (September 2012 to August 2016) at 68 operational stations. In addition, same calculations are conducted for 12 years (October 2005 to September 2017) at four stations among the 68 stations. These high vertical‐resolution radiosonde data have a vertical resolution of approximately 5 m and extend to an altitude of approximately 33 km, and thus, turbulence can be retrieved in the entire troposphere and lower stratosphere. There are thicker and stronger turbulent layers in the troposphere than in the stratosphere, with mean ε values of 1.84 × 10−4 and 1.37 × 10−4 m2/s3 in the troposphere and stratosphere, respectively. The vertical structure of ε exhibits strong seasonal variations, especially in the upper troposphere and lower stratosphere, with the largest ε values in summer and the smallest in winter. In the horizontal distribution of ε, large ε is seen mainly above the mountainous region in the troposphere, but this pattern is not seen in the stratosphere. Although ε is estimated by the square of LT multiplied by the cube of the Brunt‐Väisälä frequency (N), the regions of large ε are matched with large LT regions where N is relatively small. For the time series of ε near the tropopause for 12 years at four stations, an annual variation is prominent at all stations without significant interannual variations. There is, however, a slightly increasing trend of ε at two stations.

Description: Abstract Abrupt swings in temperature can exert negative impacts, ranging from human health to agricultural production. Here, we focus on a global assessment of the extremes in the temperature swings at sub‐daily scales using Modern‐Era Retrospective analysis for Research and Applications, Version 2 (MERRA‐2) data. Overall, the regions with extremely large swings in hourly temperature (i.e., 99th percentile) are located in desert or arid regions, and the land masses exhibit larger temperature swings than the oceans. In contrast, the first percentile of the hourly temperature swings exhibits a different spatial pattern, with the lowest values (i.e., largest negative swings) located in the Rocky Mountain, South Australia, South and North Africa and some regions in Northwestern China. We identify a significant downward/upward trend in the 99th/1st percentile of sub‐daily (i.e., hourly and 12 hr) temperature changes in the midlatitudes in the Northern Hemisphere, particularly during boreal summer. Overall, the regions with significant trends in the Northern Hemisphere are collocated with the paths of the jet streams and storm tracks. The significant downward/upward trends in the 99th/1st percentile of the sub‐daily temperature swings over the Northern Hemisphere can be explained by a weakening in the Northern Hemisphere's summer circulation, as suggested by the downward trend in the eddy kinetic energy. These results indicate that a weak/strong persistence in the circulation may lead to less/more abrupt temperature swings (i.e., increase or decrease) caused by horizontal temperature advection.

Description: Abstract The presence of a seasonal snowpack determines the hydrology, geomorphology and ecology of wide parts of the Iberian Peninsula, with strong implications for the economy, transport and risk management. Thus, reliable information on snow is necessary from a scientific and operational point of view. This is the case of the Iberian Peninsula where, lack of observation has impeded proper analysis of snowpack duration, magnitude and interannual variability. In this study we present the first snow climatology of the entire Iberian Peninsula. The scarcity of in situ observations has been overcome, using a newly developed remote sensing snow database from MODIS satellite sensors for the period 2000 ‐ 2014 and a physically based snow model (Factorial Snow Model‐ FSM), driven by a regional atmospheric model (Weather Research and Forecast model‐ WRF) over the Iberian Peninsula for the period 1980 ‐ 2014. The snowpack of the main mountain areas (Pyrenees, Cantabrian, Central, Iberian range and Sierra Nevada) are described, estimated from the generated databases. The information has been processed using a k‐means cluster algorithm, looking for similarities in snow indices at different elevation bands. Results show four different types of snowpack in terms of depth, duration and interannual variability, lying over different elevation bands in the different ranges, proving the variability of the snowpack over Iberia. Analyses reveal areas characterised by ephemeral snowpacks, while in some sectors snowpack lasts, on average, 198 days per year with 3.02 meters of peak snow depth. The coefficient of variation of interannual peak snow depth oscillated between 35.2% and 162.4%. All the analysed indices show that at common elevations the Cantabrian range and the Pyrenees host the deepest and longest lasting snowpacks, followed by the Central and Iberian ranges. The Sierra Nevada exhibits the shortest, shallowest snowpack and more year‐to‐year variability. This article is protected by copyright. All rights reserved.

Description: Abstract The mechanical dynamics of volcanic systems can be better understood with detailed knowledge on strength of a volcanic edifice and subsurface. Previous work highlighting this on Mt. Etna has suggested that its carbonate basement could be a significant zone of widespread planar weakness. Here, we report new deformation experiments to better quantify such effects. We measure and compare key deformation parameters using Etna basalt (EB), which is representative of upper edifice lava flows, and Comiso limestone (CL), which is representative of the carbonate basement, under upper crustal conditions. These data are then used to derive empirical constitutive equations describing changes in rocks strength with pressure, temperature and strain rate. At a constant strain rate of 10‐5 s‐1 and an applied confining pressure of 50 MPa the brittle to ductile transitions were observed at 975 °C (EB) and 350 °C (CL). For the basaltic edifice of Mt. Etna, the strength is described with a Mohr‐coulomb failure criterion with μ ~0.704, C = 20 MPa. For the carbonate basement, strength is best described by a power law‐type flow in two regimes: a low‐T regime with stress exponent n ~5.4 and an activation energy Q ~ 170.6 kJ/mol and a high‐T regime with n~ 2.4 and Q ~ 293.4 kJ/mol. We show that extrapolation of these data to Etna's basement predicts a brittle to ductile transition that corresponds well with the generally observed trends of the seismogenic zone underneath Mt. Etna. This in turn may be useful for future numerical simulations of volcano‐tectonic deformation of Mt. Etna, and other volcanoes with limestone basements.

Description: Abstract We analyze the spatial variation in the response of the surface geomagnetic field (or the equivalent ionospheric current) to variations in the solar wind. Specifically, we regress a reanalysis of surface external and induced magnetic field (SEIMF) variations onto measurements of the solar wind. The regression is performed in monthly sets, independently for 559 regularly spaced locations covering the entire northern polar region above 50° magnetic latitude. At each location, we find the lag applied to the solar wind data that maximizes the correlation with the SEIMF. The resulting spatial maps of these independent lags and regression coefficients provide a model of the localized SEIMF response to variations in the solar wind, which we call “Spatial Information from Distributed Exogenous Regression.” We find that the lag and regression coefficients vary systematically with ionospheric region, season, and solar wind driver. In the polar cap region the SEIMF is best described by the By component of the interplanetary magnetic field (50–75% of total variance explained) at a lag ∼20–25 min. Conversely, in the auroral zone the SEIMF is best described by the solar wind ϵ function (60–80% of total variance explained), with a lag that varies with season and magnetic local time (MLT), from ∼15–20 min for dayside and afternoon MLT (except in Oct–Dec) to typically 30–40 min for nightside and morning MLT and even longer (60–65 min) around midnight MLT.

Description: Geophysical Prospecting, Volume 0, Issue ja, -Not available-.

Description: Abstract Tectonic extension of continental lithosphere creates accommodation space in which sediments are deposited. Climate‐driven processes provide the mechanism by which mass is detached from hillslopes and sediments are transported into this accommodation space. These two forcings, climate and tectonics, act together to create either endorheic (internally drained) or exorheic (externally drained) rift basins. Here we use a large‐scale dynamic landscape evolution‐tectonics model to understand the contribution of tectonic processes in endorheic‐exorheic transitions. In the model, extension results in opening of an asymmetric half‐graben along a listric normal fault. Rift opening occurs in the models in wet, temperate, or semi‐arid climates where runoff and evapotranspiration are varied. Our numerical experiments show that slow rift‐opening rates, a slowing‐down of rift opening, or increase of headwater topography (e.g., upstream epeirogenic uplift), are tectonic situations that can cause a transition from an endorheic to an exorheic drainage state in a rift basin. Our results also show that wet climate conditions lead to a permanent exorheism that persists regardless of rift opening rates. In semi‐arid climates, endorheic conditions are favored, and may last for the duration of rifting except for when rift opening is very slow. These results form an interpretive framework to study endorheic and exorheic drainage systems in natural continental rifts. In the slow‐opening Rio Grande rift, the endorheic‐exorheic transition may have occurred without dramatic climate changes. Lake‐level variations in East African rift basins are predicted by our models to result from variations in climate.

Description: A climatology of the boundary‐layer wind‐turning angle over land is presented based on radiosonde observations from 800 stations in the Integrated Global Radiosonde Archive (IGRA). The dependence of the wind turning on a suite of parameters is analyzed. Results from previous studies indicating the importance of the planetary boundary layer (PBL) stratification for the angle of wind turning are confirmed here. A clear increase in the wind‐turning angle with wind speed, particularly for stratified conditions, is also evident. According to Rossby number similarity theory, the cross‐isobaric angle for a neutral and barotropic boundary layer decreases with the surface Rossby number, Ro. The IGRA observations indicate that this dependence on Ro might partly be linked to the dependence of the stratification on the wind speed, a dependence that seems to prevail even for the high wind speeds, a criteria that traditionally is used to approximate a neutral PBL. The vertical distribution of the turning of the wind is analyzed using the high resolution Stratospheric Processes And their Role in Climate (SPARC) data. For unstable cases, there is a maximum in the directional wind shear around the PBL top, whereas for the most stable class of cases there is a maximum near the surface. The wind‐turning angles from observations are also compared with values obtained from ERA‐Interim reanalysis fields, also presented over ocean. ERA‐Interim underestimates the magnitude of the wind‐turning angles as well as the range. Furtheremore, the midlatitude cross‐isobaric mass transport is estimated using the IGRA data. This transport is generally underestimated by ERA‐Interim, likely related to the too small wind‐turning angles. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Transverse isotropy with a vertical axis of symmetry is a common form of anisotropy in sedimentary basins, and it has a significant influence on the seismic amplitude variation with offset. Although exact solutions and approximations of the PP‐wave reflection coefficient for the transversely isotropic media with vertical axis of symmetry have been explicitly studied, it is difficult to apply these equations to amplitude inversion, because more than three parameters need to be estimated, and such an inverse problem is highly ill‐posed. In this paper, we propose a seismic amplitude inversion method for the transversely isotropic media with a vertical axis of symmetry based on a modified approximation of the reflection coefficient. This new approximation consists of only three model parameters: attribute A, the impedance (vertical phase velocity multiplied by bulk density); attribute B, shear modulus proportional to an anellipticity parameter (Thomsen's parameter ε−δ); and attribute C, the approximate horizontal P‐wave phase velocity, which can be well estimated by using a Bayesian‐framework‐based inversion method. Using numerical tests we show that the derived approximation has similar accuracy to the existing linear approximation and much higher accuracy than isotropic approximations, especially at large angles of incidence and for strong anisotropy. The new inversion method is validated by using both synthetic data and field seismic data. We show that the inverted attributes are robust for shale‐gas reservoir characterization: the shale formation can be discriminated from surrounding formations by using the crossplot of the attributes A and C, and then the gas‐bearing shale can be identified through the combination of the attributes A and B. We then propose a rock‐physics‐based method and a stepwise‐inversion‐based method to estimate the P‐wave anisotropy parameter (Thomsen's parameter ε). The latter is more suitable when subsurface media are strongly heterogeneous. The stepwise inversion produces a stable and accurate Thomsen's parameter ε, which is proved by using both synthetic and field data.

Description: Abstract In this work, cobalt phosphide (CoP) nanoparticles were successfully decorated on an ultrathin g‐C3N4 nanosheet photocatalysts by in situ chemical deposition. The built‐in electric field formed by heterojunction interface of the CoP/g‐C3N4 composite semiconductor can accelerate the transmission and separation of photogenerated charge‐hole pairs and effectively improve the photocatalytic performance. TEM, HRTEM, XPS, and SPV analysis showed that CoP/g‐C3N4 formed a stable heterogeneous interface and effectively enhanced photogenerated electron‐hole separation. UV‐vis DRS analysis showed that the composite had enhanced visible light absorption than pure g‐C3N4 and was a visible light driven photocatalyst. In this process, NaH2PO2 and CoCl2 are used as the source of P and Co, and typical preparation of CoP can be completed within 3 hours. Under visible light irradiation, the optimal H2 evolution rate of 3.0 mol% CoP/g‐C3N4 is about 15.1 μmol h−1. The photocatalytic activity and stability of the CoP/g‐C3N4 materials were evaluated by photocatalytic decomposition of water. The intrinsic relationship between the microstructure of the composite catalyst and the photocatalytic performance was analyzed to reveal the photocatalytic reaction mechanism.

Description: Abstract High volume fraction SiC nanowires‐reinforced SiC composites (SiCNWs/SiC) were prepared by hybrid process of chemical vapor infiltration and polymer impregnation/pyrolysis in this research. SiCNWs networks are first to be made promising a high volume fraction (20 vol%), and the pyrolytic carbon (PyC) interphase with 5 nm is designed on SiCNWs surface to optimize the bonding condition between SiCNWs and SiC matrix. Nanoindentation shows a modulus of 494 ± 14 GPa of SiCNWs/SiC composites without interphase comparing to the one with PyC interphase of 452 ± 13 GPa. However, the 3‐point bending test shows a higher strength of the composite with PyC interphase (273 ± 32 MPa) comparing with the one without interphase (240 ± 38 MPa). The fracture surface is observed under SEM, which shows a longer SiCNWs pullout of the composite with PyC interphase. The energy dissipation during the 3‐point bending test is calculated by the length of nanowire pull‐out, it demonstrates that the SiCNWs with PyC interphase possess better performance for toughening composite. Further characterization proves that the PyC interphase can give SiCNWs/SiC composites higher fracture toughness (4.49 ± 0.44 MPa·m1/2) than the composites without interphase (3.66 ± 0.28 MPa·m1/2).

Description: Here, we report a multicolor PersL phosphor Sr Ga GeO :Pr .The PersL color can be tuned from deep red to blue. It reveals that the multicolor luminescence of the phosphor is essentially associated with the crossrelaxation effect of Pr . What's more,the PersL lifetime of the multicolor phosphor can be also tuned. Based on the unique features of Sr Ga GeO :Pr phosphor, some luminescent images are fabricated for dynamic multicolor anticounterfeiting. Abstract Persistent luminescence (PersL) phosphor is a glow‐in‐the‐dark material that has been widely applied. Here, we report a multicolor PersL phosphor Sr2Ga2GeO7:Pr3+. The PersL color can be tuned from deep red to blue. It reveals that the luminescent color modulation of the Sr2Ga2GeO7:Pr3+ phosphor is essentially associated with the cross‐relaxation effect of Pr3+ in the host with low‐phonon assistance energy. The PersL lifetime of the multicolor phosphors can be also tuned. Based on the unique features of Sr2Ga2GeO7:Pr3+ phosphor, some simple PersL images are fabricated to emit dynamic multicolor information, and it shows that the PersL image even depicts dynamic multicolor anticounterfeiting.

Description: Abstract Extreme El Niño events affect the number of intense tropical cyclones (ITCs) over the western North Pacific (WNP). In 1997 and 2015, both extreme El Niño years, ITC numbers were above normal in the WNP. In order to clarify how, and to what extent, sea surface temperature anomaly (SSTA) distributions control the ITCs genesis, the authors conducted 50‐member ensemble simulations using a high‐resolution global nonhydrostatic model that explicitly simulates ITCs. The ensemble simulations showed a clear relationship between the number of ITCs and their genesis locations in the WNP. However, the authors found that the simulated numbers of ITCs in the WNP were also closely related to the strength of the monsoon trough, which varies under given SSTA conditions. This indicates that reliable seasonal forecasting of ITCs depends on our ability to accurately reproduce the monsoon trough, whose strength is modulated mainly by internal atmospheric variability but also by SSTA.

Description: Abstract Chorus emissions composed of coherent whistler mode waves are responsible for pitch angle scattering of energetic electrons. This scattering is closely related to energetic electron precipitation into the atmosphere, contributing to pulsating auroras. Conventionally, energetic electrons are considered to satisfy the cyclotron resonance condition over the energy range of a few to tens of kiloelectron volts and are scattered toward the loss cone by waves. However, previous simulation studies indicate that low pitch angle electrons tend to be scattered away from the loss cone by coherent whistler mode waves. We examine the mechanism of anomalous trapping at low pitch angles, deriving a particle equation with low pitch angle assumptions. An additional term that is conventionally neglected represents the Lorentz force caused by the wave magnetic field and the parallel particle velocity. Therefore, due to the large v‖×Bw Lorentz force, low pitch angle electrons satisfying the cyclotron resonant condition are scattered away from the loss cone and effectively trapped by waves. We perform test particle simulations in a one‐dimensional dipole magnetic field with a whistler mode wave model and reproduce the anomalous trapping of electrons. The simulation results show that the majority of electrons at high and moderate pitch angles are scattered toward low pitch angle regions while low pitch angle electrons are strongly scattered toward high pitch angle regions. Consequently, a coherent chorus element produces a bump in the electron pitch angle distribution.

Description: Abstract This paper describes a novel technique that allows separation and quantification of different sources of convection in the high‐latitude ionosphere. To represent the ionospheric convection electric field, we use the Spherical Elementary Convection Systems representation. We demonstrate how this technique can separate and quantify the contributions from different magnetospheric source regions to the overall ionospheric convection pattern. The technique is in particular useful for distinguishing the contributions of high‐latitude reconnection associated with lobe cells from the low‐latitude reconnection associated with Dungey two‐cell circulation. The results from the current paper are utilized in a companion paper (Reistad et al., 2019, https://doi.org/10.1029/2019JA026641) to quantify how the dipole tilt angle influences lobe convection cells. We also describe a relation bridging other representations of the ionospheric convection electric field or potential to the Spherical Elementary Convection Systems description, enabling a similar separation of convection sources from existing models.

Description: Abstract We analyzed new recordings of SPdKS seismic waveforms from a global set of broadband seismograms and horizontal tiltmeters from the Hi‐net array in Japan from 26 earthquakes in the Central American region. The anomalous waveforms are consistent with the presence of at least three ultralow‐velocity zones (ULVZs), on the core‐mantle boundary beneath northern Mexico and the southeastern United States. These ULVZs ring an area of high seismic wave speeds observed in tomographic models that has long been associated with past subduction. Waveform modeling using the PSVaxi method suggests that the ULVZs have S and P wave velocity decreases of 40% and 10%, respectively. These velocity decreases are likely best explained by a partially molten origin where the melt is generated through melting of mid‐ocean ridge basalt atop the subducted slab.

Description: Abstract Sea level rise after the Last Glacial Maximum inundated several million square kilometers of Arctic permafrost, while estimates of organic carbon (OC) quantity and vulnerability to mineralization are exceedingly uncertain. We compiled geophysical measurements from Arctic continental shelves to estimate current subsea permafrost OC stocks. We found that marine transgression since the Last Glacial Maximum inundated approximately 3.92×106 km2 of permafrost, which contained 1,460±1,010 Pg OC in the top 25 m of sediment. We estimated that current subsea permafrost underlies an area of 2.30×106 km2 and contains 860±590 Pg OC, not including methane hydrates. Most of the ~600 Pg of OC that thawed after the marine transgression is still present on the continental shelves. Although our estimates of subsea OC storage remain highly uncertain due to the sparse and uneven distribution of data, they suggest that current estimates of subsea OC substantially underestimate a major component of the global carbon cycle.

Description: Abstract Using the bow shock crossing events from four spacecraft: IMP 8, Geotail, Magion‐4, and Cluster 1, a new three‐dimensional asymmetric bow shock model is constructed. The model is parameterized by the solar wind dynamic pressure, the interplanetary magnetic field, magnetosonic Mach number, solar wind β, and the Earth's dipole tilt angle. It is shown that the shape and size of bow shock are both affected by the dipole tilt angle. The dipole tilt angle causes asymmetries in the meridional plane: (1) the bow shock subsolar standoff distance and the north‐south asymmetry increase with the dipole tilt angle; (2) as the dipole tilt angle increases, the shock flaring angle in the equatorial plane is slightly reduced, while in the meridional plane the flaring angle obviously decreases in Southern Hemisphere and keeps almost unchanged in the Northern Hemisphere. The flaring angle in the Northern Hemisphere is larger than in the Southern Hemisphere; (3) the effects of negative dipole tilt angle on shock flaring are just the opposite of those for positive tilt, and the effects of dipole tilt angle on the shape of the bow shock are north‐south symmetric. The model results are also validated by comparing with one previous empirical model and with observational crossings, and it is demonstrated that the new model is able to predict the observed crossings more accurately and can better describe the rotational asymmetry and north‐south asymmetry of the Earth's bow shock.

Description: Abstract MMS3 spacecraft passed the vicinity of the electron diffusion region of magnetotail reconnection on 3 July 2017, observing discrepancies between perpendicular electron bulk velocities and drift, and agyrotropic electron crescent distributions. Analyzing linear wave dispersions, Burch et al. (2019, https://doi.org/10.1029/2019GL082471) showed the electron crescent generates high‐frequency waves. We investigate harmonics of upper‐hybrid (UH) waves using both observation and particle‐in‐cell (PIC) simulation, and the generation of electromagnetic radiation from PIC simulation. Harmonics of UH are linearly polarized and propagate along the perpendicular direction to the ambient magnetic field. Compared with two‐dimensional PIC simulation and nonlinear kinetic theory, we show that the nonlinear beam‐plasma interaction between the agyrotropic electrons and the core electrons generates harmonics of UH. Moreover, PIC simulation shows that agyrotropic electron beam can lead to electromagnetic (EM) radiation at the plasma frequency and harmonics.

Description: Abstract We study, numerically, the behavior of capillary pressure (Pc) during slow immiscible displacement in a rough fracture as a function of the degree of fracture aperture heterogeneity that results from two distinct mechanisms: normal confining stress and fracture surface correlation. We generate synthetic self‐affine rough fractures at different correlation scales, solve the elastic contact problem to model the effect of confining stress, and simulate slow immiscible displacement of a wetting fluid by a nonwetting one using a modified invasion percolation model that accounts for in‐plane curvature of the fluid‐fluid interface. Our modeling results indicate that the power spectral density, S(f), of Pc, can be used to qualitatively characterize fracture aperture heterogeneity. We show that the distribution of forward avalanche sizes follows a power law , with exponent α=2, in agreement with previously reported values for porous media and equal to the expected theoretical exponent for a self‐organized criticality process.

Description: Abstract In this letter, detailed evolution process of parallel electromagnetic ion cyclotron waves in the inner magnetosphere has been investigated through quasilinear theory. A new saturation has been found to occur after the usual first saturation. During the interval between these two saturations, the energy transfers from H+ band to He+ band electromagnetic ion cyclotron waves. Moreover, through a best fitting, we obtain new model parameters for the anisotropy‐beta inverse relation of hot H+, which identifies the threshold of ion cyclotron instabilities in the inner magnetosphere. In situ observations of the Van Allen Probe mission also verify these new model parameters. Therefore, our results reveal the evolution process and saturation characteristics of parallel electromagnetic ion cyclotron waves in the inner magnetosphere.

Description: Abstract The processes that accompany the death of an oceanic plate, as a ridge nears a trench, remain enigmatic. How the plate might reorganize, fragment, and eventually be captured by one of the bounding plates are among the unresolved details. We present a tomographic model of the Pacific Northwest from onshore and offshore seismic data that reveals a hole in the subducted Juan de Fuca plate. We suggest that this hole is the result of a tear along a preexisting zone of weakness, is causing volcanism on the North American plate, and is causing deformation in the Juan de Fuca plate offshore. We propose that in the final stages of an oceanic plate's life, deformation on the surface can be driven by deeper dynamics and that the fragmentation and the eventual capture of oceanic plate fragments may be governed by a process that operates from the bottom up.

Description: Abstract Observations by the Lightning and Airglow Camera on Japan's Venus Climate Orbiter “Akatsuki” over its first 3 years in orbit are reported. Forty‐two opportunities during low‐altitude nightside passes have accumulated 16.8 hr of observation, yielding an area‐time product of 81.6 ×106 km2‐hr, by far the largest at Venus itself to date. No flashes attributable to lightning have been detected, whereas similar observations at Earth would yield thousands of detections. A low flash rate of ~0.005 per million km2‐hr indicated in ground‐based observations is not excluded (but would require that there are not many more smaller flashes). The allowable flash rate is incompatible with the much higher rates of bursts recorded by magnetic and electric field sensors at Venus, indicating that electrical discharges at Venus lack optical emission or that the electromagnetic detections have a nonlightning explanation or both.

Description: Abstract Retrievals of nitrogen dioxide (NO2) and other trace gases from satellite measurements rely on accurate calculation of an air mass factor (AMF) to account for the atmospheric light path. Scattering and absorption of sunlight by aerosols affects AMFs by impacting the sensitivity of satellite‐observed radiances to NO2 at different altitudes. Current NO2 retrievals either do not explicitly account for these effects or rely on aerosol information from an external source. Here we investigate a method for quantifying the impact of aerosols on NO2 AMFs using the Absorbing Aerosol Index, a satellite‐based measure of light absorption and scattering by aerosols. We find a robust relationship between the Absorbing Aerosol Index and the aerosol correction to NO2 AMFs using the GEOS‐Chem chemical transport model and the LIDORT radiative transfer model. This relationship enables estimating the impact of aerosols on AMFs using observed Absorbing Aerosol Index values, thus yielding an observation‐based aerosol correction for NO2 retrievals.

Description: Abstract The Global Moving Hotspot Reference Frame (GMHRF) has been claimed to fit hot spot tracks better than the fixed hot spot approximation mainly because the GMHRF predicts ≈1,000 km southward motion through the mantle of the Hawaiian mantle plume over the past 80 Ma. As the GMHRF is determined by starting at present and calculating backward in time, it should be most accurate and reliable for the recent geologic past. Here we compare the fit of the GMHRF and of fixed hot spots to the observed trends of young tracks of hot spots. Surprisingly, we find that the GMHRF fits the data significantly worse (p = 0.005) than does the fixed hot spot approximation. Thus, either plume conduits are not passively advected with the mantle flow calculated for the GMHRF or Earth's actual mantle velocity field differs substantially from that calculated for the GMHRF.

Description: Abstract Submesoscale processes are key in understanding physical and biological phenomena near the surface, but there remains a lack of observational evidence over large areas. We used hourly images from a geostationary satellite that can resolve variation in surface ocean color over an area of few hundred kilometers. The temporal variation in the surface chlorophyll a distribution captured by the satellite images was first used to generate a submesoscale‐permitting velocity field, from which we calculated the turbulence statistics such as kinetic energy spectra, velocity structure functions, and energy flux. Application to the April scenes in the East/Japan Sea showed that the kinetic energy spectra had a transition scale at 50 km that suggested two spectral regimes following k−3 and k−5/3, implying the coexistence of quasi‐geostrophic turbulence and surface quasi‐geostrophic turbulence. The chlorophyll a scalar spectrum suggested two spectral regimes of k−5/3 and k−1 with a transition at 3 km.

Description: Abstract Although great advance has been made in glass science, predicting luminescence properties of laser glass poses a significant challenge for scientists due to the complex relationship between the composition, structure, and properties of the rare earth ions doped laser glasses. The development of high‐performance laser glass usually relies on intuition and trial‐and‐error. Recently, with the proposal of the materials genome engineering, the “glass genome” has also attracted much attention. Here, the structure of the Nd3+ doped B2O3‐Li2O laser glasses was analyzed using Fourier transform infrared spectra and nuclear magnetic resonance, revealing that the glass contains similar glass‐forming ion‐centered coordination polyhedron structure groups to the neighbor congruent glassy compounds. The structure and properties of glass largely depend on the neighbor congruent glassy compounds. Therefore, the structure and luminescence properties of Nd3+ doped B2O3‐Li2O and B2O3‐MgO‐Li2O laser glasses can be quantitatively predicted via the neighbor congruent glassy compounds. The predictive values are in good agreement with the experimental data, which indicates that our approach is an effective way to predict the structure and luminescence properties of Nd3+ doped borate laser glasses.

Description: Abstract High‐speed solar wind streams that originate from coronal holes play an important role in space weather disturbances, especially during the declining phase of the solar cycle. Space weather forecasters attempt to find good coronal hole indices that can be used to predict high‐speed streams days in advance. Several indices related to the coronal hole area, brightness, or magnetic field expansion factor have been reported in the literature. Empirical solar wind forecast models have been developed and used in operational service by several organizations by constructing prediction functions that relate the coronal hole index to the solar wind speed. In this paper, we present a new empirical modeling method and test its validity by comparing it with a previously reported method when applied to different coronal hole indices. In total, six empirical models are tested for a long period of time (2011–2018), with a 27‐day persistence model as a comparison benchmark. The results show that while all these empirical models can capture the temporal patterns of the solar wind observations well, the new modeling method and utilization of a composite coronal hole index PCH as an input parameter indeed improves the forecast accuracy. The high‐speed streams can be predicted approximately 3 days in advance, with a probability of detection of 0.78, a positive predictive value of 0.73, and a threat score of 0.61. The uncertainty of the high‐speed stream arrival time is approximately 1 day and the uncertainty of the peak speed is approximately 80 km/s.

Description: Abstract We describe a metric that has been repeatedly applied to assess the performance of models aimed at predicting geomagnetically induced currents from Space Weather events. The used parameterization, based on the well‐known root‐mean‐square error between model and observations, is simple and intuitive. Its use is exemplified, and its advantages and disadvantages are discussed, as well as its relationship with the widely extended correlation coefficient, r. Although the use of r alone is inappropriate for purposes of evaluating the agreement between model and observations, its use is recommended to complement the described performance parameter.

Description: Abstract Branching to sinuous ridges systems, hundreds of kilometers in length and comprising layered strata, are present across much of Arabia Terra, Mars. These ridges are interpreted as depositional fluvial channels, now preserved as inverted topography. Here we use high‐resolution image and topographic data sets to investigate the morphology of these depositional systems and show key examples of their relationships to associated fluvial landforms. The inverted channel systems likely comprise indurated conglomerate, sandstone, and mudstone bodies, which form a multistory channel stratigraphy. The channel systems intersect local basins and indurated sedimentary mounds, which we interpret as paleolake deposits. Some inverted channels are located within erosional valley networks, which have regional and local catchments. Inverted channels are typically found in downslope sections of valley networks, sometimes at the margins of basins, and numerous different transition morphologies are observed. These relationships indicate a complex history of erosion and deposition, possibly controlled by changes in water or sediment flux, or base‐level variation. Other inverted channel systems have no clear preserved catchment, likely lost due to regional resurfacing of upland areas. Sediment may have been transported through Arabia Terra toward the dichotomy and stored in local and regional‐scale basins. Regional stratigraphic relations suggest these systems were active between the mid‐Noachian and early Hesperian. The morphology of these systems is supportive of an early Mars climate, which was characterized by prolonged precipitation and runoff.

Description: Abstract The Surface Water Ocean Topography (SWOT) satellite mission is planned for launch in 2021. It will use the technique of radar interferometry to measure sea surface height over a 120‐km‐wide swath with a 20‐km gap around the satellite's nadir track. The oceanographic objectives of the mission are to study ocean circulation at scales down to 15 km. To prepare for the evaluation of the mission's performance, we are undertaking a series of studies to explore the efficacy of an assimilative high‐resolution modeling system for estimating the state of the ocean based on independent observations from both spaceborne and in situ measurements. The system is based on the heritage of a multiscale approach to data assimilation by the Regional Ocean Modeling System. Observing System Simulation Experiments were first conducted in the setup of an identical twin experiment to assess the system's performance near the calibration/validation site of SWOT off the coast of California. The system was applied to a nested model domain with 1‐km resolution. Simulated satellite observations of SSH, sea surface temperature, salinity, in situ observations of upper ocean temperature, and salinity by profiling floats and a dedicated notional array of station‐keeping gliders were assimilated by the system. The results indicate that such an observing system can accurately estimate the state of the ocean, and in particular SSH for the evaluation of SWOT performance.

Description: Abstract On 20 April 2013, an Mw 6.6 Lushan earthquake occurred on the southwestern segment of the Longmen Shan fault belt, which is the tectonic block boundary between the eastern Tibetan plateau and the Sichuan basin. Seismic reflection profiles and aftershock relocation indicate that there exists a back thrust fault in the source region but whether it is ruptured during the Lushan earthquake remains controversial. Here the precise leveling data are firstly used together with Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), and strong motion data to invert for the fault geometry and slip distribution associated with the earthquake. The joint inversion result shows that the Lushan earthquake occurred on a blind thrust fault with strike N208.5 °E and dip 42.1° to the NW and did not rupture the back reverse fault. The coseismic slip model reveals the Lushan earthquake involves the rupture of one major asperity. The coseismic slip is mainly concentrated on a steeply dipping fault plane. The coseismic rupture terminates on the southwestern side of the seismic gap between the Wenchuan and Lushan earthquakes. Topographic stress may be the dominant mechanism of coseismic rupture termination.

Description: Abstract Experimental data show that inelastic straining occurs even at very low pressure before and during “brittle” fracturing. This process is therefore investigated within the framework of elastoplasticity using 2D, 3‐layer FD modeling. The constitutive model includes both tensile and shear failure mechanisms coupled at the level of the strain softening law. The modeling results show that sets of parallel joints initiate as pure dilation bands, the narrow σ3‐normal bands of localized dilatant damage (inelastic deformation). The band thickness, length, and the initial strain softening degree within it are proportional to the ductility of the material, which increases with the effective stress level (σ1) or pressure. The strength reduction within the bands is accelerated at a certain stage, and the strength locally reaches zero resulting in fracture initiation. The initial fracture then propagates in mode I following the propagating band. The fracture (joint) appears thus as a band of damaged material with the increased porosity, which is maximum along the axial zone of the band where the material is completely broken. The damage is due to both tensile and shear mechanisms. The role of shear failure increases with the ductility (pressure) increase, which also leads to the band thickness increase. These processes can result in small (band thickness)‐scale shear fractures within the band, causing the increase in the roughness of fracture walls organized in plumose patterns typical of both natural and experimentally generated joints.

Description: Abstract With simultaneous ionospheric measurements from ROCSAT‐1 Satellite and ground ionosondes/GPS receivers, three cases of concurrent plasma blobs and bubbles in the same magnetic meridian were observed around 22:30 LT in Asian‐Oceanian sector during solar maximum. Two cases were observed Equatorial Spread F (ESF) over Vanimo station (geog. 2.7°S, 141.3°E; geom. 11.2°S, 146.2°W) and plasma blobs around 8.0°S (geom.) on June 1 and October 6, 2003. The other case observed ESF over Hainan station (geog. 19.5°N, 109.1°E; geom. 9.1°N, 179.1°W) and plasma blob near the dip equator on March 8, 2004. Plasma blobs were all observed near 600 km height near the equator. ESF and amplitude scintillations from the ground stations were observed near the same magnetic meridian, indicating the existence of bubbles. Considering that both plasma bubbles and blobs are field‐aligned elongated structures, magnetic field line mapping shows that in the two cases at Vanimo, blobs were above bubbles, providing direct observational evidence for blob formation in the intermediate stage of plasma bubble evolution; in the case at Hainan the blob and bubble were likely at similar height, and it could be generated by gravity wave.

Description: Abstract The magnetic field records of the magnetometer networks in the American, East Asian‐Australian, and European‐African sectors were employed in this present work. We used them to investigate equatorial electrojet (EEJ), counter electrojet (CEJ), tidal variability in EEJ strength and ionospheric current during the 2005/2006 and 2008/2009 sudden stratospheric warming (SSW) events. In addition to the well‐investigated tidal variability in EEJ strength over the American and East Asian sectors, we investigated that of the African sector for the first time. Interestingly, the tidal components in EEJ strength during both SSW events clearly exhibit marked longitudinal differences with high, moderate, and low amplitudes in the American, East Asian, and African sectors, respectively. An exception found around day 71 in the African sector after the 2008/2009 SSW event had higher solar diurnal tidal component as compared to that of the Asian sector. Over the American sector, solar and lunar semidiurnal tides were strongly associated with CEJ current during both SSW events, whereas at the African and East Asian sectors such variabilities are not evident. A solar diurnal tidal component was strongly related to a reduction in the EEJ strength over the East Asian sector. In addition, a prolonged period of CEJ occurrence that begins during the SSW precondition and ends when the SSW was evolving characterized the African sector during both SSW events. There is a steady shift in phase at later hours when both SSW events are evolving.

Description: Abstract A growing body of research has underscored the radiative impact of mineral dust in influencing Indian summer monsoon rainfall variability. However, the various aerosol‐cloud‐precipitation interaction mechanisms remain poorly understood. Here we analyze multisatellite observations to examine dust‐induced modification in ice clouds and precipitation susceptibility. We show contrasting dust‐induced changes in ice cloud regimes wherein despite a 25% reduction in ice particle radius in thin ice clouds, we find ~40% increase in ice particle radius and ice water path in thick ice clouds resulting in the cloud deepening and subsequently strengthened precipitation susceptibility, under strong updraft regimes. The observed dust‐ice cloud‐precipitation interactions are supported by a strong correlation between the interannual monsoon rainfall variability and dust frequency. This microphysical‐dynamical coupling appears to provide negative feedback to aerosol‐cloud interactions, which acts to buffer enhanced aerosol wet scavenging. Our results underscore the importance of incorporating meteorological regime‐dependent dust‐ice cloud‐precipitation interactions in climate simulations.

Description: Abstract The paleogeographic relationship between South China and Gondwana is critical for understanding the dispersion of Gondwana, accretion of Asia, and evolution of the Paleo‐Tethys. However, the lack of robust Devonian paleomagnetic data prevents a confirmative reconstruction of South China's connection to Gondwana and its subsequent separation during the Paleozoic. Here we report a new paleopole (33.6°N, 236.4°E; A95 = 3°) from the Givetian red beds (~385 Ma) in central South China. Fitting apparent polar wander paths between South China and Gondwana suggests that South China was connected to East Gondwana from the earliest Cambrian to Early Devonian, with its position closed to NW Australia. Thereafter, South China separated from Gondwana during ~400–385 Ma, as evidenced by their decoupled apparent polar wander paths. The paleomagnetic data suggest that the Paleo‐Tethys Ocean between South China and East Gondwana had been up to ~1,600 km latitudinally wide by ~360 Ma.

Description: Abstract Simulations of stratospheric aerosol geoengineering have typically considered injections at a constant rate over the entire year. However, the seasonal variability of both sunlight and the stratospheric circulation suggests seasonally dependent injection strategies. We simulated single‐point injections of the same amount of SO2 in each of the four seasons and at five different latitudes (30°S, 15°S, equator, 15°N, and 30°N), 5 km above the tropopause. Our findings suggest that injecting only during one season reduces the amount of SO2 needed to achieve a certain aerosol optical depth, thus potentially reducing some of the side effects of geoengineering. We find, in particular, that injections at 15°N or 15°S in spring of the corresponding hemisphere results in the largest reductions in incoming solar radiation. Compared to annual injections, by injecting in the different seasons we identify additional distinct spatiotemporal aerosol optical depth patterns, thanks to seasonal differences in the stratospheric circulation.

Description: Abstract An experimental investigation of droplet generation by a plunging breaking wave is presented. In this work, simultaneous measurements of the wave crest profile evolution and of droplets ranging in radius down to 50 μm for a mechanically generated plunging breaker during many repeated breaking events in freshwater are performed. We find three distinct time zones of droplet production, first when the jet impacts the free surface upstream of the wave crest, second when the large air bubbles entrapped by the plunging jet impact reach the free surface and burst, and third when smaller bubbles burst upon reaching the free surface later in the breaking process. These subprocesses account for 22%, 44%, and 34%, respectively, of the average of 653 droplets produced per breaking event. The probability distributions of the ranges of large and small droplet radii are well represented by power law functions that intersect at a radius of 418 μm.

Description: Abstract Modes of climate variability are known to influence rainy season onset, but there is less understanding of how they impact flood timing. We use streamflow reanalysis and gauged observation datasets to examine the influence of the Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) across sub‐Saharan Africa. We find significant changes in flood timing between positive and negative phases of both IOD and ENSO; in some cases the difference in the timing of annual flood events is more than 3 months. Sensitivity to one or other mode of variability differs regionally. Changes in flood timing are larger than variability in rainy season onset reported in the literature, highlighting the need to understand how the hydrological system alters climate variability signals seen in rainy season onset, length and rainfall totals. Our insights into flood timing could support communities who rely on flood‐based farming systems to adapt to climate variability.

Description: Abstract Fluid‐induced stress perturbations in the crust at seismogenic depths caused by sources such as tidal or seasonal loading may trigger earthquakes. We investigate the role of small periodic pore pressure (Pp) perturbation in rupture nucleation by performing laboratory triaxial creep experiments on Fontainebleau sandstone, saturated in water, under sinusoidal Pp variations. Results show that recorded acoustic emissions (AEs) correlate with Pp as the rock approaches failure. More interestingly, AEs occur significantly more when Pp is decreasing, that is, when strain rate is maximum with a progressive increase of Pp‐AEs correlation in time as the rock approaches failure. This suggests that the correlation of small stress perturbations and AEs not only depends on Pp amplitude but also on the criticality of the rock. Observations at the laboratory scale support field observations where tidal loading may have modulated seismic rates during the nucleation phase of the 2004 Sumatra‐Andaman and 2011 Tohoku‐Oki earthquakes.

Description: Abstract The Lunar Reconnaissance Orbiter/Lyman Alpha Mapping Project (LAMP) UV instrument detected a 0.5‐2% icy regolith mix on the floor of some of the southern pole permanently shadowed craters of the Moon. We present calculations indicating that most or all of this icy regolith detected by LAMP (sensed to a depth of 〈 1 micron) has to be relatively young – less than 2000 years old‐ due to the surface erosional loss by plasma sputtering (external ionized gas‐surface interactions), meteoric impact vaporization, and meteoric impact ejection. These processes, especially meteoric impact ejection, will disperse water along the crater floor, even onto warm regions where it will then undergo desorption. We have determined that there should be a water exosphere over polar craters (e.g., like Haworth crater) and calculated that a model 40 km diameter crater should emit ~1019 H2O/s into the exosphere in the form of free molecules and ice‐embedded particulates.

Description: Abstract A recent airborne study obtained extensive measurements in the tropical tropopause layer (TTL) over the western Pacific and provided the first opportunity to examine the relationship between water vapor and temperature in the coldest region and season of the TTL using high‐resolution in situ data. Analysis of this data set verifies key hypotheses in Lagrangian simulations of TTL transport and freeze drying. Furthermore, the observations provide a number of new insights into the transport process: In the layer below the lapse rate tropopause, vertical transport from upward motion dominates the relative humidity structure; final dehydration, dominated by large‐scale horizontal advection, occurs in the layer transacting the cold point tropopause that is often above the lapse rate tropopause, resulting in water vapor mixing ratios with corresponding frost points consistent with the coldest temperatures of the region, lower than the temperatures of the local cold points.

Description: Abstract We study the influence of the solar extreme ultraviolet (EUV) flux intensity on the precipitating ion fluxes as seen by the Solar Wind Ion Analyzer, an energy and angular ion spectrometer aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We defined three periods with significantly different EUV flux intensity (1.6 and 3.2 times the lowest EUV intensity) and compare the precipitating ion flux measured by MAVEN/Solar Wind Ion Analyzer during each period. At low energy [30–650] eV, we find that the median (average) precipitating ion flux during the medium and low EUV periods are, respectively, 1.7 (2.1) and 3 (3.5) times more intense than the flux during the high EUV period. At high energy [650–25,000] eV, a similar trend in the intensity of the precipitating ion flux is observed but with an increase by 50% (46%) and 70% (79%), respectively. A larger EUV flux does therefore not seem to favor heavy ion precipitation into Mars's atmosphere, contrary to modeling prediction and overall expectations.

Description: Abstract A single‐column model approach conducted in the context of the Madden–Julian Oscillation through the CINDY2011/Dynamics of the Madden–Julian Oscillation field campaign is used to disentangle the respective role of the parameterizations of surface turbulent fluxes and of model atmospheric physics in controlling the surface latent heat flux. The major differences between the models used in this study occur during the suppressed phases of deep convection. They are attributed to differences in model atmospheric physics which is shown to control the near‐surface relative humidity and thereby the surface latent heat flux. In contrast, during active phases of deep convection, turbulent air‐sea flux parameterizations impact the latent heat flux through the drag coefficient and can represent two thirds of the divergence caused by the different atmospheric physics. The combined effects need to be accounted for to improve both the representation of latent heat flux and the atmospheric variables used to compute it.

Description: Abstract Small, deep low‐frequency earthquakes (LFEs) with dominant frequencies of 2–8 Hz occur at depths of 20–40 km and are thought to be related to movement of magma or crustal fluid, but their physical source processes remain largely unknown. Therefore, we determine the focal mechanisms of LFEs beneath Zao volcano, Northeast Japan, using the S/P amplitude ratio. Most focal mechanisms are classified into five groups: three types of double couples, a compensated linear vector dipole group, and a single force group. Double couples in 2007–2012 are consistent with those expected for the regional stress field, but no such events have been observed since 2013. This transition in focal mechanisms was simultaneous with a rapid increase in LFE activity beneath Zao. Our results suggest that LFEs beneath Zao were controlled mainly by the local stress field, but the stress field changed about two years after the 2011 Tohoku earthquake.

Description: Abstract The sensitivity of gravity‐wave momentum flux in the Mei‐Yu front systems to moisture is investigated via idealized simulations with various degrees of initial moisture content. Gravity waves generated in moist experiments result in net northward momentum flux and drag forcing, and the drag is indistinctive in the lower stratosphere near the tropopause but strengthens with height. As moisture content increases, the meridional flux intensifies remarkably in both physical and spectral space and extends to smaller spatiotemporal scales. However, the change of moisture has little effect on the selectivity of the strongest flux to relatively large scales and specific phase speeds. At slow phase speeds, the fanlike waves excited by the front are effectively coupled with the convective waves excited by the prefrontal moist convection, which leads to the weakening of the coupled wave flux.

Description: ABSTRACT The subsurface media are not perfectly elastic, thus anelastic absorption, attenuation and dispersion (aka Q filtering) effects occur during wave propagation, diminishing seismic resolution. Compensating for anelastic effects is imperative for resolution enhancement. Q values are required for most of conventional Q‐compensation methods, and the source wavelet is additionally required for some of them. Based on the previous work of non‐stationary sparse reflectivity inversion , we evaluate a series of methods for Q‐compensation with/without knowing Q and with/without knowing wavelet. We demonstrate that if Q‐compensation takes the wavelet into account, it generates better results for the severely attenuated components, benefiting from the sparsity promotion. We then evaluate a two‐phase Q‐compensation method in the frequency domain to eliminate Q requirement. In phase 1, the observed seismogram is disintegrated into the least number of Q‐filtered wavelets chosen from a dictionary by optimizing a basis pursuit denoising problem, where the dictionary is composed of the known wavelet with different propagation times, each filtered with a range of possible values. The elements of the dictionary are weighted by the infinity norm of the corresponding column and further preconditioned to provide wavelets of different values and different propagation times equal probability to entry into the solution space. In phase 2, we derive analytic solutions for estimates of reflectivity and Q and solve an over‐determined equation to obtain the final reflectivity series and Q values, where both the amplitude and phase information are utilized to estimate the Q values. The evaluated inversion‐based Q estimation method handles the wave‐interference effects better than conventional spectral‐ratio‐based methods. For Q‐compensation, we investigate why sparsity promoting does matter. Numerical and field data experiments indicate the feasibility of the evaluated method of Q‐compensation without knowing Q but with wavelet given.

Description: Abstract Sea ice data assimilation can greatly improve forecasts of Arctic sea ice evolution. Many previous sea ice data assimilation studies were conducted without assimilating ocean state variables, even though the sea ice evolution is closely linked to the oceanic conditions, both dynamically and thermodynamically. Based on the method of a localized ensemble error subspace transform Kalman filter, satellite‐retrieved sea ice concentration and sea ice thickness are assimilated into an Arctic sea ice‐ocean model. As a new addition, sea surface temperature (SST) data are also assimilated. The additional assimilation of SST improves not only the simulated ocean temperature in the mixed layer of the ocean substantially but also the accuracy of sea ice edge position, sea ice extent, and sea ice thickness in the marginal sea ice zone. The improvement in the simulated potential temperature in the upper 1,000 m can be attributed to the enhanced vertical convection processes in the regions where the assimilated observational SST is colder than the simulated SST without assimilation. The improvements in the sea ice edge position and sea ice thickness simulations are primarily caused by the SST data assimilation reducing biases in the simulated SST and the associated coupled ocean‐sea ice processes. Our investigation suggests that, due to the complex interaction between the sea ice and ocean, assimilating ocean data should be an indispensable component of numerical polar sea ice forecasting systems.