ALBERT

Description: The spectral model turbulence analysis technique is widely used to derive kinetic energy dissipation rates of turbulent structures (ɛ) from different in situ measurements in the Earth's atmosphere. The essence of this method is to fit a model spectrum to measured spectra of velocity or scalar quantity fluctuations and thereby to derive ɛ only from wavenumber dependence of turbulence spectra. Owing to the simplicity of spectral model of Heisenberg (1948), https://doi.org/10.1007/bf01668899 its application dominates in the literature. Making use of direct numerical simulations which are able to resolve turbulence spectra down to the smallest scales in dissipation range, we advance the spectral model technique by quantifying uncertainties for two spectral models, the Heisenberg (1948), https://doi.org/10.1007/bf01668899 and the Tatarskii (1971) model, depending on (a) resolution of measurements, (b) stage of turbulence evolution, (c) model used. We show that the model of Tatarskii (1971) can yield more accurate results and reveals higher sensitivity to the lowest ɛ‐values. This study shows that the spectral model technique can reliably derive ɛ if measured spectra only resolve half‐decade of power change within the viscous (viscous‐convective) subrange. In summary, we give some practical recommendations on how to derive the most precise and detailed turbulence dissipation field from in situ measurements depending on their quality. We also supply program code of the spectral models used in this study in Python, IDL, and Matlab.

Description: Plain Language Summary: Turbulence plays a central role in most geophysical fluids, but our understanding of it remains limited. Atmospheric turbulence plays roles as diverse as dispersion of pollutants in the boundary layer to strong influences on the thermal and wind fields on global scales from the surface to above 100 km. It also is key to transports in, and the large‐scale circulation and structure of, Earth's oceans. Measurements quantifying turbulence intensities and their environments are key to understanding its many effects but remain challenging. In situ measurements of various quantities enable estimates of turbulence intensities but must be calibrated to be of optimal benefit. This study employs a direct numerical simulation of Kelvin‐Helmholtz instabilities that quantifies the associated turbulence dynamics exactly over the range of scales simulated to evaluate theoretical spectral forms enabling the best estimates of the known turbulence intensities.

Description: Key Points: Accuracies of spectral model turbulence analysis techniques are evaluated using high‐resolution direct numerical simulation data. The Tatarskii model shows very accurate results if measured spectra resolve the viscous subrange for more than 2 decades. The Heisenberg model yields less accurate results that are almost independent of measurement resolution.

Description: National Science Foundation (NSF) http://dx.doi.org/10.13039/100000001

Description: DOD | USAF | AFMC | Air Force Office of Scientific Research (AFOSR) http://dx.doi.org/10.13039/100000181

Description: The Earth's mass redistribution due to deglaciation and recent ice sheet melting causes changes in the Earth's gravity field and vertical land motion in Greenland. The changes are because of ongoing mass redistribution and related elastic (on a short time scale) and viscoelastic (on time scales of a few thousands of years) responses. These signatures can be used to determine the mantle viscosity. In this study, we infer the mantle viscosity associated with the glacial isostatic adjustment (GIA) and long-wavelength geoid beneath the Greenland lithosphere. The viscosity is determined based on a spatio-spectral analysis of the Earth's gravity field and the land uplift rate in order to find the GIA-related gravity field. We used different land uplift data, that is, the vertical land motions obtained by the Greenland Global Positioning System (GPS) Network (GNET), gravity recovery and climate experiment (GRACE) and glacial isostatic adjustment (GIA) data, and also combined them using the Kalman filtering technique. Using different land uplift rates, one can obtain different GIA-related gravity fields.

Description: Earthquakes associated with fluid injection in various geo-energy settings, such as shale gas and deep geothermal energy, have shelved many projects with great potential. However, the injection-rate dependence of earthquake nucleation length, i.e., the slowly slipping (creeping) fault length in preparation for a subsequent earthquake (Kaneko & Lapusta, 2008), remains elusive. In this study, we take a step towards this issue by performing fluid injection experiments on low-permeability granite samples containing a critically stressed sawcut fault at different local injection rates (0.2 mL/min and 0.8 mL/min) and confining pressures (31 MPa and 61 MPa) (c. f., Ji & Wu, 2017; Wang et al., 2020). An array of local strain gauges and acoustic emission (AE) hypocenter locations were used to monitor the precursory slip of critically stressed faults before injection-induced stick-slip failure (c. f., Passelègue et al., 2020; Wang et al., 2020). The nucleation length was determined for each injection-induced stick-slip event, and its dependence on effective normal stress and injection rate was explored. Herein, we compile the processed data obtained from the experiments in four Excel worksheets. The full description of the methods is provided in Ji et al. (2022).

Description: Understanding the physical mechanisms governing fluid‐induced fault slip is important for improved mitigation of seismic risks associated with large‐scale fluid injection. We conducted fluid‐induced fault slip experiments in the laboratory on critically stressed saw‐cut sandstone samples with high permeability using different fluid pressurization rates. Our experimental results demonstrate that fault slip behavior is governed by fluid pressurization rate rather than injection pressure. Slow stick‐slip episodes (peak slip velocity 〈 4 μm/s) are induced by fast fluid injection rate, whereas fault creep with slip velocity 〈 0.4 μm/s mainly occurs in response to slow fluid injection rate. Fluid‐induced fault slip may remain mechanically stable for loading stiffness larger than fault stiffness. Independent of fault slip mode, we observed dynamic frictional weakening of the artificial fault at elevated pore pressure. Our observations highlight that varying fluid injection rates may assist in reducing potential seismic hazards of field‐scale fluid injection projects.

Description: To understand the physical mechanisms governing fluid-induced seismicity at field-scale fluid injection projects, we conducted fluid-induced fault slip experiments in the laboratory on critically stressed saw-cut sandstone samples with high permeability using different fluid pressurization rates. The data archived here acts as supplementary material to Wang et al. (2020; https://doi.org/10.1029/2019GL086627). Experiments were conducted at room temperature using a servo-hydraulic tri-axial deformation apparatus (MTS) equipped with a pore pressure system (Quizix pumps) at Experimental Rock Deformation Laboratory, GFZ. To investigate the correlation between fault slip and fluid pressure, we applied two different fluid injection schemes (hereafter tests “SC1” and “SC2”, respectively). ‘TestSC1’ refers to the fluid-induced fault slip experiment performed at fluid pressurization rate of 2 MPa/min while ‘TestSC2’ indicates the fluid-induced fault slip experiment performed at fluid pressurization rate of 0.5 MPa/min. The other boundary conditions for both experiments are similar. In addition, to simultaneously record acoustic emission (AE) events induced by artificial fault slip, 16 piezoelectric transducers (PZTs, resonance frequency ~1 MHz) contained in brass cases were directly mounted to the surface of samples, ensuring full azimuthal coverage for AE events. AE waveforms were amplified first by 40 dB using preamplifiers equipped with 100‐kHz high‐pass filters and then recorded at a sampling rate of 10 MHz with 16‐bit amplitude resolution. Each experiment lasted for about 4 hours. Throughout the experiment, mechanical data (measured by MTS) and hydraulic data (measured by Quizix pump) were all synchronously monitored with a sampling rate of 10 Hz whereas acoustic emission data were recorded with a sampling rate of 10 MHz. All results shown are recorded as a function of experimental time. The data are provided in tab-separated ASCII-Format (.txt). 2020-002_Wang-et-al_TestSC1.zip and 2020-002_Wang-et-al_TestSC2.zip are composed of 7 txt files and 8 txt files, respectively, as described below in Table 1. The first column represents time in second and the subsequent columns are indicated by the corresponding header at the first row. The second row indicates the unit for each column data. The raw data was processed with MATLAB. The algorithms we implemented include the moving average method, statistical regression and our developed MATLAB-based codes.

Description: Understanding injection‐induced seismic moment release with operational parameters is crucial for early identification of possible seismic hazards associated with fluid‐injection projects. We conducted laboratory fluid‐injection experiments on permeable sandstone samples containing a critically stressed fault at different fluid pressurization rates. The observed fluid‐induced fault deformation is dominantly aseismic. Fluid‐induced stick‐slip and fault creep reveal that total seismic moment release of acoustic emission (AE) events are related to total injected volume, independent of respective fault slip behavior. Seismic moment release rate of AE scales with measured fault slip velocity. For injection‐induced fault slip in a homogeneous pressurized region, released moment shows a linear scaling with injected volume for stable slip (steady slip and fault creep) while we find a cubic relation for dynamic slip. Our results highlight that monitoring evolution of seismic moment release with injected volume in some cases may assist in discriminating between stable slip and unstable runaway ruptures.

Description: Based on Swarm satellite data from 2015 through 2018, we present the mean characteristics of magnetic field fluctuations at midlatitudes and low latitudes. It is the first comprehensive study focusing on small‐scale variations (〈10 km). Events are observed on about 35% of the orbits. The highest occurrence rates are detected after sunset, in the East Asian/Australian sector, and during months around June solstice. Low occurrence rates are found at low magnetic latitudes (below ±10° quasi‐dipole latitude), in the region of the South Atlantic Anomaly, and during equinox seasons. All these occurrence features compare well with those of medium‐scale traveling ionospheric disturbances. We therefore term our small‐scale events small‐scale traveling ionospheric disturbances (SSTIDs). SSTIDs exhibit high field‐aligned current (FAC) densities connected to narrow current sheets with meridional width of typically 4 km. The intense FACs of several μA/m2 flow typically between the hemispheres. Return currents are distributed over larger scales and thus have smaller amplitudes. Peak current densities get larger toward lower latitudes. There are two groups of events, around morning‐noontime and evening‐night, which are separated by demarcation lines near 04 and 15 magnetic local time. The magnetic amplitudes of the small‐scale fluctuations are larger in sunlight than in darkness, indicating larger total currents in the loops. But the FAC peak current densities are larger in darkness, inferring a stronger squeezing of the current sheet under low‐conductivity conditions. We suggest that our SSTIDs are an evolutional state of medium‐scale traveling ionospheric disturbances.

Description: Knowledge of pressure-dependent static and dynamic moduli of porous reservoir rocks is of key importance for evaluating geological setting of a reservoir in geo-energy applications. We examined experimentally the evolution of static and dynamic bulk moduli for porous Bentheim sandstone with increasing confining pressure up to about 190 MPa under dry and water-saturated conditions. The static bulk moduli (Ks) were estimated from stress–volumetric strain curves while dynamic bulk moduli (Kd) were derived from the changes in ultrasonic P- and S- wave velocities (~ 1 MHz) along different traces, which were monitored simultaneously during the entire deformation. In conjunction with published data of other porous sandstones (Berea, Navajo and Weber sandstones), our results reveal that the ratio between dynamic and static bulk moduli (Kd/Ks) reduces rapidly from about 1.5 − 2.0 at ambient pressure to about 1.1 at high pressure under dry conditions and from about 2.0 − 4.0 to about 1.5 under water-saturated conditions, respectively. We interpret such a pressure-dependent reduction by closure of narrow (compliant) cracks, highlighting that Kd/Ks is positively correlated with the amount of narrow cracks. Above the crack closure pressure, where equant (stiff) pores dominate the void space, Kd/Ks is almost constant. The enhanced difference between dynamic and static bulk moduli under water saturation compared to dry conditions is possibly caused by high pore pressure that is locally maintained if measured using high-frequency ultrasonic wave velocities. In our experiments, the pressure dependence of dynamic bulk modulus of water-saturated Bentheim sandstone at effective pressures above 5 MPa can be roughly predicted by both the effective medium theory (Mori–Tanaka scheme) and the squirt-flow model. Static bulk moduli are found to be more sensitive to narrow cracks than dynamic bulk moduli for porous sandstones under dry and water-saturated conditions.

Description: The increased fraction of first year ice (FYI) at the expense of old ice (second-year ice (SYI) and multi-year ice (MYI)) likely affects the permeability of the Arctic ice cover. This in turn influences the pathways of gases circulating therein and the exchange at interfaces with the atmosphere and ocean. We present sea ice temperature and salinity time series from different ice types relevant to temporal development of sea ice permeability and brine drainage efficiency from freeze-up in October to the onset of spring warming in May. Our study is based on a dataset collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) Expedition in 2019 and 2020. These physical properties were used to derive sea ice permeability and Rayleigh numbers. The main sites included FYI and SYI. The latter was composed of an upper layer of residual ice that had desalinated but survived the previous summer melt and became SYI. Below this ice a layer of new first-year ice formed. As the layer of new first-year ice has no direct contact with the atmosphere, we call it insulated first-year ice (IFYI). The residual/SYI-layer also contained refrozen melt ponds in some areas. During the freezing season, the residual/SYI-layer was consistently impermeable, acting as barrier for gas exchange between the atmosphere and ocean. While both FYI and SYI temperatures responded similarly to atmospheric warming events, SYI was more resilient to brine volume fraction changes because of its low salinity (〈 2). Furthermore, later bottom ice growth during spring warming was observed for SYI in comparison to FYI. The projected increase in the fraction of more permeable FYI in autumn and spring in the coming decades may favor gas exchange at the atmosphere-ice interface when sea ice acts as a source relative to the atmosphere. While the areal extent of old ice is decreasing, so is its thickness at the onset of freeze-up. Our study sets the foundation for studies on gas dynamics within the ice column and the gas exchange at both ice interfaces, i.e. with the atmosphere and the ocean.

Description: Climate oscillations with seasonal and longer periods drive surface water cycles and quasi-cycles at regional and global scales. Changes in terrestrial water storage produce responses in the Earth's gravitational field and crustal deformation. Here, we use techniques from the Global Positioning System (GPS) and the Gravity Recovery and Climate Experiment (GRACE) to reveal a normal pattern of inter-annual fluctuations in the water level in the North American Great Lakes (GL). The GRACE-estimated time series was in good agreement with in situ water level measurements from 2002 to 2018, especially in terms of phases. The amplitude of 3–4 inter-annual signals in water thickness was 4–6 cm, which is equivalent to a 50–75 km3 oscillation in surface water volume within the entire GL. The slightly larger annual and inter-annual fluctuation amplitudes estimated using GRACE data indicate that the aquifer system of the GL and its surroundings also contributes to seasonal mass changes. After 2013, water levels in the GL region rose abruptly, and the water mass increased by nearly 270 km3 until the end of 2018. We also used GPS- and GRACE-derived three-component displacements (vertical, northward, and eastward) to identify load patterns. GPS- and GRACE-estimated maximum probability source directions based on multi-channel singular spectrum analysis showed that most of the selected GPS sites point to the GL region, although the direction deviations of GPS results at a few sites are mainly caused by the combined effect of the local load and superposition of the distant GL. Our findings indicate that inter-annual displacement changes at different frequencies (1- to 8-year cycle) are primarily due to water volume fluctuations in the GL. The amplitudes estimated by GPS are greater than the GRACE-based and GL-modeled results at most stations, which reflects sensitivity differences between these geodetic solutions to the surface load, as hydrological processes in the local area around a station are difficult to identify using GRACE data with a resolution of ~300 km.