ALBERT

Beschreibung: Abstract We integrate paleoseismic datasets along the Mt. Vettore‐Mt. Bove normal fault‐system (VBFS) rupturing at surface in the 30 October 2016 Norcia earthquake. Through the analysis of new trenches from this work and a review of the pre‐existing data, we correlate events among trench sites along antithetic and synthetic fault splays. We recognize seven M6.5, 2016 Norcia‐type (or larger) surface‐faulting events in the last ~22 kyr, including 2016. Before 2016, one event occurred in the past two millennia (260‐575 CE), and possibly corresponds to the event damaging Rome in 443 CE or 484/508 CE. Three previous events occurred between 10590 BCE and 415 BCE, whereas the two oldest ones date between 19820 BCE and 16540 BCE. The average recurrence time is 3360–3640 yrs for the last ~22 kyr, and 1220‐1970 yrs for the last ~4 kyr. We infer a minimum dip‐slip rate of 0.26‐0.38 mm/yr on the master fault in the central portion of the VBFS, and a dip‐slip rate of at least 0.10 mm/yr on the southernmost portion. We infer a Middle‐Late Pleistocene inception of the long‐term scarp of the investigated splays. The along‐strike variation of slip rates well reproduces the trend of the 2016 surface slip, thus the time window exposed in the trenches is representative for the present fault activity. Based on trenching data, different earthquake rupture scenarios should be also considered for local hazard assessment.

Beschreibung: Abstract Soot particles are generally considered to be poor ice nucleating particles. Involvement of soot in clouds and their release back into the atmosphere can form residual particles with altered cloud forming potential. The impact and extent of such different cloud processing scenarios on ice nucleation is however not well understood. In this work, we present the impact of cloud processing of soot aerosols on subsequent ice nucleation cycles at T ≤ 233 K. Coupling of two continuous flow diffusion chambers allows the simulation of different cloud processing scenarios and investigation of subsequent ice nucleation activity of the processed particles. The processing scenarios presented here encompass contrail, cirrus and mixed‐phase cloud processing, mimicking typical pathways that soot particles can be exposed to in the atmosphere. For all scenarios tested, the processed particles showed an enhanced ice active fraction for T 〈 233 K. The relative humidity with respect to water for the ice nucleation onset was observed to be on average approximately 10% (relative humidity with respect to ice, RHi ≈ 16 %) lower for the cloud processed particles compared to the unprocessed soot, for which ice nucleation was observed close to or at homogeneous freezing conditions of solution droplets. We attribute the enhanced ice nucleation abilities of the cloud processed soot to a pore condensation and freezing mechanism and have identified key parameters governing these changes. Enhanced ice nucleation abilities of soot in cirrus clouds can have significant impacts, given the importance of the atmospheric ice phase for precipitation formation and global climate.

Beschreibung: Abstract The mobile south flank of Kīlauea Volcano hosts two normal fault systems, the Koa'e fault system (KFS) and the Hilina fault system (HFS). In historical time, at least three M〉6.5 earthquakes have occurred on the basal detachment of the Kīlauea Volcano's south flank, with the most recent being the May 4, 2018 M6.9 earthquake. Here we analyze kinematic GPS data collected from 2001 to 2017, and InSAR data before, during and after the 2018 M6.9 earthquake to determine the crustal motion across the HFS and KFS faults. Our results indicate that the HFS faults did not significantly slip during the interseismic period from 2007 to 2011. Despite its substantial magnitude, InSAR shows that the 2018 M6.9 earthquake triggered sub‐cm level slip along sections of the previously mapped HFS branches. Up to 20 cm of offset occurred on what appears to be a newly formed (or previously unknown) fault near the eastern end of the HFS. During the 3 months following the M6.9 earthquake, up to more than 30 cm of slip occurred along the KFS, which helps accommodate rapid large‐scale subsidence of Kīlauea's summit region as large volumes of summit reservoir magma fed the lower East Rift Zone eruption. The HFS appears to activate only in concert with large earthquakes on the basal detachment. The KFS, on the other hand, moves both seismically during small local earthquakes, and aseismically in response to nearby earthquakes and caldera subsidence.

Beschreibung: Abstract Correlations within and between Precambrian basins are heavily reliant on precise dating of volcanic units (i.e., tuff beds and lava flows) in the absence of biostratigraphy. However, felsic tuffs and lavas are rare or absent in many basins and direct age determinations of Precambrian basaltic lavas have proven to be challenging. In this paper, we report the first successful application of 40Ar/39Ar dating to pyroxene from a Neoproterozoic basalt unit, the Keene Basalt in the Officer Basin of central Australia. 40Ar/39Ar analyses of igneous pyroxene crystals yielded an age of 752 ± 4 Ma (MSWD = 0.69, probability = 72%), which is underpinned by 40Ar/39Ar plagioclase age (753.04 ± 0.84 Ma) from the basalt. This age is significant because the Keene Basalt is one of the very few extrusive igneous rocks identified within the Neoproterozoic successions of central Australia, and is potentially an important time marker for correlating the Neoproterozoic stratigraphy within, and beyond, the central Australian basins. Our geochronological and geochemical data show that the Keene Basalt, which is characterized by enriched elemental and Nd‐Pb isotopic signatures, is strikingly similar to, and coeval with, the 755 ± 3 Ma Mundine Well Dolerite in northwestern Australia. Here, we suggest that both are part of the same large igneous province (~6.5 × 105 km2) related to breakup of the supercontinent Rodinia. This study demonstrates the potential of pyroxene 40Ar/39Ar geochronology to date ancient flood basalts, and to provide pivotal time‐constraints for stratigraphic correlations of Precambrian basins.

Beschreibung: Abstract Brucite, Mg (OH)2, is an important analog for studying the thermodynamics of hydrous silicate minerals in the deep Earth, as well as H/D isotope fractionation between minerals and water. In this study, we measured in situ Raman and Fourier transform infrared spectra for the natural and deuterated brucite samples, at high temperatures to 650 K, just before the dehydration of brucite at ambient pressure. All of the optical modes systematically shift to lower frequencies at elevated temperature, while deuterium substitution reduces the magnitudes of the temperature dependence. The isobaric mode Grüneisen parameters (γiP), as well as the intrinsic anharmonic parameters (ai), have been evaluated for the vibrational modes between Mg (OH)2 and Mg (OD)2. The anharmonic contribution to the thermodynamic properties (such as internal energy, isochoric and isothermal heat capacities, and entropy) is negative and severe at high temperature. The difference in the heat capacity is up to ~7% at 700 K due to the anharmonic effect. The deuterium isotopic effect on the thermodynamics is positive, and the magnitude of the isotopic effect is comparable to that from the anharmonic effect. On the other hand, the anharmonicity significantly increases the magnitude of the positive pressure dependence of the D/H fractionation β factor for brucite, and this correction could be more important at elevated temperature. At the temperature of 800 K, 103·(∂lnβ/∂P)T increases from +0.23 GPa−1 (for quasi‐harmonic approximation) to +0.44 GPa−1, due to the anharmonic correction.

Beschreibung: No abstract is available for this article.

Beschreibung: Abstract To evaluate the effect of melt viscosity on bubble nucleation, we formulated the homogeneous nucleation rate of water bubbles to explicitly include melt viscosity. The viscosity coefficient appears in the preexponential factor of the nucleation rate in terms of the Péclet number: the ratio of the bubble growth timescale by molecular diffusion and the viscous relaxation timescale. The preexponential factor is almost constant when viscosity is low (or a high Péclet number), whereas it linearly decreases with increasing viscosity (or a decreasing Péclet number) exceeding the crossover value of viscosity, under a given supersaturation. The crossover point depends on whether homogeneous or heterogeneous nucleation takes place. We numerically solved the evolution of bubble nucleation and growth processes in ascending magmas by using the new nucleation rate formula and a precise approximation of moment equations of the bubble size distribution function. The resultant bubble number density has two regimes, similar to the previous study, but the transition point between the diffusion‐controlled regime and the viscosity‐controlled regime moves to higher viscosity or higher decompression rates by 0.6 log units at the maximum. In the viscosity‐controlled regime, the effect of the better approximation of bubble size distribution moment equations reduces bubble number density by a few orders of magnitude compared with the previous study. As a result of compiling the past laboratory experimental data, it turned out that all the experiments are conducted under the conditions equivalent to the diffusion‐controlled regime. We propose an experimental condition to confirm the presence of the viscosity‐controlled regime.

Beschreibung: Abstract Seismic observations suggest (1) significant accumulation of subducted slabs above the 670‐km discontinuity in many subduction zones, (2) possible structure change at ~1,000‐km depth, and (3) the large low shear wave velocity provinces above the core‐mantle boundary in the African and Pacific lower mantle be associated with chemical heterogeneity. Global mantle convection models with realistic plate motion history reproduce most of these structures. However, it remains unclear how the convection models compare with seismic models at different spatial wavelengths and depths. By conducting quantitative analysis between mantle convection and seismic models, we found that mantle convective structures show significant correlations with seismic structures in the upper mantle and mantle transition zone for wavelengths up to spherical harmonic degree 20. However, the global correlation is weak at intermediate to short wavelengths (for degrees 4 and higher) in the lower mantle below ~1,000‐km depth. A weak layer beneath the spinel‐to‐postspinel phase change help consistently reproduce stagnant slabs in the western Pacific, while having insignificant effects elsewhere, that is, the large low shear wave velocity province structures. The cold slab structures and their correlations with the seismically fast anomalies are nearly identical for our convection models with and without the plumes, indicating that seismically fast anomalies in the mantle mainly result from the subducted slabs. Models with viscosity increase at 1,000‐km depth and the 670‐km depth phase change may reproduce seismic slab structures including the stagnant slabs in the mantle transition zone equally well as models with a thin weak layer below the 670‐km phase boundary.

Beschreibung: Abstract Based on a successful cloud‐resolving simulation with the Weather Research and Forecasting Model, this study examines the evolution and the role of midtropospheric mesoscale cyclonic vortex in the formation of Super Typhoon Nepartak (2016). The midtropospheric vortex is correlated with the convective activity in pre‐Nepartak. Once the deep convection outbreaks, the midtropospheric vortex intensifies first via the vertical advection associated with the severe updrafts and then through the midlevel convergence associated with stratiform precipitation. As the stratiform precipitation dissipates, the midlevel vortex weakens slightly in the following shallow convection phase. The above‐described processes recur sequentially during the pregenesis of Nepartak, and the midtropospheric vortex demonstrates diurnal variations. Its intensification usually corresponds to the weakening of low‐level cyclonic circulation except for the deep convection phase, indicating that the development of midtropospheric vortex can inhibit the development of self‐sustained low‐level cyclonic circulation. Although the midtropospheric vortex is not always a quasi‐balanced perturbation, a cold core can be found in the lower troposphere below it during the most of the pregenesis stage. The appearance of the cold core enhances the low‐level temperature gradient around it, which favors convection burst. In addition, the closed cyclonic circulation associated with the midlevel vortex can serve as a pouch protecting the vorticity, moisture, and convection inside from the vertical wind shear and dry air intrusion when the low‐level and midlevel vortices are overlapped in the late pregenesis stage, which facilitates the sustained deep convection and the formation of Nepartak.

Beschreibung: Abstract The dynamical behavior of the mesosphere and lower thermosphere (MLT) region during strongly disturbed wintertime conditions commonly known as polar‐night jet oscillations (PJOs) is described in detail and compared to other wintertime conditions. For this purpose, wind measurements provided by two specular meteor radars located at Andenes (69°N, 16°E) and Juliusruh (54°N, 13°E) are used to estimate horizontal mean winds and tides as an observational basis. Winds and tidal main features are analyzed and compared for three different cases: major sudden stratospheric warming (SSW) with (a) strong PJO event, (b) non‐PJO event, and (c) no major SSWs. We show that the distinction into strong PJOs, non‐PJOs, and winters with no major SSWs is better suited to identify differences in the behavior of the mean winds and tides during the boreal winter. To assess the impact of the stratospheric disturbed conditions on the MLT region, we investigate the 30‐year nudged simulation by the Extended Canadian Middle Atmosphere Model. Analysis of geopotential height disturbances suggests that changes in the location of the polar vortex at mesospheric heights are responsible for the jets observed in the MLT mean winds during strong PJOs, which in turn influence the evolution of semidiurnal tides by increasing or decreasing their amplitudes depending on the tidal component.

Beschreibung: Abstract Forced by Pacific Decadal Oscillation‐related sea surface temperature (SST) anomalies with the same pattern but opposite signs in the western‐central North Pacific, nonlinear wintertime atmospheric responses are produced by a state‐of‐the‐art atmospheric general circulation model (GFDL AM2.1); that is, an obvious equivalent barotropic geopotential low appears over the cold SST forcing (“CSST”), whereas a weak baroclinic structure shows up corresponding to the warm SST forcing (“WSST”), and both of them have similar characteristics in the lower troposphere. Specifically, because of the relatively dry environment in the central North Pacific, nonlinear responses of moisture process including latent heat flux and low‐level atmosphere moisture advection induce asymmetric diabatic heating (Qd): in WSST, Qd tends to increase in the middle‐lower troposphere but decrease in the middle‐upper level, whereas it always increases in the whole troposphere in CSST. Thus, Qd has the same low‐level positive vertical gradient in both CSST and WSST, which produces similar atmospheric circulation anomalies in the lower troposphere. In turn, the asymmetric responses of low‐level temperature advection further modify air temperature meridional gradient as well as atmospheric baroclinicity in the lower troposphere, significantly shifting the transient eddy activities southward in CSST and greatly weakening their intensity in WSST, respectively. Accordingly, the transient eddy vorticity forcing primarily determines the upper‐level atmospheric responses in CSST, but it has unsystematic effects in WSST that are overtaken by Qd. Therefore, the dominance of diabatic heating in WSST and transient eddy forcing in CSST over the central North Pacific lead to the asymmetric atmospheric responses among which the asymmetry of moisture plays an essential role.

Beschreibung: Abstract Thirty southerly low‐level jet (LLJ) events were observed during the Plains Elevated Convection at Night (PECAN) field campaign in the Great Plains region of the United States during summer 2015. Here we present Doppler lidar wind data from three PECAN instrumentation sites to explore characteristics of LLJs and the boundary layer as well as some of the heterogeneities possible within the wind field of a LLJ. Southerly LLJs were observed on 66% of nights at the southwestern site (Greensburg, KS) but only 56% and 53% of nights at the eastern and northern sites, respectively (Hesston and Ellis, KS). The northernmost site had a relative abundance of weaker jets or nonjet conditions due to fronts or convective systems that only affected part of the observation domain. Plotting mean wind fields of each LLJ type reveals that the strongest LLJs tend to develop under very similar conditions but begin to show variability in wind profile evolution after several hours. A robust mixed layer height retrieval algorithm is used to investigate the interplay between the jets and the turbulent convective boundary layer, showing that stronger LLJs are preceded by deeper afternoon mixed layers and often have a later decoupling of mixing between the upper convective mixed layer and the near‐surface layer. Only the strongest LLJs generated a shallow mixing layer overnight. Comparing jet strength and direction to pristine nocturnal convection initiation shows that the strongest southerly LLJs yielded the most pristine nocturnal convection initiation events per night, and the pristine nocturnal convection initiation occurred farther north.

Beschreibung: Abstract In this study, the interannual variations in the tropical cyclone (TC) over the western North Pacific (WNP) and the influences of regional sea surface temperature (SST) anomalies are documented by separating the WNP into four quadrants considering nonuniform SST‐induced environmental changes. Our analysis shows that the TC variations in the northwest and southeast quadrants are related to both equatorial central‐eastern Pacific Ocean (EPO) and tropical Indian Ocean (TIO) SST anomalies. The TC variation in the northeast quadrant is mainly related to tropical North Atlantic Ocean SST anomalies. The main environmental variables differ for the TC variations in the four quadrants. Lower‐level (850‐hPa) vorticity is important for the TC variations in the northwest, southwest, and southeast quadrants. Midlevel (700‐hPa) humidity contributes to the TC variations in the northwest, northeast, and southeast quadrants. The vertical shear has a supplementary contribution to the TC variation in the southeast quadrant. The potential intensity (PI) negatively affects the TC variations in the southwest and southeast quadrants. The remote SST anomalies modulate different environmental variables over the WNP. The TIO SST influence is manifested in the lower‐level vorticity and vertical motion. The tropical North Atlantic SST impact occurs through the lower‐level vorticity change. The EPO SST effect occurs via changing the lower‐level vorticity and vertical motion as well as the midlevel moisture and vertical shear. The environmental variables experience more prominent changes when SST anomalies coexist in two remote regions. Numerical experiments confirm the EPO and TIO SST anomaly impacts on the environmental conditions affecting the WNP TC variations.

Beschreibung: Abstract An anomalous “north‐south” dipole mode of the snow water equivalent (SWE) persisting from winter to spring is detected over the Eurasian mid‐to‐high latitudes in this study. Using observational data sets and numerical experiments of the Community Atmospheric Model (5.0), we show that this mode contributes to prolonged winter‐springtime coldness in midlatitude Eurasia and is closely linked to the declining November Arctic sea ice concentration. The decline in the sea ice concentration over the Barents‐Laptev Seas can induce a teleconnection pattern over the mid‐to‐high latitudes in the following winter, accompanied by an anomalous ridge over the Ural Mountains and an anomalous trough over Europe and East Asia. Such changes in the large‐scale circulation lead to more cold surges and heavy snowfall in the midlatitudes and light snowfall in the high latitudes, forming an anomalous north‐south dipole mode of the SWE, which further reduces the temperature through thermodynamic feedback. Due to seasonal memory, this SWE pattern can persist into the following spring and can lead to springtime midlatitude coldness via thermodynamic and dynamic processes. For the thermodynamic process, the anomalous SWE condition can lead to anomalous wet soil, reduced incoming surface solar radiation, and cooling air in the midlatitudes. This phenomenon induces an enhanced Siberian High and a deepened East Asian trough via the snow‐Siberian high‐feedback mechanism, which favors a cold spring in northern East Asia. Further analysis suggests that an empirical seasonal prediction model based on the SWE can reasonably predict East Asian spring temperature anomalies.

Beschreibung: Abstract A complete and quantitative understanding of cumulus entrainment remains elusive, in part due to the difficulty of directly observing cloud entrainment rates. Multiple approaches to ground‐based observational retrieval of bulk fractional entrainment rates (ε) within cumuli have been developed, such as the parcel model by Jensen and Del Genio (JDG, 2006, https://doi.org/10.1175/JCLI3722.1) and Entrainment Rate In Cumulus Algorithm (ERICA) by Wagner et al. (2013, https://doi.org/10.1175/JTECH-D-12-00187.1). In this paper, a new cumulus entrainment retrieval based on a turbulent kinetic energy (TKE) similarity theory is presented. This method estimates ε based on only the environmental and subcloud conditions. By conducting large‐eddy simulations of a range of continental and maritime shallow cumulus convection cases as Observing System Simulations Experiments, the first numerical verification of the three retrieval methods is produced. These simulations consider a broad range of shallow cumulus environments along with variations of the numerical configuration. The diagnosed ε from these simulations is found to be robustly larger in cumuli over the ocean than in cumuli over land. For continental cumuli, the experiments also reveal a diurnal cycle with increasing ε in the late afternoon. These diagnosed ε serve as the “truth” against which the pseudo‐retrieved entrainment rates from several different implementations of each retrieval are verified. Overall, the simpler JDG and TKE retrievals outperform the more sophisticated ERICA method and better capture the sensitivity to continentality. Only the TKE method reproduces the diurnal variations in ε within continental cumuli. The mean error in the ε retrievals are between 20% and 30% for the TKE and JDG methods, but 50% for ERICA.

Beschreibung: Abstract The present study documents intraseasonal snow cover variations over western Eurasia and associated atmospheric processes using the latest Moderate Resolution Imaging Spectroradiometer/Terra daily snow cover product and National Centers for Environmental Prediction/National Center for Atmospheric Research atmospheric reanalysis. It is found that 9‐ to 30‐day variation dominates total intraseasonal snow cover variations over western Siberia. Composite analysis based on 69 positive snow events over western Siberia reveals that atmospheric circulation anomalies control the 9‐ to 30‐day snow variation over western Siberia. A zonal wave train associated with the North Atlantic Oscillation leads to the development of an anomalous cyclone over western Siberia. The associated anomalous ascending motion, anomalous water vapor convergence, and water vapor increase in the atmosphere provide a favorable condition for snowfall. The snowfall starts when anomalous ascending motion reaches the strongest. The maximum snow cover appears about 1 day after the peak of anomalous descending motion and water vapor flux divergence. The surface air temperature tends to vary out of phase with snow cover over western Siberia. Surface air temperature anomalies over western Siberia are contributed by horizontal advection and diabatic heating. The adiabatic heating has a damping effect in surface air temperature variation.

Beschreibung: Abstract With a six‐year (2009–2014) summer climate simulation using the Weather Research and Forecasting model at convection‐permitting resolution (4‐km grid spacing), the effects of microphysics parameterization (MP) schemes on precipitation characteristics are investigated in this study. The convection‐permitting simulations employ three popular MP schemes, namely, Lin (single‐moment bulk MP), Weather Research and Forecasting Single‐Moment 5‐class (one‐moment and mixed‐phased MP), and Thompson (two‐moment and mixed‐phase MP) scheme. By evaluating the simulations against the CMORPH, rain gauge (Station), and ERA‐Interim data, it is found that the convection‐permitting model reproduce well the summer precipitation amount and the associated large‐scale atmospheric circulations, which are insensitive to the choice of MP schemes. The simulations with three MP schemes overestimate the precipitation amount, especially over the Yangtze‐Huaihe River Valley. The overestimations may be due to the systematic biases, and cannot be significantly reduced by using different MP schemes. Moreover, all simulations capture well the major features of precipitation diurnal variations and their transition characteristics, but they significantly overestimate the precipitation frequency while underestimate the precipitation intensity. The analysis on the microphysical hydrometeors shows that the model‐simulated precipitation amount is considerably affected by the vertical profiles of solid hydrometeors, especially the snow and graupel particles. The Thompson scheme creates more snow particles and less graupel than the other schemes, while produces the least precipitation amount that best matches the CMORPH.

Beschreibung: Abstract Carbonaceous matter in the atmosphere has an important influence on climate change. Currently, the deposition of carbonaceous matter is one of the largest uncertainties in the climate system. This phenomenon is common in remote regions, such as the Himalayas and Tibetan Plateau. In this study, for the first time, we reported in situ measurements of wet and dry deposition rates of carbonaceous matter at three remote stations: Nam Co, Lulang, and Everest. The results showed that the annual wet deposition rates of water‐insoluble organic carbon (WIOC) and black carbon (BC) were 60.2 and 5.8 mg·m−2·year−1, 330 and 34.6 mg·m−2·year−1, and 47.0 and 2.6 mg·m−2·year−1 at the Nam Co, Lulang, and Everest stations, respectively. Seasonal variations in the wet deposition rates of WIOC and BC were controlled by precipitation amount and their atmospheric concentrations. In addition, the wet scavenging ratios of WIOC and BC at Nam Co Station were close to those observed in other remote areas. The total BC deposition at Nam Co Station (15.3 mg·m−2·year−1) was higher than that from chemical transport models, implying a dominant role of dry deposition of BC in the total deposition at this station and an urgent need to improve the aerosol deposition in models for the Himalayas and Tibetan Plateau. It was found that the deposition rates of carbonaceous matter in the Himalayas and Tibetan Plateau had large spatial variation; thus, high‐resolution models need to be applied in the future.

Beschreibung: Abstract It is important but difficult to measure the shortwave radiative forcing of the dust aerosols over land from satellite‐observed radiance because the inhomogeneous surface albedo varies in a large dynamic range. In this study, we proposed a satellite‐based equi‐albedo method to derive the dust aerosol shortwave direct forcing over land. In the method, an equal radiance at the top of atmosphere was assumed for the region with the similar surface albedo and the similar solar zenith angle. The aerosol optical depth (AOD) from Moderate Resolution Imaging Spectroradiometer and the shortwave radiance product from Clouds and Earth's Radiant Energy System were used to derive the dust aerosol radiative forcing. The dust storm events outbroken on 9 and 24 April 2010 in Taklimakan desert were selected as study cases. The mean dust shortwave direct forcing efficiency is −35.08 W/m2 per unit of dust AOD during the dust storm events. The results were validated with the calculated radiative forcing from Moderate Resolution Imaging Spectroradiometer AOD product by the radiative transfer model. It shows that the derived radiative forcing is well correlated with the simulated one. The mean difference is 10.57 and the standard deviation is 1.35. Moreover, uncertainty has been estimated. The regional mean‐directed radiative forcing due to dust are −28.98 ± 7.99 and −35.76 ± 10.61 W/m2 of these two cases directly from satellite observations. This research indicates that the proposed method is reliable and effective, which can be used to estimate the shortwave direct radiative forcing of the dust storm event.

Beschreibung: Abstract Orientations of natural fault systems are subject to large variations. They often contradict classical Coulomb failure theory as they are misoriented relative to the regional Andersonian stress field. This is ascribed to local effects of structural or stress heterogeneities and reorientations of structures or stresses on the long term. To better understand the relation between fault orientation and regional stresses, we simulate spontaneous fault growth and its effect on the stress field. Our approach incorporates earthquake rupture dynamics, viscoelastoplastic brittle deformation and a rate‐ and state‐dependent friction formulation in a continuum mechanics framework. We investigate how strike‐slip faults orient according to local and far‐field stresses during their growth. We identify two modes of fault growth, seismic and aseismic, distinguished by different fault angles and slip velocities. Seismic fault growth causes a significant elevation of dynamic stresses and friction values ahead of the propagating fault tip. These elevated quantities result in a greater strike angle relative to the maximum principal regional stress than that of a fault segment formed aseismically. When compared to the near‐tip time‐dependent stress field the fault orientations produced by both growth modes follow the classical failure theory. We demonstrate how the two types of fault growth may be distinguished in natural faults by comparing their angles relative to the original regional maximum principal stress. A stress field analysis of the Landers‐Kickapoo fault suggests that an angle greater than ∼25° between two faults indicates seismic fault growth.

Beschreibung: Abstract Multiyear droughts are a common occurrence in southwestern North America (SWNA), but it is unclear what causes these persistent dry periods. The ocean‐atmosphere conditions coinciding with droughts have traditionally been studied using correlation and composite methods, which suggest that cool conditions in the tropical Pacific are associated with SWNA droughts and warm conditions are associated with wet periods in SWNA. Nevertheless, the extent to which multiyear droughts are truly consistent with this paradigm remains unknown. This is, in part, because the temporal trajectory of ocean‐atmosphere conditions during these dry periods have not been sufficiently characterized. Here we examine the continuum of ocean‐atmosphere trajectories before, during, and after multiyear droughts in SWNA using observation‐based data and an ensemble of climate model simulations from the Community Earth System Model. An examination of sea surface temperature patterns at the beginning, middle, and end of SWNA droughts shows that an El Niño event tends to precede SWNA droughts, a cool tropical Pacific occurs during droughts, and central Pacific El Niño events end droughts. However, moderate El Niño events can occur in the middle of persistent droughts, so a warm tropical Pacific does not always end these dry periods. These findings are important for drought predictability and emphasize the need to improve simulations of the magnitude, life cycle, and frequency of occurrence of El Niño events.

Beschreibung: Abstract Earthquake ruptures dynamically activate coseismic off‐fault damage around fault cores. Systematic field observation efforts have shown the distribution of off‐fault damage around main faults, while numerical modeling using elastic‐plastic off‐fault material models has demonstrated the evolution of coseismic off‐fault damage during earthquake ruptures. Laboratory scale micro‐earthquake experiments have pointed out the enhanced high‐frequency radiation due to the coseismic off‐fault damage. However, the detailed off‐fault fracturing mechanisms, subsequent radiation and its contribution to the overall energy budget remain to be fully understood because of limitations of current observational techniques and model formulations. Here, we constructed a new physics‐based dynamic earthquake rupture modeling framework, based on the combined finite‐discrete element method (FDEM), to investigate the fundamental mechanisms of coseismic off‐fault damage, and its effect on the rupture dynamics, the radiation and the overall energy budget. We conducted a 2‐D systematic case study with depth and showed the mechanisms of dynamic activation of the coseismic off‐fault damage. We found the decrease in rupture velocity and the enhanced high‐frequency radiation in near‐field due to the coseismic off‐fault damage. We then evaluated the overall energy budget, which shows a significant contribution of the coseismic off‐fault damage to the overall energy budget even at depth, where the damage zone width becomes narrower. The present numerical framework for the dynamic earthquake rupture modeling thus provides the insight into the earthquake rupture dynamics with the coseismic off‐fault damage.

Beschreibung: Abstract Pulse‐like ruptures arise spontaneously in many elastodynamic rupture simulations and seem to be the dominant rupture mode along crustal faults. Pulse‐like ruptures propagating under steady state conditions can be efficiently analyzed theoretically, but it remains unclear how they can arise and how they evolve if perturbed. Using thermal pressurization as a representative constitutive law, we conduct elastodynamic simulations of pulse‐like ruptures and determine the spatiotemporal evolution of slip, slip rate, and pulse width perturbations induced by infinitesimal perturbations in background stress. These simulations indicate that steady state pulses driven by thermal pressurization are unstable. If the initial stress perturbation is negative, ruptures stop; conversely, if the perturbation is positive, ruptures grow and transition to either self‐similar pulses (at low background stress) or expanding cracks (at elevated background stress). Based on a dynamic dislocation model, we develop an elastodynamic equation of motion for slip pulses and demonstrate that steady state slip pulses are unstable if their accrued slip b is a decreasing function of the uniform background stress τb. This condition is satisfied by slip pulses driven by thermal pressurization. The equation of motion also predicts quantitatively the growth rate of perturbations and provides a generic tool to analyze the propagation of slip pulses. The unstable character of steady state slip pulses implies that this rupture mode is a key one determining the minimum stress conditions for sustainable ruptures along faults, that is, their “strength.” Furthermore, slip pulse instabilities can produce a remarkable complexity of rupture dynamics, even under uniform background stress conditions and material properties.

Beschreibung: Abstract Atmospheric rivers (ARs) can significantly modulate surface hydrological processes through the extreme precipitation they produce. However, there is a lack of comprehensive evaluation of ARs' impact on surface hydrology. This study uses a high‐resolution regional climate simulation to quantify the impact of ARs on surface hydrological processes across the western U.S. watersheds. The model performance is evaluated through extensive comparison against observations. Our analysis indicates that ARs produce heavy precipitation but suppress evapotranspiration. Snowpack ablates more during ARs, with higher air temperature and increased longwave radiation playing the primary and secondary roles, respectively. At the 0 °C to 10 °C temperature range, ARs increase the probability of snow ablation from 0.33 to 0.57. The runoff‐to‐precipitation ratio is primarily controlled by antecedent soil moisture, but it almost doubles in the northwestern watersheds due to the intensification of snow ablation during AR events. From the analysis of the relationship between the hydrological responses and different meteorological factors, precipitation, temperature, and radiation are identified as the key drivers that distinguish the hydrologic responses between AR and non‐AR events. Lastly, analysis of ARs and total runoff at annual scale and 1 April snowpack and winter precipitation shows that ARs explain 30% to 60% of the variability of annual total runoff and sharpen the seasonality of water resources availability in the west coast mountain watersheds.

Beschreibung: Abstract As a widespread landscape, rugged terrain significantly distorts the land surface albedo. Simply neglecting the topographic effects in the land surface albedo modeling and retrievals can lead to large biases and uncertainties over rugged terrain. In spite of gradually increasing research about the topographic effects on the albedo, the albedo sensitivities over rugged terrain to different variables remain unclear. In this paper, the sensitivities of coarse‐scale snow‐free albedo to topography were quantitatively investigated using the Moderate Resolution Imaging Spectroradiometer (MODIS) land surface albedo data and the variance‐based global sensitivity analysis based on a well‐established mechanistically‐based land surface albedo parameterized model over rugged terrain. The results based on the MODIS data revealed that MODIS land surface albedo over the Tibetan Plateau was highly sensitive to the topographic distribution, and the differences of the spatially averaged snow‐free black‐sky albedos over grasslands induced by different terrain could reach up to 0.10 in winter. The topographic effects on MODIS albedo are tightly relevant to the land cover type, solar illumination geometries, and vegetation characteristics. The global sensitivity analysis results underscored that topography was an important driving factor of the snow‐free albedo, and it accounted for more than 30% of the total variance, respectively. These results highlight the necessities for the topographic consideration in the land surface albedo modeling and retrievals even though the terrain is gentle (10–20°) and advance our understanding of the albedo sensitivities to different variables over rugged terrain, which will facilitate the improvement of land surface albedo parameterization in the land surface models.

Beschreibung: Abstract An anelastic numerical model is used to study the influences of fine structure (FS) in the wind and stability profiles on gravity wave (GW) propagation in the Mesosphere and Lower Thermosphere (MLT). Large amplitude GWs interacting with FS, that is, thin regions of enhanced wind and stability, evolve very differently depending on the precise vorticity source and sink terms for small‐scale motions induced by the FS gradients. The resulting small‐scale dynamics are deterministic, promoting local instabilities, dissipation, and momentum deposition at locations and orientations determined by the initial FS. The resulting momentum depositions yield significant changes to the background wind structure, having scales and amplitudes comparable to the effects of large‐scale features in the ambient atmosphere. The deterministic nature of the large‐scale impacts further suggests that they can be estimated without fully resolving the underlying instability dynamics. Given the significant amplitudes and ubiquitous occurrence of FS throughout the atmosphere, the influences of these important and diverse flow evolutions merit inclusion in broader modeling efforts.

Beschreibung: Abstract One unique long‐range transport event with multiple layers of aerosol plumes was observed over Taiwan during 29–31 March 2006. A synergy of ground‐based observation, remote sensing, and backward trajectory simulation collectively indicated the high‐altitude (above 3 km) plume originated from biomass burning in Southeast Asia while the midaltitude (around 0.8–2 km) plume was attributed to dust from the Gobi Desert. Aerosol optical properties measured at a low‐altitude site were characterized of abundant coarse mode particles and increased single scattering albedo as a function of increased wavelength, indicating the influence from dust particles. While at a high mountain site (elevation of ~3 km), aerosol optical depth was elevated by a factor of 3–4 compared to its background value and mainly comprised of fine particles. It was diagnosed that the high‐altitude aerosols were influenced by the transported smoke plumes but exempted from dust. Simulation of the meteorological conditions against a Taiwan‐wide meteorology network showed strong near surface temperature rise of more than 2° during this long‐range transport event as well as for the vertical temperature profiles. Both dust and biomass burning aerosol plumes via long‐range transport contributed significantly to the atmospheric warming, resulting in strong instantaneous aerosol radiative forcing of 46.0 W/m2 in the atmosphere. A “double dome” warming effect mechanism was proposed that both biomass burning and dust plumes above the boundary layer could efficiently reserve the solar energy and heat the lower troposphere.

Beschreibung: Abstract Accurate representation of cloud vertical overlap in climate models is particularly significant for predicting the total cloud fraction (TCF) and calculating radiative budget. It refers to the parameterization of overlap parameter—decorrelation length‐scale L, but the potential of dynamical factors in developing parameterization of L has still received far less attention. Using ground‐based radar observation over Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site, here long‐term seasonal‐averaged L is retrieved and shows a very high anti‐correlation with TCF from different datasets, indicating that TCF is sensitive to the way of cloud overlap. Therefore, combined with meteorological reanalysis dataset, a robust multiple regression model between L and dynamical factors is built, and exhibits smaller TCF bias compared with previous parameterization of L. Contribution calculation further verifies that atmospheric instability contributes 70% of L variation, indicating it dominates the long‐term variation of L over SGP site. This finding implies that dynamical factors should be taken into account in the parameterization of L.

Beschreibung: Abstract The Baiyun slide complex contains geological evidence for some of the largest landslide ever discovered in the continental slopes of the South China Sea. High‐resolution seismic data suggest that a variety of landslides with varied scales have occurred repeatedly in this area. The largest landslide reconstructed from bathymetric and seismic data has an estimated spatial coverage of ~5,500 km2 and a conservative volume of ~1,035 km3. Here, using geomorphological and geotechnical data, we construct a series of probable landslide scenarios and assess their tsunamigenic capacity. By treating the slides as deformable mudflows, we simulate the dynamics of landslide movements. The simulated landslide motions match the geophysical observations interpreted in previous studies. Particularly, we are able to reproduce the spatial distribution of observed runout, including the distance, shape, and deposit thickness, for the most credible slide scenario. We investigate tsunami impacts generated by different slide scenarios and highlight the importance of initial water depth, sliding direction, and nearshore bathymetry. The worst‐case scenario is capable of producing basin‐wide tsunami, with maximum wave amplitudes reaching ~5 m near Hong Kong and Macau, 1–3 m in western Philippines, and at least 1 m along central Vietnam, southeast Hainan, and southern Taiwan. The most noticeable phenomenon we observed is that the southern Chinese coast is the hardest‐hit region in all the simulated scenarios regardless of the diverse slide features. We conclude that the persistence of high tsunami impact is caused by the unique bathymetric feature of the wide continental shelf in front of southern China.

Beschreibung: Abstract Previous work has shown that the Madden‐Julian Oscillation (MJO) can influence the North Atlantic Oscillation (NAO) via a Rossby wave teleconnection that propagates through the troposphere (i.e., a tropospheric pathway). In addition, recent work suggests that the MJO can influence the stratospheric polar vortex, which is also known to influence the tropospheric NAO—thus, there likely exists a stratospheric pathway for MJO influence as well. Here, we apply two methods to shed more light on the pathways linking the MJO to the NAO. First, we use a traditional approach in climate science based on analyzing conditional probabilities. Second, we use methods from causal discovery theory based on probabilistic graphical models. Together, these two analysis approaches reveal that the MJO can impact the NAO via both a tropospheric and stratospheric pathway. The stratospheric pathway is shown to come about in two ways: First, both methods show that the MJO itself influences the strength of the stratospheric polar vortex on a timescale of ∼10 days, and then 5 days later the vortex can drive changes in the NAO. Second, the state of the stratospheric polar vortex acts to condition the NAO to be conducive (or not) to MJO influence. When the vortex is in a state that opposes the expected NAO response to the MJO, we find little influence of the MJO on the NAO, however, when the vortex supports the expected NAO response, the NAO is up to 30% more likely to be in a particular state following active MJO periods.

Beschreibung: Abstract ITSG‐Grace2018 is a new series of GRACE‐only gravity field solutions based on reprocessed GRACE observation data (L1B RL03) and the latest atmosphere and ocean dealiasing product (AOD1B RL06). It includes unconstrained monthly and constrained daily solutions, as well as a high‐resolution static gravity field. Compared to the previous ITSG release, we implemented a number of improvements within the processing chain and use updated background models. In an effort to better model all known error sources, we propagate synthetic orientation uncertainties of the star camera assembly to the antenna offset correction for intersatellite ranging observations. This enables the disentanglement of the stationary noise of the K‐Band system and the nonstationary noise of the antenna offset correction. We further incorporated uncertainties of the atmosphere and ocean dealiasing product to reduce temporal aliasing effects. To mitigate errors in the applied ocean tide model, we used constrained GRACE estimates of selected tidal constituents as an additional background model. Variability over quiet ocean areas suggests a 27% to 46% lower noise level compared to the current spherical harmonic solutions of the official processing centers (300 km Gaussian filter applied). To ensure that the low noise floor is not accompanied by signal loss, we examined drainage basin averages, which showed consistent amplitudes with the official GRACE time series. These evaluations lead to the conclusion that ITSG‐Grace2018 is a state‐of‐the‐art GRACE time series which exhibits an excellent signal‐to‐noise ratio.

Beschreibung: Abstract The northern Hikurangi plate boundary fault hosts a range of seismic behaviors, of which the physical mechanisms controlling seismicity are poorly understood, but often related to high pore fluid pressures and conditionally stable frictional conditions. Using 2D marine seismic streamer data, we employ full‐waveform inversion (FWI) to obtain a high‐resolution 2D P‐wave velocity model across the Hikurangi margin down to depths of ~2 km. The validity of the FWI velocity model is investigated through comparison with the pre‐stack depth migrated seismic reflection image, sonic well data, and the match between observed and synthetic waveforms. Our model reveals the shallow structure of the overriding plate, including the fault plumbing system above the zone of SSEs to theoretical resolution of a half seismic wavelength. We find that the hanging walls of thrust faults often have substantially higher velocities than footwalls, consistent with higher compaction. In some cases, intra‐wedge faults identified from reflection data are associated with low‐velocity anomalies, which may suggest they are high‐porosity zones acting as conduits for fluid flow. The continuity of velocity structure away from IODP drill site U1520 suggests that lithological variations in the incoming sedimentary stratigraphy observed at this site continue to the deformation front and are likely important in controlling seismic behavior. This investigation provides a high‐resolution insight into the shallow parts of subduction zones, which shows promise for the extension of modeling to 3D using a recently‐acquired, longer‐offset, seismic dataset.

Beschreibung: Abstract Studies of mechanical responses of the Earth crust to large earthquakes can provide us with unique insights into the processes of stress buildup and release. As a complement to geodetic methods that derive crustal strain dynamics from surface observations (e.g., GPS, InSAR), noise‐based seismic velocity monitoring directly probes the mechanical state of the crust, at depth and continuously in time. We investigate the responses of the crust to the Mw 9.0, 2011 Tohoku‐oki earthquake. In addition to the Hi‐net short‐period sensors, we use Hi‐net tiltmeters as long‐period seismometers (8–50 s) to sample the crust below 5 km in depth. The spatial distribution of the strong velocity decreases at short periods appears to be limited to the region of strong ground shaking induced by the 2011 Tohoku‐oki earthquake, while the long‐period velocity changes correlate well with the modeled static strain induced by viscoelastic relaxation and afterslip at depth. Amplitudes of coseismic velocity changes decrease with increasing depth. The temporal evolution of velocity changes in different period bands shows that the maximum drops in the velocity at long periods are delayed in time with respect to the occurrence of the Tohoku‐oki earthquake. The inversion of seismic velocity changes at depth illustrates how S wave velocities evolve down to 40 km at a regional scale after a major earthquake.

Beschreibung: Abstract Unconventional reservoirs comprise a growing portion of producible reserves due to increasing knowledge of their nature as well as significant advances in production technology. The development of advanced pore‐scale modeling techniques presents potential for better estimation of reservoir flow characteristics including relative permeability, saturation distributions, and capillary pressure. Although pore‐scale network models take into account the pore throat connections and the appropriate fluid properties, highly simplified pore cross‐sectional shapes are still employed when estimating the threshold capillary pressure for fluid‐fluid displacements in each pore element. As a result, there is a growing need for more realistic threshold capillary pressure estimates generated using pore geometries that honor the real pore topology. To this end, a semi‐analytical model is presented that allows the prediction of threshold capillary pressure as well as the capillary pressure vs. saturation relationship for piston‐like fluid displacements using images of unconventional reservoir rock samples. The model was validated on three different idealized pore geometries and compared against available analytical solutions, resulting in an error of less than 1% for all cases. The model was compared to experimental data using fluid occupancy maps obtained using an X‐ray nano‐CT scanner during an oil imbibition sequence into a miniature reservoir shale sample. The capillary pressure versus wetting phase saturation relationship was also determined for a 2D FIB‐SEM image slice. The presented model shows promise for enabling more advanced pore‐scale modeling of shale rock.

Beschreibung: Abstract The National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2) has a large cold bias in the model's deep soil temperature during summer. This study explores the potential triggering effect of that bias on excessive Eurasian snow cover in early winter. Snow cover appears erroneously early in the fall, especially in western Eurasia, in long simulations with CFSv2. The seasonal transition may be too early because the model land surface temperature (LST) reaches its freezing point earlier than observed, so that new snow cannot melt. This process initiates snow‐albedo feedback too early. The early cooling of LST is partially influenced by a seasonal resurfacing of the cold bias in the deep soil layer. From winter to early spring, a cold bias prevails in LST and upper soil temperature as snow cover remains. During this season, the temperature in the deep soil is generally warmer than in the upper soil and has relatively little bias. From spring to summer, the cold bias in the upper soil becomes smaller as it warms up in response to solar heating. On the other hand, the deep soil temperature has a noticeably smaller seasonal change than observed, resulting in a severe cold bias during summer. As the solar radiation declines quickly in early fall, the cold deep soil temperature causes additional cooling in the upper soil layer and helps to bring LST to the freezing point early in the western Eurasia, which leads to enhanced bias in the snow properties.

Beschreibung: Abstract Season‐dependent interannual variability of surface winds across Antarctica and the Southern Ocean is investigated using European Centre for Medium‐Range Weather Forecasts Interim (ERA‐Interim) reanalysis for 1979–2017 and the empirical orthogonal function (EOF) technique. The first three EOF modes, which account for 18–23%, 11–15%, and 8–10% of the total variances for the four austral seasons, are related to several known modes of Southern Hemisphere large‐scale circulation including the southern annular mode, Pacific South American (PSA), and Zonal Wave 3 (ZW3) and to sea surface temperature anomalies in eastern (Niño 3) and central (Niño 4) tropical Pacific Ocean. Specifically, EOF1 and EOF2 are significantly correlated with southern annular mode in all seasons except for autumn EOF2. Neither EOF1 nor EOF2 are significantly correlated to Niño 3, but Niño 4 is significantly correlated to winter EOF1 and to EOF2 in all seasons but winter. EOF1 is significantly related to ZW3 for all seasons but summer; however, no significant connection is found between EOF2 and ZW3. Significant correlations exist between spring and winter EOF2 and PSA1 and between spring EOF1 and PSA2. EOF3 is significantly correlated with Niño 3, Niño 4, and PSA2 in all seasons but summer. The spatial patterns of all three modes can be largely explained by the anomalous mean sea level pressure fields that strengthen or weaken climatological winds in different regions of the Antarctica and the Southern Ocean, although variations in katabatic forcing also plays a role in the variations of surface winds especially in East Antarctica and during winter season where and when katabatic forcing dominates.

Beschreibung: Abstract To investigate the mechanisms for the record‐breaking rainfall in the coastal metropolitan city of Guangzhou, China during 6–7 May 2017, budget analyses of advection and source/sink terms of the water vapor, potential temperature, and vertical momentum equations were conducted using the model output of a nested very large eddy simulation with the Weather Research and Forecasting model. Results show that the warm and moist air flows from the south and east onshore in the lower troposphere provided the main moisture source for the heavy rainfall. The structure of vertical velocity and hydrometeors (low‐echo centroid structure), in which the heavy rainfall was separated from the low‐level updraft, was favorable for the formation and maintenance of a heavy precipitation rate. The removal of the heat due to the advection (cooling tendency) in the upper troposphere increased the convective available potential energy of parcels rising from the lower troposphere, maintaining the development of updrafts. Although the total buoyancy forcing was the main contribution term for maintaining the updrafts, total dynamic acceleration played an important role in the vertical acceleration below the maximum vertical velocity core. In particular, the nonlinear dynamic perturbation pressure gradient force in the lower troposphere induced by the rotations aloft maintained the strong updrafts.

Beschreibung: Abstract We study the 3‐D P wave velocity structure of the crust and mantle down to 1,000‐km depth beneath the central and eastern United States. A 3‐D velocity model is obtained by conducting a joint inversion of 236,670 arrival times of local earthquakes and 870,455 relative traveltime residuals of teleseismic events recorded by the EarthScope/USArray Transportable Array. Significant low‐velocity (low‐V) anomalies are revealed in the crust beneath the eastern arm of the Midcontinent Rift and the Triassic Basins along the East Coast, whereas a prominent high‐velocity (high‐V) anomaly is visible beneath the Llano Uplift in central Texas. The stable North American Craton exhibits high‐V anomalies at depths of 65–250 km. Low‐V anomalies exist along the eastern and southern margins of the craton, which may reflect relatively thin lithosphere there. A prominent low‐V anomaly is revealed at depths of 50–200 km beneath the New Madrid Seismic Zone, which is bounded by high‐V anomalies to its southeast and northwest. This feature reflects a weak lithosphere surrounded by relatively strong cratonic regions and stress concentration caused intraplate seismicity in the New Madrid region. Two high‐V bodies appear in the mantle transition zone (410‐ to 660‐km depths) beneath the Interior Low Plateaus, the central Great Plains, and the Central Lowland, which may reflect the subducted Farallon plate or delaminated lithosphere. At depths of 800–1,000 km, a high‐V anomaly is visible beneath the southeast United States, which may be the subducted Hess Rise conjugate.

Beschreibung: Abstract The 24 May 2013 earthquake beneath the Sea of Okhotsk (610 km, Mw 8.3) produced significant ground motion across the whole span of the Japanese islands, from 1,300‐4,200 km epicentral distance. The largest shaking was concentrated along the back‐arc side of the subduction zone, which is the opposite of the normal pattern for deep earthquakes in the Pacific slab. Observations from the dense Hi‐net and F‐net arrays across Japan show that the largest shaking in northern Japan (near 2,000 km epicentral distance) was caused by near‐caustic S waves, with triplication of upgoing and downgoing waves from the deep source and reflected waves from the 660 km discontinuity. 3‐D finite‐difference‐method simulations confirm that the anti‐waveguide effect of the high‐wavespeed slab is to push the zone of larger intensity 300 km further to south than might be expected. The S wavefont distorted by the slab has near‐critical incidence at the free surface producing large sP and generating shear‐coupled PL (s‐PL) waves with period 〉3 s. With increasing epicentral distance the S incident angle exceeds critical, then total sS reflection creates large ground motion at large distance (〉3,000 km) and even further (〉6,000 km) with sSS. The propagation of sS, sSS linking to sS‐PL and sSS‐PL wavetrains is very efficient in continental structures with thicker crust. The felt reports at large (4,000‐8000 km) distances from the 2013 Sea of Okhotsk earthquake can be explained by lengthy, long‐period ground motion in the continental environment with amplification in sedimentary basins and in tall buildings.

Beschreibung: Abstract Global reanalysis products are extensively used for hydrologic applications in sparse data regions. ERA‐5, among the new‐era reanalysis products, has been significantly improved for horizontal and vertical resolutions and data assimilation. However, the new‐era reanalysis products (ERA‐5, CFSR, ERA‐Interim, MERRA‐2, and JRA‐55) have not been evaluated for the hydrologic applications in India specially to understand if ERA‐5 outperforms the other reanalysis products or not. Here, we use the five new‐era reanalysis products for the monsoon (June‐September) season precipitation, maximum (Tmax) and minimum (Tmin)temperatures, total runoff, evapotranspiration (ET), and soil moisture against the observations from India Meteorological Department (IMD) in India for 1980‐2018. We use a well calibrated and evaluated hydrological model [the Variable Infiltration Capacity (VIC) model] to simulate hydrologic variables using the forcing from IMD and reanalysis products. In addition, we evaluated the reanalysis products for streamflow and annual water budget for the two basins located in the diverse climatic settings in India. Our results show that ERA‐5 outperforms the other reanalysis products for the monsoon season precipitation, Tmax, ET, and soil moisture. However, CFSR performs better than ERA‐5 for the monsoon season total runoff in India. Performance for streamflow and annual water budget for ERA‐5 is either better or comparable to the other reanalysis products in the two river basins. Overall, we find that ERA‐5 performs better than the other reanalysis products and can be used for the hydrologic assessments in India.

Beschreibung: Abstract The Madden‐Julian oscillation (MJO) is the leading source of global subseasonal predictability; however, many dynamical forecasting systems struggle to predict MJO propagation through the Maritime Continent. Better understanding the biases in simulated physical processes associated with MJO propagation is the key to improve MJO prediction. In this study, MJO prediction skill, propagation processes, and mean state biases are evaluated in reforecasts from models participating in the Subseasonal Experiment (SubX) and Subseasonal to Seasonal (S2S) prediction projects. SubX and S2S reforecasts show MJO prediction skill out to 4.5 weeks based on the Real‐time Multivariate MJO index consistent with previous studies. However, a closer examination of these models' representation of MJO propagation through the Maritime Continent reveals that they fail to predict the MJO convection, associated circulations, and moisture advection processes beyond 10 days with most of models underestimating MJO amplitude. The biases in the MJO propagation can be partly associated with the following mean biases across the Indo‐Pacific: a drier low troposphere, excess surface precipitation, more frequent occurrence of light precipitation rates, and a transition to stronger precipitation rates at lower humidity than in observations. This indicates that deep convection occurs too frequently in models and is not sufficiently inhibited when tropospheric moisture is low, which is likely due to the representation of entrainment.

Beschreibung: Abstract Preparatory mechanisms accompanying or leading to nucleation of larger earthquakes have been observed at both laboratory and field scales, but conditions favoring the occurrence of observable preparatory processes are still largely unknown. In particular, it remains a matter of debate why some earthquakes occur spontaneously without noticeable precursors as opposed to events that are preceded by an extended failure process. In this study, we have generated new high‐resolution seismicity catalogs framing the occurrence of 20 ML 〉 2.5 earthquakes at The Geysers geothermal field in California. To this end, a seismicity catalog of the 11 days framing each large event was created. We selected 20 sequences sampling different hypocentral depths and hydraulic conditions within the field. Seismic activity and magnitude frequency distributions displayed by the different earthquake sequences are correlated with their location within the reservoir. Sequences located in the northwestern part of the reservoir show overall increased seismic activity and low b values, while the southeastern part is dominated by decreased seismic activity and higher b values. Periods of high injection coincide with high b values and vice versa. These observations potentially reflect varying differential and mean stresses and damage of the reservoir rocks across the field. About 50% of analyzed sequences exhibit no change in seismicity rate in response to the large main event. However, we find complex waveforms at the onset of the main earthquake, suggesting that small ruptures spontaneously grow into or trigger larger events.

Beschreibung: Abstract Distributions of eight targeted current‐use pesticides (CUPs) chloroneb, simazine, atrazine, alachlor, dacthal, chlorobenzilate, methoxychlor, and permethrin were investigated in seawater and the atmosphere in a region covering the North Pacific to the Arctic Oceans during the 7th and 8th Chinese National Arctic Research Expedition (2016 and 2017) voyages of the research vessel R/V Xuelong (Snow Dragon in English). Total CUP concentrations in seawater have prominent seasonal and latitudinal trends, with higher concentrations occurring at lower latitudes in early summer. The major contributors of the CUPs (∑8CUP) did not alter over different seasons with dominant chloroneb, alachlor, and atrazine accounting for more than 90%, but their concentrations did have marked seasonal changes. However, the compositions of the eight analyzed CUPs varied indistinctly between seasons indicating a possible combined environmental impact on these compounds rather than the effects of individual chemical properties. Three‐day backward air mass trajectories indicate that atmospheric masses from northeastern China are responsible for the high concentrations of CUPs in East China and Japan Seas, whereas those from the North Atlantic Ocean contribute to the low levels in local area. Fugacity ratios indicate potential volatilization and equilibrium of chloroneb in the Canada Basin of the Arctic Ocean and Japan Sea, respectively, and deposition of other CUPs in both regions. However, atmospheric concentrations are decoupled from those in seawater, which indicates a low exchange rate.

Beschreibung: Abstract On 2 October 2016, a significant seismic swarm of long‐period events was recorded on Tenerife (Canary Islands, Spain). The swarm lasted more than 5 hr and consisted of at least 766 detected events. We found a positive correlation between the amplitude of each event and the preceding interevent time together with a stability of the spectral properties and waveform similarity during most of the swarm duration. Toward the end of the swarm, individual events merged into a continuous tremor. These observations can be explained by postulating an unsteady transonic choked flow within a crack‐like conduit as a source mechanism for this swarm. The flow resulted from a sudden discharge of magmatic fluids from a pressurized reservoir into the hydrothermal system of Tenerife. The injected fluids reached the surface starting about 1 month after the swarm, as evidenced by the macroscopic increase in the diffuse CO2 emissions from the crater of Teide volcano. The lack of ground deformation and the absence of relevant seismicity at depths greater than 10 km exclude the ascent of a basaltic magma batch as a causative source mechanism. Instead, we hypothesize the sudden release of fluids accumulated at the top of a magma chamber as a possible mechanism. Another possibility is the injection of a small batch of mafic magma into a cooling magma chamber, triggering a convective mixing. Both cases imply the presence of a magma chamber at depths greater than 8.6 km. These results have important implications for the development of the volcano monitoring system of Tenerife.

Beschreibung: Abstract In this study, we examine the impacts of urbanization and open water surface on heavy convective rainfall based on numerical modeling experiments using the Weather Research and Forecasting model. We focus on a severe storm event over the emerging Xiong'an City in northern China. The storm event consists of two episodes and features intense moisture transport and strong large‐scale forcing. A set of Weather Research and Forecasting simulations were implemented to examine the sensitivity of spatiotemporal rainfall variability in and around the urban area to different land use scenarios. Modeling results highlight contrasting roles of open water and urban surface in dictating space‐time organizations of convective rainfall under strong large‐scale forcing. Dynamic perturbation to atmospheric forcing dominates the impacts of open water and urban surface on spatial rainfall distribution during the second storm episode, while urban surface promotes early initiation of convection during the first storm episode through enhanced buoyant energy. Open water surface contributes to convective inhibition through evaporative cooling but can enhance moist convection when the impact of urban surface is also considered. The synergistic effect of open water and urban surface leads to rainfall enhancement both over and in the downwind urban area. Changes in rainfall accumulation with different spatial extents of urban coverage highlight strong dependence of urban‐induced rainfall anomalies on urbanization stages. Our results provide improved understandings on hydrometeorological impacts due to emerging cities in complex physiographic settings and emphasize the importance of atmospheric forcing in urban rainfall modification studies.

Beschreibung: Abstract Numerical modeling is applied to elucidate the formation mechanism of the large lower positive charge centers (LPCCs) observed during thunderstorms over the Tibetan Plateau based on the simulation of a storm at the northeastern boundary of the plateau. Four sensitivity tests were carried out to explore the impacts of inductive charging, reversal temperature, and the choice of noninductive charging scheme. The results show that the unique environmental conditions of the Qinghai‐Tibet Plateau, which include weak convection and low freezing level, are fundamental to the formation of large LPCC. A weakened charge density in the upper positive charge center highlights the role of a LPCC in lightning initiation although the charge density of the LPCC has no obvious change compared to that in the LPCC of the typical tripole structure. This accounts for Tibetan Plateau thunderstorms having low frequencies of lightning flashes, which occur mainly in the lower dipole. Inductive electrification, which provided more than 50% of the positive charge on graupel and increased the positive and negative charge on cloud drops by 2 orders of magnitude, is an important complement to the lower dipole of the tripole charge structure originally established by noninductive electrification. The inductive electrification also evidently enhances the LPCC and the middle negative charge center while slightly reducing the upper positive charge center. Subsequently, the lightning activity is strengthened, and lightning flashes are more likely to be initiated at the lower dipole. Varying the reversal temperatures and noninductive charging scheme does not fundamentally affect the formation of the LPCC.

Beschreibung: Abstract The melting layer of precipitation has a major impact on remote sensing and telecommunications. However, there is a shortage of observational studies to validate and constrain the melting layer models especially for high‐frequency radar bands. In this paper, we report how multifrequency radar Doppler spectra can be used to retrieve the melting layer attenuation at Ka‐ and W‐bands. The presented analysis is based on identifying Rayleigh scattering regions in radar Doppler spectra measurements where dual‐wavelength spectral ratios can be related to differential attenuation. We show that the estimated attenuation at Ka‐ and W‐bands agrees reasonably well with previously reported studies, but there are indications of differences at higher rain rates. We advocate that this technique can be applied to long‐term observations to advance our knowledge of the melting process. The parameterizations of melting layer attenuation as a function of rain rate and radar reflectivity are also presented.

Beschreibung: Abstract The Cenozoic convergence between India and Asia has created Earth's thickest crust in the Pamir‐Tibet Plateau by extreme crustal shortening. Here we study the crustal structure of the Pamir and western Tian Shan, the adjacent margins of the Tajik, Tarim, and Ferghana Basins, and the Hindu Kush, using data collected by temporary seismic experiments. We derive, compare, and combine independent observations from P and S receiver functions. The obtained Moho depth varies from ~40 km below the basins to a double‐normal thickness of 65–75 km underneath the Pamir and Hindu Kush. A Moho doublet—with the deeper interface down to a depth of ~90 km—coincides with the arc of intermediate‐depth seismicity underneath the Pamir, where Asian continental lower crust delaminates and rolls back. The crust beneath most of the Central and South Pamir has a low Vp/Vs ratio (〈1.70), suggesting a dominantly felsic composition, probably a result of delamination/foundering of the mafic rocks of the lower crust. Beneath the Cenozoic gneiss domes of the Central and South Pamir, which represent extensional core complexes, the Vp/Vs ratios are moderate to high (~1.75), consistent with the previously observed, midcrustal low‐velocity zones, implying the presence of crustal partial melts. Even higher crustal average Vp/Vs ratios up to 1.90 are found in the sedimentary basins and along the Main Pamir Thrust. The ratios along the latter—the active thrust front of the Pamir—may reflect fluid accumulations within a strongly fractured fault system.

Beschreibung: Abstract The large magnitude of the 2011 Mw 9.0 Tohoku‐Oki earthquake, which occurred off the east coast of Japan, was not expected or predicted by any previous studies. One surprising observation was the sudden change in stress state; local earthquakes confirmed a compressional stress state before the main shock, whereas an extensional stress state was evident after the main shock. Using discrete element method modeling, this project attempts to reproduce the stress change after the main shock, and explores the conditions that can cause stress switching both onshore and offshore Tohoku. Our simulations demonstrate that rapid fault weakening can produce stress change and predominant normal‐fault earthquake mechanisms in the upper plate of Tohoku‐Oki. Several specific conditions seem to favor such stress switching; the megathrust fault must have been frictionally strong before the main shock, and comparable values of internal (μinternal) and basal friction (μbasal) are necessary to cause the formation of widespread normal faults within the wedge. Furthermore, dynamic extension during elastic unloading appears to play an important role in accommodating stress changes and wedge deformation in the Tohoku area; these cannot be explained solely based on Critical Coulomb Wedge models, but instead require dynamic unloading processes.

Beschreibung: Abstract Passive seismic methods for imaging the discontinuity structure of Earth have primarily focused on differences in vertically and radially polarized energy in the coda of earthquake‐generated body waves (e.g., receiver functions). To convert the timing of scattered wave arrivals to depth, three parameters must be known or inferred: depth or layer thickness (H), P‐wave velocity (VP), and S‐wave velocity (VS). A common way to solve for these parameters is through H‐κ stacking analysis, in which layer thickness and the ratio between VP and VS (κ) is calculated while holding one of the velocity parameters constant. However, this assumption biases estimates of layer properties and leads to uncertainties that are not appropriately quantified. As these results are commonly used as starting models for more complex seismic or geodynamic analyses, these assumptions can propagate much further than the initial study. In this study, we introduce independent observations from body‐wave autocorrelations that can help constrain this underdetermined problem. P‐wave autocorrelation allows for the recovery of the Moho‐reflected P‐wave phase from teleseismic earthquakes, which is removed during deconvolution in the calculation of receiver functions. As the Moho‐reflected P‐wave is independent of VS, this constraint allows us to create a system of equations that better quantifies the thickness, VP, and VS of a layer and produces a more appropriate estimation of associated uncertainties. We apply this to 88 seismic stations that are spatially distributed throughout the United States to obtain a model of crustal variability that is unbiased by a priori assumptions of velocity structure.

Beschreibung: Abstract Lightning channels are made of plasma. As a consequence, the driving electrical current changes the channel's resistance in a nonlinear fashion. The resistance has an intricate dependence on the history of Joule heating and various cooling processes, as well as on the various kinetic processes that dictate the population balance of electrons within the channel. Such dependence cannot be captured by an analytic function, as often attempted. In this paper, we introduce a minimal numerical model that can qualitatively capture the temporal dynamics of the key plasma properties of a lightning channel, including its electric field, temperature, plasma density, radius, and the resulting nonlinear resistance. Through a series of novel parameterizations, we introduce six zero‐dimensional equations that can capture both nonequilibrium/low‐temperature and local thermodynamic equilibrium/high‐temperature plasma regimes. In this manuscript, we go to great lengths to validate the model, showing that it can reproduce the finite time scale of streamer‐to‐leader transition, replicate the negative differential resistance behavior of steady‐state plasma arcs, and properly describe the temporal evolution of temperature in a return stroke channel. Finally, the model is applied to the simulation of optical emissions from rocket‐triggered lightning strikes, explaining the measured delay between the rise of current and visible light, as well as reproducing the direct relationship between peak current and peak radiated power and between charge transferred to ground and total radiated energy.

Beschreibung: Abstract Observations of shallow fault creep reveal increasingly complex time‐dependent slip histories that include quasi‐steady creep and triggered as well as spontaneous accelerated slip events. Here we report a recent slow slip event on the southern San Andreas fault (SSAF) triggered by the 2017 Mw8.2 Chiapas (Mexico) earthquake that occurred 3000 km away. Geodetic and geologic observations indicate that surface slip on the order of 10 mm occurred on a 40‐km‐long section of the SSAF between the Mecca Hills and Bombay Beach, starting minutes after the Chiapas earthquake and continuing for more than a year. Both the magnitude and the depth extent of creep vary along strike. We derive a high‐resolution map of surface displacements by combining Sentinel‐1 Interferometric Synthetic Aperture Radar (InSAR) acquisitions from different lines of sight. InSAR‐derived displacements are in good agreement with the creepmeter data and field mapping of surface offsets. Inversions of surface displacement data using dislocation models indicate that the highest amplitudes of surface slip are associated with shallow (〈1 km) transient slip. We performed 2‐D simulations of shallow creep on a strike‐slip fault obeying rate‐and‐state friction to constrain frictional properties of the top few kilometers of the upper crust that can produce the observed behavior.

Beschreibung: Abstract Various factors influence particle breakage in the shear zone of silica sands, including particle shape characteristics, loading path, shear displacement, normal load, initial density and boundary conditions. The present study focuses on particle breakage occurring in the shear zone of a crushed silica sand under shear loading. Several ring shear tests were conducted to measure shear stress‐displacement response of a sand. Grain‐size distribution curves of the original sand (prior to shearing) and the sheared sand from the shear zone are then compared. The mechanism of particle breakage is determined by a thorough examination of particle damage under constant normal load (CNL) and constant volume (CV) tests, considering both loose and dense samples. The results show that void ratio is the dominant factor in particle breakage. On the other hand, it is observed that particle breakage resulting from particle shape properties accrue within the initial loading stages. It is observed that the boundary condition may also affect the magnitude of particle breakage, i.e. particle damage under CNL boundary condition is found to be significantly higher than that under CV condition for the same initial normal stress and void ratio.

Beschreibung: Abstract This study analyzes the summertime precipitation bias over the Central United States and its relationship to the simulated large‐scale environment and the convection scheme in the Energy Exascale Earth System Model Atmosphere Model version 1. This relationship is mainly examined in a set of short‐term hindcasts initialized with realistic large‐scale conditions for the summer of 2011. Besides the uniform 1° model resolution, we adopt Regionally Refined Meshes to increase the model resolution to 0.25° over the contiguous United States. Additional five‐year Atmospheric Model Intercomparison Project simulations are conducted to confirm that the results from the hindcasts are consistent with the climate runs. We find that the summertime dry precipitation bias over the Great Plains and the wet bias over the Rockies cannot be reduced simultaneously by changing resolution or tuning parameters. As for the diurnal cycle, Energy Exascale Earth System Model Atmosphere Model version 1 captures the general diurnal variation of the large‐scale moisture transport and the large‐scale upward motion over the Central United States. However, the diurnal cycle of precipitation over the Great Plains is out of phase with the diurnal variation of the large‐scale environment because the convective precipitation dominates the total precipitation and its diurnal cycle, and it does not directly respond to the local moisture convergence and the large‐scale upward motion. These results reemphasize the importance of improving the coupling of the convection to the large‐scale environment in reducing the summer precipitation bias over the Central United States in climate models with the resolution of ~0.25°.

Beschreibung: Abstract The gradual collapse of the subglacial Bárdarbunga caldera in 2014–2015 provided an opportunity to explore the geothermal signals produced by large‐scale volcanic events. In August 2014, four ice cauldrons (shallow depressions on the ice surface) formed on the caldera flank. These cauldrons reached their maximum volume rapidly and then shrank, indicating that they were created during minor subglacial eruptions. Several weeks after the start of the collapse, three cauldrons on the caldera rim grew in volume, with four smaller cauldrons forming in 2015–2017. The cauldrons reached volumes in the range of 1.0 ± 0.2 to 27 ± 3 million m3. HYDROTHERM numerical simulations of fluid flow and heat transport in the uppermost 1 km of the crust were performed to explore possible causes for these thermal signals and in particular assess the role of shallow magmatic intrusions. The heat transfer required to create the more rapidly formed caldera‐rim cauldrons can be reproduced with shallow intrusions into high‐permeability pathways, which greatly enhance the surface thermal signal. The preintrusion temperature of the surrounding bedrock has a major effect on heat transfer to the surface, with cold bedrock causing a buffering effect, whereas temperature conditions close to the boiling point of water produce far more efficient heat transfer due to the formation of steam plumes. Not all observed behavior is reproduced by our models, suggesting that geothermal reservoirs below 1‐km depth may play a significant role in the observed thermal anomalies.

Beschreibung: Abstract We describe a suite of repeating long‐period seismic events at Fuego volcano in Guatemala. These events, recorded on a temporary network over a period of 8 days during January 2012, did not occur with any visibly or audibly detectable emissions from the volcano. Events are separated into families based on different correlation coefficient thresholds. A correlation coefficient threshold of 0.70 yields two families with 123 events and 25 events, respectively. These two event families share enough common features that if the correlation coefficient threshold is 0.65, the families merge and grow to include an additional 226 events. The short duration and frequency content concentrated below 2 Hz of the second family allow us to create a phase‐weighted stack which we then inverted for source mechanism and location using unconstrained full‐waveform moment‐tensor inversion. The eigenvalue decompositions of the best‐fit models indicate the source is a crack with some volume change. The short duration of the modeled source time function and the slight variability of signal shape within the suite of repeating events indicate the events are caused by rapid pressurization of cracks or series of connected cracks. The interevent times of these events appear clustered, indicating driving processes more complex than continual degassing of magma in the conduit would allow. Better understanding of these events may shed light on processes not captured by real‐time seismic amplitude measurements or gas flux measurements alone.

Beschreibung: Abstract Natural materials contain small grains of magnetic iron oxides that can record information about the magnetic field of the Earth when they form and can be used to document changes in the geomagnetic field through time. Thermoremanent magnetization is the most stable type of remanent magnetization in igneous rocks and can be carried by particle sizes above the upper size limit for single‐domain behavior. To better understand thermoremanent magnetization in particles larger than single domain, we imaged the thermal dependence of magnetic structures in ~1.5‐μm grains of titanomagnetite (Fe2.46Ti0.54O4) using variable‐temperature magnetic force microscopy. At room temperature, grains displayed single‐vortex and multivortex states. Upon heating, the single‐vortex state was found to be stable up to the Curie temperature (~215 °C), whereas multivortex states unblocked between 125 and 200 °C by transitioning into single‐vortex states. During cooling in a weak field (~0.1 mT), single‐vortex states nucleated just below the Curie temperature and remained unchanged to room temperature. The single‐vortex state was the only magnetic state observed at room temperature after weak field thermoremanent magnetization acquisition experiments. These observations indicate that single‐vortex states occur in titanomagnetite and, like single‐domain particles, have high thermal stability necessary for carrying stable paleomagnetic remanence.

Beschreibung: Abstract The rarity of reports in the literature of brief and spatially limited observations of drizzle at temperatures below ‐20°C suggest that riming and other temperature‐dependent cloud microphysical processes such as heterogeneous ice nucleation and ice crystal depositional growth prevent drizzle persistence in cold environments. In this study, we report on a persistent drizzle event observed by ground‐based remote‐sensing measurements at McMurdo Station, Antarctica. The temperatures in the drizzle‐producing cloud were below ‐25°C and the drizzle persisted for a period exceeding 7.5 hours. Using ground‐based, satellite, and reanalysis data we conclude that drizzle was likely present in parts of a widespread cloud field, which stretched more than ~1000 km along the Ross Ice Shelf coast. Parameter space sensitivity tests using two‐moment bulk microphysics in large‐eddy simulations constrained by the observations suggest that activated ice freezing nuclei (IFN) and accumulation‐mode aerosol number concentrations aloft during this persistent drizzle period were likely on the order of 0.2 L‐1 and 20 cm‐3, respectively. In such constrained simulations, the drizzle moisture flux through cloud base exceeds that of ice. The simulations also indicate that drizzle can lead to the formation of multiple peaks in cloud water content profiles. This study suggests that persistent drizzle at these low temperatures may be common at the low aerosol concentrations typical of the Antarctic and Southern Ocean atmospheres.

Beschreibung: Abstract We studied long‐term evolution of non‐transform discontinuities (NTDs) on the Mid‐Atlantic Ridge from 0 to ~20‐25 Ma crust using plate reconstructions of multibeam bathymetry, long‐range HMR1 sidescan sonar, residual mantle Bouguer gravity anomaly, and gravity‐derived crustal thickness. NTDs have propagated north and south with respect to flowlines of relative plate motion and both rapidly and slowly compared to the half spreading rate; at times they have been quasi‐stable. Fast, short‐term (〈2 m.y.) propagation is driven by reduced magma supply (increased extension) in the propagating ridge tip when NTD ridge‐axis offsets are small (〈~ 5 km). Propagation at larger offsets generally is slower and longer‐term. These NTDs can show classic structures of rift propagation including inner and outer pseudofaults and crustal blocks transferred between ridge flanks by discontinuous jumps of the propagating ridge tip. In all cases crustal transfer occurs within the NTD valley. Aside from ridge‐axis offset, the evolution of NTDs appears to be controlled by three factors: (1) Gross volume and distribution of magma supplied to ridge segments as controlled by 3D heterogeneities in mantle fertility and/or dynamic upwelling; this controls fundamental ridge segmentation. (2) The lithospheric plumbing system through which magma is delivered to the crust. (3) The consequent focusing of tectonic extension in magma‐poor parts of spreading segments, typically at segment ends, which can drive propagation. We also observe long‐wavelength (5‐10 m.y.) residual mantle Bouguer anomaly (RMBA) asymmetry between the conjugate ridge flanks, and we attribute this to asymmetric distribution of density anomalies in the upper mantle.

Beschreibung: Abstract In the parallel paper by Li et al. (2019), an effluent chemistry monitoring system was designed and used in core flood experiments to continuously measure the effluent concentration and study the evolution of the rock‐fluid system. In this study, the results from the parallel paper were used for interpretation and modeling of the dissolution and wormhole formation. Based on the behavior of the effluent concentration, two transient states and two quasi‐steady states were defined to describe the dissolution in the rock‐fluid system. Dimensional analysis was used to identify the controlling mechanisms of the dissolution and transport in the matrix and the wormholes. The dimensional analysis showed that the dissolution in the matrix was reaction‐controlled, while the dissolution in the wormholes was diffusion‐controlled. It also showed that the rock‐fluid system evolved from reaction‐controlled dissolution to diffusion‐controlled dissolution during the core flood tests. A continuum model and the extended Graetz solution were used to model the dissolution in the matrix and in the wormholes, respectively. In the continuum model, this study estimates the effective surface area as a function of the flow rate (injection flux), to account for the effect of flow conditions on dissolution. Finally, a semi‐empirical model combining the continuum model and the extended Graetz solution was developed to simulate the formation of wormholes and the evolution of the dissolution kinetics during core flood tests.

Beschreibung: Abstract Core flood tests were conducted to study the effect of flow rate on the dissolution of the gypsum rock matrix and the formation of wormholes. An effluent chemistry monitoring system (ECMS) was designed and integrated into a triaxial system to provide continuous effluent concentration measurements, in addition to the pressure and flow measurements during the core flood tests. X‐ray computed tomography (CT) was used to study the geometry of the wormholes after the tests. The core flood tests showed agreement with experiments reported in the literature regarding permeability evolution and wormhole breakthrough. By continuously monitoring the effluent concentration, the ECMS advanced the experimental study by showing how the dissolution kinetics evolved with the formation of wormholes. 3D topological and morphological algorithms were developed to analyze the CT data and provide quantitative descriptions for the wormhole geometry. The CT analysis showed that higher flow rates resulted in more complex wormhole geometries regarding the number of wormholes and branches.

Beschreibung: ABSTRACT 36Cl production in the atmosphere is modulated by the magnetic field intensity of both the Sun and the Earth. The record of 36Cl concentration along with that of 10Be in ice cores may therefore provide information as to their variability. To better understand the 36Cl signal in glaciological archives, we measured its concentration in Talos Dome snow samples (mean accumulation rate of 8 g.cm‐2.yr‐1 water equivalent) spanning the 1955 to 1980 C.E. period with a resolution of one sample every 3 years, and in Vostok snow samples (mean accumulation rate of 1.96 g.cm‐2.yr‐1 water equivalent) spanning the 1949 to 2007 C.E. period with a six‐month resolution that had never before been obtained. Marine nuclear bomb tests in the late 1950s produced anthropogenic 36Cl which was injected into the stratosphere and spread around the globe. In the late 1950s this anthropogenic pulse led to an increase of 36Cl concentration at Talos Dome that was more than 100 times higher than the pre‐ and post‐ bomb values. It is noteworthy that the atmosphere of Vostok remains polluted by anthropogenic 36Cl today. This pollution results from gaseous H36Cl mobility at low accumulation sites and implies re‐emission of 36Cl from the snowpack that is not observed at Talos Dome. The 36Cl/Cl‐ ratio may be used to discriminate the stratospheric anthropogenic 36Cl source from the tropospheric natural 36Cl source, which allows us to discuss the immobile vs. mobile 36Cl in the Vostok snowpack.

Beschreibung: Abstract We use World Wide Lightning Location Network (WWLLN) data on the radiated radio frequency electromagnetic energy per stroke to identify the upper tip of the global lightning stroke energy distribution. The mean stroke energy is about 1,000 J per stroke in the very low frequency band between 5 and 18 kHz, while the distribution used in this paper is limited to strokes in that band above 1 MJ, about 3 orders of magnitude above the mean. It is shown that these energies are representative of the tip of the optical distribution, first identified by Turman (1977) above 10 GW per stroke, which he termed “superbolts.” The distribution peaks globally in the Northern Hemisphere winter (November–February) with most superbolts being found in the North Atlantic west of Europe, the winter Mediterranean Sea, and a strong local maximum over the Andes in South America. We identify regions with somewhat fewer superbolts in the North Pacific east of Japan in winter, along the equator of the Atlantic and Indian Oceans and south of South Africa. We find very few superbolts during April–October each year. While superbolts are scattered around the globe, the local occurrence peaks do not coincide with the usual three main lightning “chimneys.” Unlike the distribution of all normal global lightning, we find superbolts predominantly over the oceans and seas, with fewer over the continents, just the opposite of all global lightning.

Beschreibung: Abstract Atmospheric rivers (ARs) manifest as transient filaments of intense water vapor transport that contribute to synoptic‐scale extremes and interannual variability of precipitation. Despite these influences, the synoptic to planetary scale processes that lead to ARs remain inadequately understood. In this study, North Pacific ARs within the November‐April season are objectively identified in both reanalysis data and the Community Earth System Model version 2 (CESM2), and atmospheric patterns preceding AR landfalls beyond one week in advance are examined. Latitudinal dependence of the AR processes is investigated by sampling events near the Oregon (45°N, 230°E) and southern California (35°N, 230°E) coasts. Oregon ARs exhibit a pronounced anticyclone emerging over Alaska 1‐2 weeks before AR landfall that migrates westward into Siberia; dual midlatitude cyclones developing over southeast coastal Asia and the northeast Pacific; and a zonally elongated band of enhanced water vapor transport spanning the entire North Pacific basin that guides anomalous moisture toward the North American west coast. The precursor high‐latitude anticyclone corresponds to a significant increase in atmospheric blocking probability, suppressed synoptic eddy activity, and an equatorward‐shifted storm track. Southern California ARs also exhibit high‐latitude blocking but have an earlier‐developing and more intense northeast Pacific cyclone. Compared to reanalysis, CESM2 underestimates Northeast Pacific AR frequencies by 5‐20% but generally captures AR precursor patterns well, particularly for Oregon ARs. Collectively, these results indicate that the identified precursor patterns represent physical processes that are central to ARs and are not simply an artifact of statistical analysis.

Beschreibung: Abstract The spatial non‐uniformity of the electric field in air discharges, such as streamers, can influence the accuracy of spectroscopic diagnostic methods and hence the estimation of the peak electric field. In this work, we use a self‐consistent streamer discharge model to investigate the spatial non‐uniformity in streamer heads and streamer glows. We focus our analysis on air discharges at atmospheric pressure and at the low pressure of the mesosphere. This approach is useful to investigate the spatial non‐uniformity of laboratory discharges as well as sprite streamers and blue jet streamers, two types of Transient Luminous Event (TLE) taking place above thunderclouds. This characterization of the spatial non‐uniformity of the electric field in air discharges allows us to develop two different spectroscopic diagnostic methods to estimate the peak electric field in cold plasmas. The commonly employed method to derive the peak electric field in streamer heads underestimates the electric field by about 40‐50 % as a consequence of the high spatial non‐uniformity of the electric field. Our diagnostic methods reduce this underestimation to about 10‐20%. However, our methods are less accurate than previous methods for streamer glows, where the electric field is uniformly distributed in space. Finally, we apply our diagnostic methods to the measured optical signals in the Second Positive System of N2 and the First Negative System of of sprites recorded by Armstrong et al. (1998) during the SPRITE's 95 and 96 campaigns.

Beschreibung: Abstract It is increasingly acknowledged that cold pools can influence the initiation of new convective cells. Yet, the full complexity of convective organization through cold pool interaction is poorly understood. This lack of understanding may partially be due to the intricacy of the dynamical pattern formed by precipitation cells and their cold pools. Additionally, how exactly cold pools interact is insufficiently known. To better understand this dynamics, we develop a tracking algorithm for cold pool gust fronts. Rather than tracking thermodynamic anomalies, which do not generally coincide with the gust front boundaries, our approach tracks the dynamical cold pool outflow. Our algorithm first determines the locus of the precipitation event. Second, relative to this origin and for each azimuthal bin, the steepest gradient in the near‐surface horizontal radial velocity vr is employed to determine the respective locus of the cold pool gust front edge. Steepest vr‐gradients imply largest updraft velocities, hence strongest dynamical triggering. Results are compared to a previous algorithm based on the steepest gradient in temperature — highlighting the benefit of the method described here in determining dynamically active gust front regions. Applying the method to a range of numerical experiments, the algorithm successfully tracks an ensemble of cold pools. A linear relation emerges between the peak rain intensity of a given event and maximal vr for its associated cold pool gust front — a relation found to be nearly independent of the specific sensitivity experiment.

Beschreibung: Abstract Subseasonal‐to‐seasonal (S2S) forecasts of Northern Hemisphere (NH) extratropical winter weather patterns continue to be a challenging venture. Past studies have considered the individual influence of two modes of climate variability – the Madden‐Julian Oscillation (MJO) and the state of the stratospheric polar vortex (SPV) – on the NH polar jet stream and associated weather regimes. This study takes a different approach and quantifies the joint influence of the SPV and the MJO on NH S2S winter weather patterns. Using ERA‐Interim, we illustrate that variability associated with the MJO primarily influences tropospheric patterns across the North Pacific and western North America 10 to 14 days later, while SPV variability has a stronger influence on tropospheric patterns over the North Atlantic and Europe for the same lags. Over the rest of North America and into the Arctic, however, constructive and destructive interference between the MJO and the SPV teleconnections yield unique jet stream and temperature patterns that differ from the single‐mode composites. As such, S2S forecasts of temperature and jet stream patterns across much of North America may improve in accuracy by considering both modes. The study also shows that MJO Phase 2,3 events influence the resulting tropospheric circulation via primarily a tropospheric pathway. By contrast, MJO Phase 7,8 events modify both the tropospheric and stratospheric circulation with potential feedbacks on the tropospheric circulation at longer lags. Hence, the study suggests another benchmark by which to test S2S dynamical prediction systems, including the importance for modeling stratosphere‐troposphere coupling dynamics correctly.

Beschreibung: Abstract Four years (201404‐201803) of Global Precipitation Measurement (GPM) Precipitation Features (PF) data along with co‐located the Modern Era Retrospective‐Analysis for Research and Applications (MERRA‐2) model data are used to identify Freezing Rain Features (FRFs). A PF with presence of both melting layer (maximum temperature of the vertical column 〉 4 0C), and a layer of subfreezing air (2 m temperature 〈 0 0C) adjacent to the surface is considered an FRF. During four years of observations, GPM and MERRA‐2 identify approximately 3096 FRFs globally (650S‐650N). Most of them are observed over northern hemispheric land in the winter season. The majority of FRFs originate through the “melting process” whereas only 35 features are associated with “warm rain” process. The locations, seasonal and diurnal distribution patterns of the FRFs over the United Stated are well matched with the ground‐based observations. The ground‐based observations verify approximately 70% of the FRFs over the United States. Ku band radar properties show that FRFs are found relatively shallower (2‐5 km) and less intense (〈 27 dBZ) than precipitation features in general, but deeper and more intense than Snow Features (SFs). Passive microwave properties show that FRFs Tbs and Polarization Corrected Temperature (PCT) are warmer than SFs at all GMI channels with the largest differences in 166 GHz. The enhancement in Tbs are more distinct with “warm rain” FRFs. FRF Tb tends to decrease as echo top height increases at all GMI channels except for 183 GHz, where Tbs have lack of dependence on echo top height.

Beschreibung: Abstract Global warming is expected to produce modifications in the intensity, as well as in the seasonality and spatiotemporal structure of extreme precipitation. In the present study, the temporal evolution of simulated daily and sub‐daily precipitation extremes was analyzed to assess how they respond to climate warming over different time horizons. Pooling series from the recent 50‐member Canadian Regional Climate Model v5 Large Ensemble (CRCM5‐LE), the probability distributions, date and time of occurrences, and spatiotemporal structure of simulated Annual Maxima (AM) precipitation were analyzed at various spatial scales and for durations between 1h and 3 days. In agreement with previous studies, the results underline the large increases in AM precipitation quantiles, especially for the shortest durations and for the more extreme events (i.e. longest return periods), and modifications in their spatiotemporal scaling properties and annual and diurnal cycles. For instance, sub‐daily AM extremes are expected to occur later in the evening, while, no matter the duration, the extremes are expected to occur over a wider period of the year in future climate. Finally, the analysis of projected AM probability distributions showed that heavy‐tail Generalized Extreme Value (GEV) distributions will most likely be observed in the future climate, with some model grid boxes experiencing a significant increase of GEV shape parameters. These results may have major consequences in terms of the occurrence and possible impacts of the most extreme precipitation events.

Beschreibung: Abstract Intergranular pressure solution is a well‐known rock deformation mechanism in wet regions of the upper crust, and has been widely studied, especially in the framework of compaction of granular materials, such as reservoir sandstones and fault rocks. Several analytical models exist that describe compaction creep by stress‐induced mass transport, and the parameters involved are relatively well constrained by laboratory experiments. While these models are capable of predicting compaction behaviour observed at relatively high porosities, they often overestimate compaction rates at porosities below 20% by up to several orders of magnitude. This suggests that the microphysical processes operating at low porosities are different and are not captured well by existing models. The implication is that available models cannot be extrapolated to describe compaction of sediments and fault rocks to the low porosities often reached under natural conditions. To address this problem, we propose a new, thermodynamic model that describes the decline of pressure solution rates within individual grain contacts as a result of time‐averaged growth of asperities or islands and associated constriction of the grain boundary diffusion path (here termed grain boundary evolution). The resulting constitutive equations for single grain‐grain contacts are then combined and solved semi‐analytically. The compaction rates predicted by the model are compared with those measured in high‐strain compaction experiments on wet granular halite. A significant reduction in compaction rate is predicted when grain boundary evolution is considered, which compares favourably with the experimental compaction data.

Beschreibung: Abstract Ambient noise is useful for characterizing frequency‐dependent noise levels and for assessing data quality for seismic stations. We use three years of ambient noise spectra from 16 stations in central Alaska to examine environmental and structural influences on seismic stations. The region contains a major river (Tanana river) that is ice‐covered for half the year and is underlain by a sedimentary basin (Nenana basin) that strongly influences the seismic wavefield. Nenana basin amplifies ambient seismic noise by 12–16 dB at 0.1–0.7 Hz and 17–30 dB at 0.7–3 Hz. A meteorological station and river gauge at Nenana provide environmental data for comparison with seismic stations. During the summer, the Tanana river produces noise levels elevated by 30–40 dB at frequencies near 10 Hz, as recorded by all stations within 100 m of the main river channel. The Tanana river lacks any sediment larger than sand in this region; therefore we attribute the 10 Hz river signal to turbulence within the water and to unsteady shearing on the river bottom. The influence of wind is apparent on seismic noise at low (〈0.05 Hz) frequencies, due to atmospheric‐induced tilting, and at high (〉2.0 Hz) frequencies, due to unsteady shearing and turbulence near the ground. Our empirical findings motivate future studies, such as how flow from air or water couples to the ground and how deep sedimentary basins influence the ambient noise wavefield. Our results have implications for seismic site selection, environmental monitoring, and detection and characterization of earthquakes.

Beschreibung: Abstract When analyzing the rupture of a large earthquake, geodetic data are often critical. These data are generally characterized by either a good temporal or a good spatial resolution, but rarely both. As a consequence, many studies analyze the co‐seismic rupture with data that also include one or more days of early post‐seismic deformation. Here, we invert simultaneously for the co‐ and post‐seismic slip with the condition that the sum of the two models remains compatible with data covering the two slip episodes. We validate the benefits of this approach with a toy model and an application to the 2009 Mw6.3 L'Aquila earthquake, using a Bayesian approach and accounting for epistemic uncertainties. For the L'Aquila earthquake, we find that if early post‐seismic deformation is not explicitly acknowledged co‐seismic signal, co‐seismic slip models may overestimate the peak amplitude while long‐term post‐seismic models may largely underestimate the total post‐seismic slip amplitude. This example illustrates how the proposed approach could improve our comprehension of the seismic cycle, of fault frictional properties, and the spatial and temporal relationship between seismic rupture, afterslip and aftershocks.

Beschreibung: Abstract Here we present high‐resolution 2‐D coupled tectonic‐surface processes modeling of extensional basin formation. We focus on understanding feedbacks between erosion and deposition and tectonics during rift and passive margin formation. We test the combined effects of crustal rheology and varying surface process efficiency on structural style of rift and passive margin formation. The forward models presented here allow to identify the following four feedback relations between surface processes and tectonic deformation during rifted margin formation. (1) Erosion and deposition promote strain localization and enhance large offset asymmetric normal fault growth. (2) Progressive infill from proximal to more distal half‐grabens promotes the formation of synthetic sets of basin ward dipping normal faults for intermediate crustal strength cases. (3) Sediment loading on top of undeformed crustal rafts in weak crust cases enhances mid and lower crustal flow resulting in sag basin subsidence. (4) Interaction of high sediment supply to the distal margin in very weak crust cases results in detachment based rollover sedimentary basins. Our models further show that erosion efficiency and drainage area provide a first order control on sediment supply during rifting where rift related topography is relatively quickly eroded. Long term sustained sediment supply to the rift basins requires elevated onshore drainage basins. We discuss similar variations in structural style observed in natural systems and compare them with the feedbacks identified here.

Beschreibung: Abstract We investigate energy partitioning using seismological methods of sources with estimated sub‐micron levels of slip in the laboratory. Estimates inferred from recorded seismic waves are founded on micro‐scale phenomenological friction experiments in the laboratory and appear to be constrained by the inherent assumptions. In this concerted study, we build on the methods used to absolutely calibrate an array of piezoelectric transducers in a direct shear laboratory apparatus. We found that flat‐broadband sensor behavior, allowed us to study source‐extent parameters using spectral source models that are typically used to interpret small to moderate‐sized earthquakes. We computed the corner frequencies, low‐frequency plateaus and high‐frequency spectral falloff exponent using single station assumptions. Moment magnitude ranged from ‐9〈Mw〈‐7.5 and slip was on the order of nanometers to micrometers ‐ extending our understanding of source parameters studied via seismic waves. A number of findings are highlighted: (i) Variations in spectral falloff with corner frequencies followed the observations made in natural conditions. (ii) Corner frequency shift phenomena was observed (fcP/fcS∼ 1.34) and was attributed to source finiteness rather than wave propagation effects. (iii) Events were stress‐overshoot as determined by the Savage‐Wood efficiency. (iv) The empirical power law scaling relationship between fracture energy and slip, given as , where nG=1.28 appears to breakdown with seismological estimates made at the mining scale (nG=1.86) and laboratory scale (nG=2.35). This break in scaling may be related to the types of off‐fault energy sinks that are inherently captured in the seismological interpretation of fracture energy.

Beschreibung: Abstract Nanograins (〈〈 1 μm) are common in the principal slip zones of natural and experimental faults, but their formation and influence on fault mechanical behaviour are poorly understood. We performed transmission Kikuchi diffraction (TKD; spatial resolution 20‐50 nm) on the principal slip zone of an experimental carbonate gouge (50 wt.% calcite, 50 wt.% dolomite) that was deformed at a maximum slip rate of 1.2 ms‐1 for 0.4 m displacement. The principal slip zone (PSZ) consists of nanogranular aggregates of calcite, Mg‐calcite, dolomite and periclase, dominated by grain sizes in the range of 100‐300 nm. Nanograins in the ultrafine (〈 800 nm) PSZ matrix have negligible internal lattice distortion, while grains 〉 800 nm in size contain subgrains. A weak crystallographic preferred orientation is observed as a clustering of calcite c‐axes within the PSZ. The high‐resolution microstructural observations from TKD, in combination with published flow laws for calcite, are compatible with high‐velocity slip in the PSZ having been accommodated by a combination of grain size sensitive creep in the ultrafine matrix, and grain size insensitive creep in the larger grains, with the former process likely controlling the bulk rheology of the PSZ after dynamic weakening. If the activation energy for creep is lowered by the nanogranular nature of the aggregates, this could facilitate grain size sensitive creep at high (coseismic) strain rates and only moderate bulk temperatures of c. 600 °C, although temperatures up to 1000 °C could be locally achieved due to processes such as flash heating.

Beschreibung: Abstract Interactions between convection and the Saharan Air Layer in the tropical Atlantic Ocean are quantified using a novel compositing technique that leverages geostationary cloud observations to add temporal context to the polar orbiting CloudSat and CALIPSO satellites allowing aerosol optical depth (AOD) changes to be tracked throughout a typical convective storm lifecycle. Four years of CALIPSO observations suggest that approximately 20% of the dust mass in every 10° longitude band between 10°W and 80°W is deposited into the ocean. Combining a new convective identification algorithm based on hourly geostationary cloud products with AODdust profiles along the CALIPSO track reveals that wet scavenging by convection is responsible for a significant fraction of this deposition across the Atlantic. Composites of four years of convective systems reveal that, on average, convection accounts for 15% ± 7% of the dust deposition in each longitude band relative to pre‐convective amounts, implying that dry deposition and scavenging by non convective events are responsible for the remaining 85% of dust removal. In addition, dust layers are detrained at upper levels of the atmosphere between 8 km and 12 km by convective storms across the Atlantic. The dust budget analysis presented here indicates that convection lofts 1.5% ± 0.6% of dust aerosol mass to altitudes greater than 6 km. This may have significant implications for cloud formation downstream of convection since lofted dust particles can act as effective ice nucleating particles (INP), altering cloud microphysical and radiative properties, latent heating, and precipitation rates.

Beschreibung: Abstract Southern African tropical lows are synoptic‐scale cyclonic vortices that propagate westward across southern Africa in the Austral summer. They strongly influence local rainfall and aggregate in the climatological DJF mean to form the Angola Low. In this study, tropical lows are identified and tracked using an objective feature tracking method. The statistics of tropical low tracks over southern Africa are presented and compared across three reanalysis products. Findings are compared to the literature of tropical low pressure areas elsewhere in the world, where it is found that most tracking statistics compare well, but that the tendency of tropical lows to become semi‐stationary over Angola is unique to southern Africa. The hypothesis that tropical lows in Angola have a causal relationship with Tropical Temperate Troughs is tested, and a correlation between occurrence frequencies is found at inter‐annual but not daily time‐scales. Precipitation is attributed to the tropical lows and it is found that tropical lows are associated with 31% of rainfall across tropical southern Africa, based on gridded precipitation products. The inter‐annual variability of the number of tropical lows that form per year (σ=6 events/annum) is linked to ENSO and the tropical easterly jet. The mean latitude of tropical lows is shifted northwards during El Niño and southwards during La Niña. Much of the inter‐annual precipitation variability maximum in Angola is attributed to rainfall associated with tropical lows. These results provide insights into the southern African response to ENSO and into the mechanisms of rainfall in the southern African tropical edge.

Beschreibung: Abstract Nevado de Toluca is a stratovolcano in the densely populated Mexican Highland and the source of three late Pleistocene Plinian eruptions. While the characterization of the magmatic processes and time scales, leading to these events is crucial to evaluate potential signals of the reawakening of this volcano, they are not well constrained. Here we present insights gathered from the analysis of mineral zoning. Abrupt changes in plagioclase compositions [anorthite (An) 〉10 mol. %] record repeated thermochemical disturbance of a shallow magma reservoir prior to the three eruptions investigated. In particular, sudden chemical changes in the outermost crystal rims indicate that the eruptions were triggered by recharge of less evolved magma. Recalculated melt compositions reveal that magma input is chemically heterogeneous. Orthopyroxene compositions span a large range [En53‐En92, Cr2O3 of up to 0.9 wt. %] resulting from magma hybridization, which is consistent with a late‐stage temperature increase recorded by amphibole chemistry. Using diffusion chronometry, we show that this recharge typically occurs decades to centuries before Plinian eruptions and hence on time scales relevant for volcano monitoring. Additionally, modelling of Mg profiles in plagioclase constraints the duration of differentiation cycles to be millennial in order of magnitude. Our study shows that similar processes preceded explosive eruptions from Nevado de Toluca, suggesting that analogous paths of unrest should be considered when evaluating potential future activity. We emphasize that signs of deep crustal activity, such as relatively broad deformation pattern surrounding the volcano, could herald the reactivation of Nevado de Toluca.

Beschreibung: Abstract To better investigate the characteristics and sources of trace elements (TEs) in PM2.5 in urban Beijing, a one‐year hourly observation was continuously made using an online multi‐element analyzer from June 1, 2016 to May 31, 2017. The average concentrations of 14 individual TEs ranged from 1.1 ng/m3 (V) to 900 ng/m3 (K). The occurrence levels of most TEs of interest in Beijing were lower than those in most domestic cities, but higher than those in most foreign countries. The formation of sulfate increased with the concentrations of all studied TEs during autumn and winter. Dust, industry, biomass burning and waste incineration, vehicle emissions, coal combustion and oil combustion were identified by the positive matrix factorization (PMF) model, which accounted for 36.3%, 10.7%, 27.1%, 13.7%, 7.6% and 4.6%, respectively, of the total elements. All factors exhibited higher concentrations on weekends than on weekdays. Local vehicular emissions and industry contributed to the loading of TEs, but dust, biomass burning and waste incineration, coal combustion and oil combustion from neighboring areas appeared to be dominant sources of TEs. Except for dust and industry, the four other sources of TEs were mainly located in the south and southeast areas of the sampling site. The analysis by conditional probability function and potential source contribution function showed that the distribution of the PMF sources of TEs roughly agreed with the location of main point sources. Overall, this work provides more detailed information on the characteristics of the TEs for the scientific community and modelling work.

Beschreibung: Abstract A major component of California's yearly precipitation comes from wintertime atmospheric river (AR) events which bring large amounts of moisture from the tropics up to the midlatitudes. Understanding these systems, specifically the effects of aerosol particles on precipitation associated with these storms, was a major focus of the 2015 Atmospheric Radiation Measurement (ARM) Cloud Aerosol Precipitation Experiment (ACAPEX), which was part of the wintertime CalWater 2015 campaign. The measurement campaign provided sampling platforms on four aircraft, including the ARM Aerial Facility G‐1, as well as the NOAA Ronald H. Brown research vessel and at a ground station in Bodega Bay, CA. Measurements of ice nucleating particles (INPs) were made with the Colorado State University (CSU) Continuous Flow Diffusion Chamber (CFDC) aboard the G‐1, and aerosol filters were collected on the G‐1, at the Bodega Bay site and on the Ronald H. Brown for post‐processing via immersion freezing in the CSU Ice Spectrometer. Aerosol composition was measured aboard the G‐1 with the Aerosol Time‐of‐Flight Mass Spectrometer (ATOFMS). Here we present INP concentrations and aerosol chemical compositions during the course of the aircraft campaign. During the AR event, we found that marine aerosol was the main aerosol type and that marine INPs were dominant at cloud activation temperatures, which is in stark contrast to the dominance of dust INPs during the AR events in the CalWater 2011 campaign.

Beschreibung: Abstract Fram Strait is a hot‐spot of Arctic cold air outbreaks (CAOs), which typically occur within the northerly flow associated with a strong low‐tropospheric east‐west pressure gradient between Svalbard and Greenland. This study investigates the processes in the inner Arctic that thermodynamically pre‐condition air masses associated with CAOs south of Fram Strait where they lead to negative potential temperature anomalies often in excess of 15 K. Kinematic backward trajectories from Fram Strait are used to quantify the Arctic residence time and to analyse the thermodynamic evolution of these air masses. Additionally, the study explores the importance of cyclonic tropopause polar vortices (TPVs) for CAO formation south of Fram Strait. Results from a detailed case study and the climatological analysis of the 100 most intense CAOs from Fram Strait in the ERA‐Interim period reveal that: (i) air masses that cause CAOs (CAO air masses) reside longer in the inner Arctic compared to those that do not (NO‐CAO air masses), and they originate from climatologically colder regions; (ii) the 10‐day accumulated cooling is very similar for CAO and NO‐CAO air masses indicating that the transport history and northerly origin of the air masses is more decisive for the formation of an intense negative temperature anomaly south of Fram Strait than an enhanced inner‐Arctic diabatic cooling; (iii) 40 % (29 %) of the top 40 (100) CAOs are related to a TPV in the vicinity of Fram Strait; (iv) TPVs confine anomalously cold air masses within their associated low‐tropospheric cold dome leading to enhanced accumulated radiative cooling.

Beschreibung: Abstract Quantifying the urbanization effect on station and regional surface air temperature (SAT) trends is a prerequisite for monitoring and detecting long‐term climate change. Based on the data set of satellite visible spectral remote sensing, a new method is developed to determine the urbanization level around observational sites on varied spatial scales and to classify the sites into different categories of stations (U1, U2, …, U6) with U1 the least and U6 the largest affected by urbanization. Urbanization effect on SAT anomaly series of urban and national stations are then evaluated for the periods of 1980–2015 and 1960–2015. Results show that the percentage of built‐up area in different circumferences of the observational sites can be considered as a good indicator of comprehensive urbanization level of station and can be used to classify stations and to determine reference stations; the largest increase in annual mean SAT (Tmean) during 1980–2015 occurred at U6 stations, and U1 stations registered the weakest annual mean warming. The urbanization level is significantly positively correlated to the linear trends of annual mean Tmean and minimum SAT (Tmin) and significantly negatively correlated to the diurnal temperature range (DTR) change. The data sets of the national reference climate station network and basic meteorological station network show large urbanization effect and contribution, with the annual mean urbanization contributions reaching 28.7% and 25.8% for the periods 1960–2015 and 1980–2015, respectively. For all the national stations (2,286 in total), the urbanization contributions are 17.1% and 14.6% for the two same periods, respectively.

Beschreibung: Abstract Starting in late 2013, the Northeast (NE) Pacific Ocean experienced anomalously warm sea surface temperatures (SSTs) that persisted for over 2 years. This marine heatwave, known as “the Blob,” produced many devastating ecological impacts with socioeconomic implications for coastal communities. The warm waters observed during the NE Pacific 2013/2016 marine heatwave altered the surface energy balance and disrupted ocean–atmosphere interactions in the region. In principle, ocean–atmosphere interactions following the formation of the marine heatwave could have perpetuated warm SSTs through a positive SST‐cloud feedback. The actual situation was more complicated. While reanalysis data show a decrease in boundary layer cloud fraction and an increase in downward shortwave radiative flux at the surface coincident with warm SSTs, this was accompanied by an increase in longwave radiative fluxes at the surface, as well as an increase in sensible and latent heat fluxes out of the ocean mixed layer. The result is a small negative net heat flux anomaly (compared to the anomalies of the individual terms contributing to the net heat flux). This provides new information about the midlatitude ocean–atmosphere system while it was in a perturbed state. More specifically, a mixed layer heat budget reveals that anomalies in both the atmospheric and oceanic processes offset each other such that the anomalously warm SSTs persisted for multiple years. The results show how the atmosphere–ocean system in the NE Pacific is able to maintain itself in an anomalous state for an extended period of time.

Beschreibung: Abstract Stable oxygen isotopes (δ18O) in the Bona‐Churchill (B‐C) ice core from southeast Alaska provide a valuable, high‐resolution history of climate variability and sea ice cover in the western Arctic over the last 800 years. Multiple ice cores have been collected from the Wrangell‐St. Elias Mountain Range; however, their δ18O records exhibit little consistency as each core offers a unique view on local, regional, and/or global climate variability. To explore the primary mechanisms influencing the isotopic signature at the B‐C site, we utilize isotope‐enabled model data, reanalysis data, and observations, which all indicate a strong connection between isotopes at the B‐C site and western Arctic climate, likely established by the location of the storm track in this region. Enriched B‐C δ18O reflects increased southerly flow and warmer waters in the Bering Sea, which modulates the heat flux through the Bering Strait and into the Arctic, thereby affecting sea ice cover in the western Arctic. The B‐C δ18O paleorecord shares some remarkable similarities (r = −0.80, p 〈 .001) with the duration of western arctic sea ice cover reconstructed from a Chukchi Sea sediment core. Interestingly, during the Little Ice Age, enriched δ18O and reduced western Arctic sea ice are observed and may be indicative of prolonged periods of the warm Arctic/cold continents pattern and a northwestward shift of the North Pacific storm track.

Beschreibung: Abstract The Tien Shan is the largest independent latitudinal mountain ranges in Eurasia, but due to a lack of long‐term instrumental data, knowledge of past temperature variability in the Tien Shan is limited. In this paper, a maximum latewood density (MXD) chronology of Schrenk spruce (Picea schrenkiana) was constructed from the Tien Shan, Central Asia. Correlation analyses indicate that the MXD variations of spruce trees in the Tien Shan are mainly limited by summer temperature variations. The July‐August mean temperature was reconstructed for the Tien Shan back to 1615 CE. The temperature reconstruction explained 42.9% of the instrumental variance (1930–2005). This reconstruction successfully captured the past temperature variabilities in the Tien Shan and enabled visualization of the cool climate before 1850s and the climatic warming after the 1850s. Agreement among the temperature series of the Tien Shan and regional temperature records based on MXDs from Eurasia suggested that our reconstruction had good reliability, captured some warm/cold periods, and first principal component (PC1) of these MXD records showed that there are strong large‐scale temperature forcings in Eurasia. Our study also showed that there is a linkage between temperature changes in Central Asia and the atmospheric circulations of the middle‐high latitude westerly region and the sea surface temperatures of both the Pacific Ocean and the Atlantic Ocean. The positive correlations among the PC1 time series of regional temperature reconstructions based on MXDs and Northern Hemisphere temperature reconstructions revealed that the MXD chronologies from Central Asia have the potential for developing large‐scale temperature reconstruction.

Beschreibung: Abstract Conduit models of volcanic eruptions simulate magma evolution through phase transitions and material changes during ascent. We present a time‐dependent one‐dimensional model of a chamber‐conduit system to examine the temporal evolution of dome‐forming eruptions. As magma ascends, volatiles exsolve and may escape vertically through the column or laterally through the conduit walls. Magma solidifies which increases viscosity, leading to a natural transition from viscous flow at depth to frictional sliding along the conduit walls near the surface, resulting in the extrusion of a semi‐solid plug. The model evaluates time‐ and depth‐dependent pressure, velocity, porosity and relative amounts of exsolved water to carbon dioxide. Transient effects arise when magma outflux from the chamber appreciably decreases pressure over the magma ascent timescale. For low magma permeability, transient effects increase porosity and velocity relative to steady‐state solutions. For high magma permeability, efficient vertical and lateral gas escape depresses porosity and velocity at later times. We use the model to predict three time‐series datasets from the 2004‐2008 eruption of Mount St. Helens: extruded volume, ground deformation and carbon dioxide emissions. We quantify sensitivity of model predictions to input parameters using the Distance‐based Generalized Sensitivity Analysis. Chamber volatile content, volume and excess pressure influence the amplitude of observables, while conduit radius, frictional rate‐dependence and magma permeability scale influence temporal evolution. High magma permeability can cause marked departures from exponentially decaying flux and may explain the unique temporal evolution of deformation observed at the only nearby continuous GPS station in operation at eruption onset.

Beschreibung: Abstract The clear sky greenhouse effect (G) is defined as the trapping of infrared radiation by the atmosphere in the absence of clouds. The magnitude and variability of G is an important element in the understanding of Earth's energy balance; yet the quantification of the governing factors of G is poor. The global mean G averaged over 2000 to 2016 is 130–133 W m−2 across data sets. We use satellite observations from Clouds and the Earth's Radiant Energy System Energy Balance and Filled (CERES EBAF) to calculate the monthly anomalies in the clear sky greenhouse effect (ΔG). We quantify the contributions to ΔG due to changes in surface temperature, atmospheric temperature, and water vapor by performing partial radiation perturbation experiments using ERA‐Interim and Geophysical Fluid Dynamics Laboratory's Atmospheric Model 4.0 climatological data. Water vapor in the middle troposphere and upper troposphere is found to contribute equally to the global mean and tropical mean ΔG. Holding relative humidity (RH) fixed in the radiative transfer calculations captures the temporal variability of global mean ΔG while variations in RH control the regional ΔG signal. The variations in RH are found to help generate the clear sky super greenhouse effect (SGE). Thirty‐six percent of Earth's area exhibits SGE, and this disproportionately contributes to 70% of the globally averaged magnitude of ΔG. In the global mean, G's sensitivity to surface temperature is 3.1–4.0 W m−2 K−1, and the clear sky longwave feedback parameter is 1.5–2.0 W m−2 K−1. Observations from CERES EBAF lie at the more sensitive ends of these ranges and the spread arises from its cloud removal treatment, suggesting that it is difficult to constrain clear sky feedbacks.

Beschreibung: Abstract Heat waves lead to major impacts on human health, food production, and ecosystems. To assess their predictability and how they are projected to change under global warming, it is crucial to improve our understanding of the underlying processes affecting their occurrence and intensity under present‐day climate conditions. Beside greenhouse gas forcing, processes in the different components of the climate system—in particular the land surface, atmospheric circulation, and the oceans—may play a key role in changing the odds for a particular event. This study aims to identify the role of the individual drivers for five heat waves (and, in some cases, of concurrent droughts) in the recent decade. Simulations are performed with the Community Earth System Model using nudging of horizontal atmospheric circulation and prescription of soil moisture. The fully constrained model accurately reproduces how anomalous an event was. Factorial experiments, which force the model toward observations for one or several key components at a time, allow us to identify how much of the observed temperature anomaly of each event can be attributed to each driver. Considering all analyzed events, atmospheric circulation and soil moisture play similarly important roles, each contributing between 20% and 70% to the events' anomalies. This highlights that the role of thermodynamics can be just as important as that of the dynamics for temperature extremes, a possibly underestimated feature. In addition, recent climate change amplified the events and contributed between 10% and 40% of the events' anomalies.

Beschreibung: Abstract The current assessment of the Antarctic surface mass balance mostly relies on reanalysis products or climate model simulations. However, little is known about the ability of models to reliably represent the microphysical processes governing the precipitation. This study makes use of recent ground‐based precipitation measurements at Dumont d'Urville station in Adélie Land to evaluate the representation of the precipitation microphysics in the Polar version of the Weather Research Forecast (Polar WRF) atmospheric model. During two summertime snowfall events, high‐resolution simulations are compared to measurements from an X‐band polarimetric radar and from a Multi‐Angle Snowflake Camera (MASC). A radar simulator and a “MASC” simulator in Polar WRF make it possible to compare similar observed and simulated variables. Radiosoundings and surface‐meteorological observations were used to assess the representation of the regional dynamics in the model. Five different microphysical parameterizations are tested. The simulated temperature, wind, and humidity fields are in good agreement with the observations. However, the amount of simulated surface precipitation shows large discrepancies with respect to observations, and it strongly differs between the simulations themselves, evidencing the critical role of the microphysics. The inspection of vertical profiles of reflectivity and mixing ratios revealed that the representation of the sublimation process by the low‐level dry katabatic winds strongly influences the actual amount of precipitation at the ground surface. By comparing the simulated radar signal as well as MASC and model particle size distributions, it is also possible to identify the microphysical processes involved and to pinpoint shortcomings within the tested parameterizations.

Beschreibung: Abstract This study investigates the dynamics that led to the repeated cold surges over midlatitude Eurasia, exceptionally warm conditions and sea ice loss over the Arctic, and the unseasonable weakening of the stratospheric polar vortex in autumn and early winter 2016–2017. We use ERA‐Interim reanalysis data and COBE sea ice and sea surface temperature observational data to trace the dynamical pathways that caused these extreme phenomena. Following abnormally low sea ice conditions in early autumn over the Pacific sector of the Arctic basin, blocking anticyclones became dominant over Eurasia throughout autumn. Ural blocking (UB) activity was four times above climatological levels and organized in several successive events. UB episodes played a key role in the unprecedented sea ice loss observed in late autumn 2016 over the Barents‐Kara Seas and the weakening of the stratospheric vortex. Each blocking induced circulation anomalies that resulted in cold air advection to its south and warm advection to its north. The near‐surface warming anomalies over the Arctic and cooling anomalies over midlatitude Eurasia varied in phase with the life cycles of UB episodes. The sea ice cover minimum over the Barents‐Kara Seas in 2016 was not observed in late summer but rather in mid‐November and December shortly after the two strongest UB episodes. Each UB episode drove intense upward flux of wave activity that resulted in unseasonable weakening of the stratospheric vortex in November. The surface impact of this weakening can be linked to the migration of blocking activity and cold spells toward Europe in early winter 2017.

Beschreibung: Abstract Arid regions are among the most sensitive areas to climate change, a better understanding of the impact of volcanic eruptions on the hydroclimate over global arid regions is helpful for adaptation but is not well studied. Here we show evidences that arid regions exhibit a wetter condition after volcanic eruptions based on reconstructions and observations, especially for volcanoes located in the northern hemisphere and tropics. Such “dry gets wetter” response is further supported by climate model simulations driven by volcanic aerosol forcing. The dynamic processes related to changes in atmospheric circulation are found to play a dominant role in precipitation responses. The wetter condition over northern hemispheric (southern hemispheric) (NH (SH)) arid regions after southern (northern) hemispheric volcanic eruptions is caused by enhanced cross equator flow, while the increased precipitation over global arid regions and NH (SH) arid regions following tropical volcanic eruptions and northern (southern) eruptions respectively, are mainly controlled by the monsoon‐desert coupling mechanism. The response of the extreme precipitation is consistent with that of the mean precipitation, but more sensitive on a regional scale. The results indicate that volcanic eruptions at different latitudes should be considered in the design of near‐term decadal climate prediction experiments and the implementation of geoengineering activities.

Beschreibung: Abstract The East Africa‐West Asia (EA‐WA) sector is a region where the skill in forecasting rainfall is unusually high in the subseasonal‐to‐seasonal (S2S) time range and where ENSO and MJO signals have significant impacts. Much of regional rainfall intra‐seasonal variability is related to tropical‐temperate interactions on synoptic time scales, and we examine whether the skill of dynamical S2S forecasts for the region exceeds the predictability associated with ENSO and the MJO. Deterministic skill (Spearman's rank correlation) of multimodel ensemble forecasts of weekly and Week 3–4 precipitation averages is examined for starts in September–April, based on three ensemble prediction systems (EPS) from the S2S prediction project over the common 1999–2010 reforecasts period. The skill of weekly forecasts decreases with increasing lead but remains statistically significant out to Week 3 and Week 4, a level of skill that has not been previously reported at such leads. The skill of Week 3–4 forecasts is higher than that of Week 3 or Week 4 forecasts. The prediction skill of weekly and 2‐week precipitation averages from ENSO and the MJO is estimated using a multiple linear regression and found to be less than that achieved by the dynamical forecasts for Week 3, Week 4, and Week 3–4 targets. This is evidence for achievable forecast skill that extends beyond typical weather‐scale lead times and that is not simply related to ENSO or MJO signals. The opportunity for skillful predictions is illustrated through successful forecasts up to 4 weeks in advance of extreme rainfall over the Arabian peninsula related to strong tropical‐temperate interactions during weak ENSO and MJO conditions.

Beschreibung: Abstract With the improvement of Global Navigation Satellite System (GNSS) observation accuracy and the establishment of large continuously operating networks, long GNSS time series are now widely used to understand a range of Earth deformation processes. The continuously operating stations of the Crustal Movement Observation Network of China capture deformation signals due to time‐dependent tectonic, nontectonic mass loading, and potentially unknown geophysical processes. In order to separate and recover these underlying sources accurately and effectively, we apply the independent component analysis (ICA) to decompose the observed time series of vertical displacements. Then, we compare these signals with those predicted from independently developed geophysical process models of atmospheric, nontidal ocean, snow, soil moisture mass loading, and the Land Surface Discharge Model, as well as with Gravity Recovery and Climate Experiment observations. For comparison, we also perform the principal component analysis decomposition of time series and find that the ICA achieves a more consistent representation of multiple geophysical contributors to annual vertical GNSS displacements. ICA can decompose the long‐term trend and different seasonal and multiannual signals that closely correspond to the independently derived mass loading models. We find that independent contributions from atmospheric, soil moisture, and snow mass loading can be resolved from the GNSS data. Discrepancies are likely due to the correlated nature of some of the loading processes and unmodeled contributions from groundwater and surface water changes in South Central China and the Ganges Basin.

Beschreibung: Abstract Single‐domain magnetite particles exhibit minimum susceptibility along their elongation, resulting in so‐called inverse fabric of the anisotropy of magnetic susceptibility (AMS). We report the discovery of inverse AMS fabrics from pelagic clay recovered by a ∼12 m long piston core from the western North Pacific. A previous study identified fossil single‐domain magnetite produced by magnetotactic bacteria (magnetofossils) as the dominant ferrimagnetic mineral in the sediment. The inverse AMS fabrics were found in a ∼2 m zone. The ∼6 and ∼4 m of sediment above and below this zone showed normal, horizontal AMS fabrics. Rock magnetic data and ferromagnetic resonance spectroscopy indicated that magnetofossils account for most of the mean susceptibility regardless of normal or inverse AMS. This was explained by the mixing models where the inverse fabric from magnetofossils is nearly balanced by the normal fabrics of terrigenous minerals. The corrected degree of AMS carried by magnetofossils in the sediment was estimated to be ∼1.01, which is comparable to that of typical pelagic sediment at shallow depth. On the other hand, terrigenous minerals in the sediment were estimated to have higher degree of anisotropy, possibly reflecting burial and subsequent erosion of 〉80 m of sediment, which was also suggested by a subbottom acoustic stratigraphy. This suggests that inverse AMS fabrics due to magnetofossils may be widespread in pelagic clay without strong compaction.

Beschreibung: Abstract We employ strong motion seismograms and static offsets from the Global Positioning System, Interferometric Synthetic Aperture Radar, and other measurements in order to derive a coseismic slip and afterslip model of the M6.0 24 August 2014 South Napa earthquake. This earthquake ruptured an ∼13‐km‐long portion of the West Napa fault with predominantly right‐lateral strike slip. In the kinematic seismic slip inversions, we couple the coseismic slip and afterslip distributions by requiring both distributions to involve right‐lateral strike slip with positive amplitude, with the net static slip being the sum of the two. We consider several candidate fault geometries: a first involving two steeply east dipping fault planes that reach Earth's surface at the western surface trace (STW), where most surface rupture was observed, a second involving a steeply west dipping plane that also reaches Earth's surface at the STW, and a third involving a combination of two variably west dipping planes constrained to pass through the locus of postseismic seismicity located ∼1 km west of the STW. The data are best fit using the model of two east dipping fault planes, with coseismic slip up to ∼1.2 m on a dominant shallow asperity about 10 km north of the hypocenter and on deeper asperities on the southern part of the rupture. Afterslip up to 1 m is concentrated along the southern part of the rupture at depths 5 km, consistent with surface observations of afterslip. Seismic moments associated with coseismic slip and afterslip are 1.13×1018 N m (Mw 6.00) and 3.64×1017 N m, respectively.

Beschreibung: Abstract Viscosity and elasticity are material properties essential for understanding the composition, dynamics, and evolution of the Earth's core, yet their intrinsic connection as embedded in the general theory of viscoelasticity is not well explored. Here we use molecular dynamics to determine the viscoelasticity of liquid iron at conditions of the Earth's outer core. The frequency‐dependent viscosity and shear modulus are determined from the power spectrum of the stress autocorrelation function. We find that the stress autocorrelation function is well characterized by a generalized Maxwell model containing two relaxation modes. The mode with shorter relaxation time (τ1) corresponds to the motion of individual atoms; the other with longer relaxation time (τ2) is associated with collective motions. As temperature (T ) decreases, the slow‐decaying mode becomes more prominent with increasingly larger τ2. In contrast, τ1 remains nearly constant (∼0.016 ps). The infinite frequency shear modulus (G∞), which characterizes the instantaneous shear response, is found to be larger than the static shear modulus of hexagonal close‐packed (hcp) iron of the same density and increases linearly with T. Based on these findings as well as seismic analyses (Tsuboi & Saito, 2002, https://doi.org/10.1186/BF03351717; Krasnoshchekov et al., 2005, https://doi.org/10.1038/nature03613), the zero frequency viscosity (η0) of the lowermost outer core is inferred as 109 Pa·s. The likely material states exhibiting such viscosities are discussed. Moreover, we show that to retain the rigidity consistent with seismic observations, the η0 of the inner core should be at least 1013 Pa·s.

Beschreibung: Abstract The H‐κ method (Zhu and Kanamori, 2000) has been widely used to estimate the crustal thickness (H) and the ratio of P to S velocities (VP/VS ratio, κ) with receiver functions. However, in regions where the crustal structure is complicated, the method may produce biased results, arising particularly from dipping Moho and/or crustal anisotropy. H‐κ stacking in case of azimuthal or radial anisotropy with flat Moho has been proposed, but not for cases with plunging anisotropy and dipping Moho. Here, we propose a generalized H‐κ method called H‐κ‐c, which corrects for these effects first before stacking. We consider rather general cases, including plunging anisotropy and dipping interfaces of multiple layers, and use harmonic functions to correct for arrival time variations of Ps and its crustal multiples with back‐azimuth (θ). Systematic synthetic tests show the arrival time variations can be well fitted by cosθ and cos2θ functions even for very complex crustal structures. Correcting for the back‐azimuthal variations significantly enhances H‐κ stacking. We verify the feasibility of the H‐κ‐c method by applying it to 40 permanent stations in various geological setting across the Mainland China. The results show clear improvement after the harmonic corrections, with clearer multiples and stronger stacking energy, as well as more reliable H‐κ values. Large differences in H (up to 5.0 km) and κ (up to 0.09) between the new and traditional methods occur mostly in mountainous regions, where the crustal structure tends to be more complex. We caution in particular about systematic bias when the traditional method is used in the presence of dipping interfaces. The modified method is simple and applicable anywhere in the world.

Beschreibung: Abstract Pore structure is a crucial attribute in characterizing fluid flow through porous media. However, direct experimental measurements or numerical reconstructions are commonly expensive and not environmentally friendly, with great uncertainty caused by the complex nature of porous media. In this study, we demonstrate that one special "bridge function", which is a function of the apparent length and tortuosity fractal dimension, can characterize the relationship of pore structures between two dimensions (2D) and three dimensions (3D), and it can serve as a conversion bridge of the radius to determine the capillary pressure curve (CPC). We compare estimations by the proposed method with experimental results obtained by mercury intrusion porosimetry in six typical natural sandstones with varying porosities and permeabilities. The result shows that cross sections of the global pore structure, such as thin section, electronic probe, and micro computed tomography slices, give a reliable estimation of the CPC using the bridge function in porous media with a medium porosity. However, in unconventional porous media with a relatively low porosity (~10%) or extra high porosity (~30%), due to the empirical nature of the equation widely used to calculate the tortuosity fractal dimension, the necessary modification is necessary to obtain the CPC when applying the bridge function in such porous media. This insight can significantly simplify the procedure for obtaining the petrophysical properties of a porous medium, which may shed light on the inherent differences and correlations between the 2D and 3D pore structures of porous media.

Beschreibung: Abstract Seaward Dipping Reflectors (SDRs) are large piles of seaward‐thickening volcanic wedges imaged seismically along most rifted continental margins. Despite their global ubiquity, it is still debated whether the primary cause of SDR formation is tectonic faulting or magmatic loading. To study how SDRs might form we developed the first two‐dimensional thermo‐mechanical model that can account for both tectonics and magmatism development of SDRs during rifting. We focus here on the magmatic loading mechanism and show that the shape of SDRs may provide unprecedented constraints on lithospheric strength at volcanic rifting margins. For mapping SDRs geometries to lithospheric strength, a sequence of model lithospheric rheologies are treated, ranging from analytic thin elastic plates to numerical thick elasto‐visco‐plastic crust and mantle layers with temperature and stress dependent viscosity. We then analyzed multi‐channel seismic depth‐converted images of SDRs from Vøring and Argentinian rifted margins in terms of geometric parameters that can be compared to our model results. This results in estimates for the lithospheric thickness during rifting at the two margins of 3.4 and 5.7 km. The plate thickness correlates inversely with mantle potential temperature at these margins during rifting, as estimated by independent studies.

Beschreibung: Abstract We examine the sensitivity of cloud heterogeneity to large‐scale meteorology in the marine mid‐latitudes using satellite observations from the MISR and MODIS instruments aboard the Terra satellite, and nudged simulations from the UK Met Office's Global Atmosphere 7.0 (GA7) for the year 2007. Using MISR observations, we quantify several sources of observational uncertainty due to cloud heterogeneity such as finite‐resolution cloud fraction biases and sub‐pixel heterogeneity. With a simple measure of cloud geometry, we show that these sources of observational uncertainty are maximized in post‐cold‐frontal conditions, where scattered, sub‐pixel clouds are frequent, and are minimized in the warm‐sector, where sub‐pixel clouds are infrequent. These results demonstrate the greater difficulty in interpreting remote sensing measurements in post‐cold‐frontal conditions compared to quiescent or warm‐sector conditions. We show that the neglect of this observational uncertainty can qualitatively alter the interpretation of how cloud properties respond to large‐scale meteorology in GA7 and General Circulation Models in general. However, conservative application of the satellite data still allows robust evaluation of the simulated clouds. For overcast domains, MODIS observations show that overcast cloud heterogeneity is independent of large‐scale meteorology. However, heterogeneity increases when moving from warm sector to post‐cold‐frontal conditions in GA7. GA7 overestimates the heterogeneity of the low‐topped clouds that populate these regimes. For overcast domains, this bias is equivalent to a 15% underestimation in the mean cloud optical depth. These results suggest a systematic error in the subgrid‐scale variability parameterization which, when corrected, will improve the simulation of the mid‐latitudes in GA7.