Recent Submissions

  • Numerical Study of CH4 Generation and Transport in XLPE-Insulated Cables in Continuous Vulcanization

    Ruslan, Mohd Fuad Anwari Che; Youn, Dong Joon; Aarons, Roshan; Sun, Yabin; Sun, Shuyu (Materials, MDPI AG, 2020-07-06) [Article]
    <jats:p>In this work, we apply a computational diffusion model based on Fick’s laws to study the generation and transport of methane (CH 4 ) during the production of a cross-linked polyethylene (XLPE) insulated cable. The model takes into account the heating process in a curing tube where most of the cross-linking reaction occurs and the subsequent two-stage cooling process, with water and air as the cooling media. For the calculation of CH 4 generation, the model considers the effect of temperature on the cross-linking reaction selectivity. The cross-linking reaction selectivity is a measure of the preference of cumyloxy to proceed either with a hydrogen abstraction reaction, which produces cumyl alcohol, or with a β -scission reaction, which produces acetophenone and CH 4 . The simulation results show that, during cable production, a significant amount of CH 4 is generated in the XLPE layer, which diffuses out of the cable and into the conductor part of the cable. Therefore, the diffusion pattern becomes a non-uniform radial distribution of CH 4 at the cable take-up point, which corresponds well with existing experimental data. Using the model, we perform a series of parametric studies to determine the effect of the cable production conditions, such as the curing temperature, line speed, and cooling water flow rate, on CH 4 generation and transport during cable production. The results show that the curing temperature has the largest impact on the amount of CH 4 generated and its distribution within the cable. We found that under similar curing and cooling conditions, varying the line speed induces a notable effect on the CH 4 transport within the cable, while the cooling water flow rate had no significant impact.</jats:p>
  • Simultaneous Bayesian Estimation of Non-Planar Fault Geometry and Spatially-Variable Slip

    Dutta, Rishabh; Jonsson, Sigurjon; Vasyura-Bathke, Hannes (Wiley, 2020-07-02) [Preprint]
    Large earthquakes are usually modeled with simple planar fault surfaces or a combination of several planar fault segments. However, in general, earthquakes occur on faults that are non-planar and exhibit significant geometrical variations in both the along-strike and down-dip directions at all spatial scales. Mapping of surface fault ruptures and high-resolution geodetic observations are increasingly revealing complex fault geometries near the surface and accurate locations of aftershocks often indicate geometrical complexities at depth. With better geodetic data and observations of fault ruptures, more details of complex fault geometries can be estimated resulting in more realistic fault models of large earthquakes. To address this topic, we here parametrize non-planar fault geometries with a set of polynomial parameters that allow for both along-strike and down-dip variations in the fault geometry. Our methodology uses Bayesian inference to estimate the non-planar fault parameters from geodetic data, yielding an ensemble of plausible models that characterize the uncertainties of the non-planar fault geometry and the fault slip. The method is demonstrated using synthetic tests considering checkerboard fault-slip patterns on non-planar fault surfaces with spatially-variable dip and strike angles both in the down-dip and in the along-strike directions. The results show that fault-slip estimations can be biased when a simple planar fault geometry is assumed in presence of significant non-planar geometrical variations. Our method can help to model earthquake fault sources in a more realistic way and may be extended to include multiple non-planar fault segments or other geometrical fault complexities.
  • Volcano-wide deformation after the 2017 Erta Ale dike intrusion, Ethiopia, observed with radar interferometry

    Xu, Wenbin; Xie, Lei; Aoki, Yosuke; Rivalta, Eleonora; Jonsson, Sigurjon (Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), 2020-06-25) [Article]
    Erta Ale volcano erupted on 16 Jan. 2017 in a difficult-to-access terrain in the Erta Ale volcanic range in Ethiopia. Like many other rifting ridge volcanoes, little is known about the properties of the deep magma plumbing system. Here, we analyze interferometric synthetic aperture radar data from different satellites between late Jan. 2017 and May 2019 to study the ground deformation after the start of the intrusion to infer the possible geometry and volume change of the magma reservoir that fed the eruption. We identified volcano-wide subsidence of up to 9 cm and horizontal contraction of up to ~5 cm that extend from Erta Ale to neighboring volcanoes. The modeling results suggest that an off-rift NE-SW elongated mid-crustal source is required to explain the observed volcano-wide deformation, but the depth is poorly constrained and the shape is complex. We suggest the presence of vertical interactions between stacked mid-crustal magma sources. Our study demonstrates that a considerable volume of melt could have been stored in mid-crustal magma reservoirs within the slow-spreading Erta Ale ridge to facilitate recent volcanic activity.
  • Weaker cooling by aerosols due to dust-pollution interactions

    Klingmüller, Klaus; Karydis, Vlassis A.; Bacer, Sara; Stenchikov, Georgiy L.; Lelieveld, Jos (Copernicus GmbH, 2020-06-23) [Preprint]
    <jats:p>Abstract. The interactions between aeolian dust and anthropogenic air pollution, notably chemical ageing of mineral dust and coagulation of dust and pollution particles, modify the atmospheric aerosol composition and burden. Since the aerosol particles can act as cloud condensation nuclei, this not only affects the radiative transfer directly via aerosol-radiation interactions, but also indirectly through cloud adjustments. We study both radiative effects using the global ECHAM/MESSy atmospheric chemistry-climate model (EMAC) which combines the Modular Earth Submodel System (MESSy) with the European Centre/Hamburg (ECHAM) climate model. Our simulations show that dust-pollution interactions reduce the cloud water path and hence the reflection of solar radiation. The associated climate warming outweighs the cooling which the dust-pollution interactions exert through the direct radiative effect. In total, this results in a net warming by dust-pollution interactions which moderates the negative global anthropogenic aerosol forcing at the top of the atmosphere by (0.2 ± 0.1) W m−2. </jats:p>
  • Molecular insight on competitive adsorption and diffusion characteristics of shale gas in water-bearing channels

    Gong, Liang; Shi, Ji Hong; Ding, Bin; Huang, Zhao Qin; Sun, Shuyu; Yao, Jun (Fuel, Elsevier BV, 2020-06-20) [Article]
    The shale gas adsorption and flow characteristics play essential roles in improving shale gas recovery. Motivated by the desire to clarify these characteristics carefully and precisely, a series of shale models with different water contents from 0.6 to 2.4 wt% were established. Presumably, these characteristics were sought to pin down answers by using the grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) methods respectively. Importantly, the analysis of the pore structure of these models is firstly taken into account considering its microstructure to meet the demand for the explanation of the adsorption characteristics of methane. The results showed that the enterable volumes decrease significantly with the increase of water content due to the diffusion and aggregation of water molecules in the middle of enterable pores. Intuitively, it could lead to a marked linear decrease in the adsorption amount of methane from 1.2 mmol/g to 0.6 mmol/g. A curiosity of this study is that the diffusion coefficients of methane increase as the increase of temperature and ensuring the low pressure outside the channel could boost the flux of methane intriguingly. Suffice to say, the optimum development shale gas conditions in this work are at the temperature of 358 K and in the presence of water content of 2.4 wt%. Hence, there is an expectation that this study would provide a guidance for the exploitation of shale gas in the presence of water.
  • A self-adaptive deep learning algorithm for accelerating multi-component flash calculation

    Zhang, Tao; Li, Yu; Li, Yiteng; Sun, Shuyu; Gao, Xin (Computer Methods in Applied Mechanics and Engineering, Elsevier BV, 2020-06-11) [Article]
    In this paper, the first self-adaptive deep learning algorithm is proposed in details to accelerate flash calculations, which can quantitatively predict the total number of phases in the mixture and related thermodynamic properties at equilibrium for realistic reservoir fluids with a large number of components under various environmental conditions. A thermodynamically consistent scheme for phase equilibrium calculation is adopted and implemented at specified moles, volume and temperature, and the flash results are used as the ground truth for training and testing the deep neural network. The critical properties of each component are considered as the input features of the neural network and the final output is the total number of phases at equilibrium and the molar compositions in each phase. Two network structures are well designed, one of which transforms the input of various numbers of components in the training and the objective fluid mixture into a unified space before entering the productive neural network. “Ghost components” are defined and introduced to process the data padding work in order to modify the dimension of input flash calculation data to meet the training and testing requirements of the target fluid mixture. Hyperparameters on both two neural networks are carefully tuned in order to ensure the physical correlations underneath the input parameters are preserved properly through the learning process. This combined structure can make our deep learning algorithm to be self-adaptive to the change of input components and dimensions. Furthermore, two Softmax functions are used in the last layer to enforce the constraint that the summation of mole fractions in each phase is equal to 1. An example is presented that the flash calculation results of a 8-component Eagle Ford oil is used as input to estimate the phase equilibrium state of a 14-component Eagle Ford oil. The results are satisfactory with very small estimation errors. The capability of the proposed deep learning algorithm is also verified that simultaneously completes phase stability test and phase splitting calculation. Remarks are concluded at the end to provide some guidance for further research in this direction, especially the potential application of newly developed neural network models.
  • Magnitude-Dependent Transient Increase of Seismogenic Depth

    Zielke, Olaf; Schorlemmer, Danijel; Jonsson, Sigurjon; Mai, Paul Martin (Seismological Research Letters, Seismological Society of America (SSA), 2020-06-10) [Article]
    <jats:title>Abstract</jats:title> <jats:p>The thickness of the seismogenic zone in the Earth’s crust plays an important role in seismotectonics, affecting fault-system architecture and relative fault activity, earthquake size and distribution within a fault system, as well as long-term accumulation of tectonic deformation. Within the last two decades, several studies have revealed that aftershocks of large continental earthquakes may occur below the background depth of the seismogenic zone, that is, below the seismic–aseismic transition zone. This observation may be explained with a strain- and strain-rate-induced shift in rheological behavior that follows large mainshocks, transiently changing the deformation style below the seismogenic zone from incipient ductile to seismically brittle failure. As large earthquakes transiently deepen the seismic–aseismic transition zone, it is plausible to assume that larger mainshocks may cause stronger deepening than smaller mainshocks. Corresponding observations, however, have not yet been reported. Here, we use well-located seismic catalogs from Alaska, California, Japan, and Turkey to analyze if mainshock size positively correlates with the amount of transient deepening of the seismic–aseismic transition zone. We compare the depths of background seismicity with aftershock depths of 16 continental strike-slip earthquakes (6≤M≤7.8) and find that large mainshocks do cause stronger transient deepening than moderate-size mainshocks. We further suggest that this deepening effect also applies to the mainshocks themselves, with larger mainshock coseismic ruptures being capable of extending deeper into the normally aseismic zone. This understanding may help address fundamental questions of earthquake-source physics such as the assumed scale invariance of earthquake stress drop and whether fault-slip scales with rupture length or rupture width.</jats:p>
  • Thermodynamically consistent modeling of two-phase incompressible flows in heterogeneous and fractured media

    Gao, Huicai; Kou, Jisheng; Sun, Shuyu; Wang, Xiuhua (Oil & Gas Science and Technology – Revue d’IFP Energies nouvelles, EDP Sciences, 2020-06-05) [Article]
    <jats:p>Numerical modeling of two-phase flows in heterogeneous and fractured media is of great interest in petroleum reservoir engineering. The classical model for two-phase flows in porous media is not completely thermodynamically consistent since the energy reconstructed from the capillary pressure does not involve the ideal fluid energy of both phases and attraction effect between two phases. On the other hand, the saturation may be discontinuous in heterogeneous and fractured media, and thus the saturation gradient may be not well defined. Consequently, the classical phase-field models can not be applied due to the use of diffuse interfaces. In this paper, we propose a new thermodynamically consistent energy-based model for two-phase flows in heterogeneous and fractured media, which is free of the gradient energy. Meanwhile, the model inherits the key features of the traditional models of two-phase flows in porous media, including relative permeability, volumetric phase velocity and capillarity effect. To characterize the capillarity effect, a logarithmic energy potential is proposed as the free energy function, which is more realistic than the commonly used double well potential. The model combines with the discrete fracture model to describe two-phase flows in fractured media. The popularly used implicit pressure explicit saturation method is used to simulate the model. Finally, the experimental verification of the model and numerical simulation results are provided.</jats:p>
  • Frequency-domain least-squares generalized internal multiple imaging with the energy norm

    Wang, Guanchao; Guo, Qiang; Alkhalifah, Tariq Ali; Wang, Shangxu (GEOPHYSICS, Society of Exploration Geophysicists, 2020-06-05) [Article]
    <jats:p> Recorded seismic data contain various types of scattered energy, including those corresponding to multiples. Traditional imaging techniques are focused on the single-scattering events, and thus, may fail to image crucial structures, such as salt flanks and faults that sometimes are only illuminated by the multiple scattered energy. The recently introduced generalized internal multiple imaging (GIMI) offers an opportunity to image multiples by projecting the recorded data back into the subsurface, followed by an interferometric cross-correlation of the subsurface wavefield with the recorded data. During this process, the interferometric step converts the first-order scattering to a tomographic component and the double-scattering forms the primary reflectivity. Dealing with a large volume of data consisting of full wavefields over the image space, renders the interferometric step computationally expensive in time-domain. To make the implementation of GIMI tractable, we formulate its frequency-domain version. Moreover, we use the energy norm imaging condition to separate the reflectivity part from the tomographic component. We demonstrate these features with numerical experiments. </jats:p>
  • Hybrid frequency-domain full-waveform inversion using ray+Born sensitivity kernels

    Djebbi, Ramzi; Alkhalifah, Tariq Ali (GEOPHYSICS, Society of Exploration Geophysicists, 2020-06-05) [Article]
    <jats:p> Full-waveform inversion (FWI) using the scattering integral (SI) approach is an explicit formulation of the inversion optimization problem. The inversion procedure is straightforward, and the dependence of the data residuals on the model parameters is clear. However, the biggest limitation associated with this approach is the huge computational cost in conventional exploration seismology applications. Modeling from each of the source and receiver locations is required to compute the update at every iteration, and that is prohibitively expensive, especially for 3D problems. To deal with this issue, we have developed a hybrid implementation of frequency-domain FWI, in which forward modeling is combined with ray tracing to compute the update. We use the sensitivity kernels computed from dynamic ray tracing to build the gradient. The data residual is still computed using finite-difference wavefield modeling. With ray theory, the Green’s function can be approximated using a coarser grid compared to wave-equation modeling. Therefore, the memory requirements, as well as the computational cost, are reduced significantly. Considering that in transmission FWI long-to-intermediate wavelengths are updated during the early iterations, we obtain accurate inverted models. The inversion scheme captured the anomaly embedded in the homogeneous background medium. For more complex models, the hybrid inversion method helps in improving the initial model with little cost compared with conventional SI inversion approaches. The accuracy of the inversion results shows the effectiveness of the hybrid approach for 3D realistic problems. </jats:p>
  • Evaluation of elastoplastic properties of brittle sandstone at microscale using micro-indentation test and simulation

    Song, Rui; Wang, Yao; Sun, Shuyu; Cui, Mengmeng; Liu, Jianjun (Energy Science & Engineering, Wiley, 2020-06-05) [Article]
    The micro-indentation test has been regarded as an efficient tool to obtain the elasticity modulus and hardness of the minerals in rock, which is essential for studying the deformation-crack mechanism of the pore structure. However, researches on microscopic plastic parameters have been rarely conducted. This paper develops a novel method to determine the microscopic initial strength and residual strength of brittle sandstone. A dimensionless analysis on the micro-indentation curve of rock is conducted to acquire its key influencing factors of the elastoplastic properties, which include the initial cohesive force and the residual cohesive force. Then, small cylindrical rock samples are prepared for micro-CT scanning and micro-indentation test by a conical indenter to acquire the microstructure, indentation curve, and the microscale elasticity. The pore scale indentation simulation is conducted using the reconstructed rock models with different strength. The function between the indentation curve and strength is deduced by the parametric finite element method (FEM) study. Based on this function, the microscale initial strength and residual strength of the brittle sandstone are determined. The proposed method is validated by comparing the microscale numerical simulation results of uniaxial compression on the representative volume element (RVE) model of rock with the experimental results. A reasonable deviation is observed compared with the experimental benchmark data for the stress-strain curves, as well as Young's modulus and uniaxial compression strength, proving the effectiveness of the proposed method.
  • Thermodynamic modeling of CO2 solubility in saline water using NVT flash with the cubic-Plus-association equation of state

    Li, Yiteng; Qiao, Zhonghua; Sun, Shuyu; Zhang, Tao (Fluid Phase Equilibria, Elsevier BV, 2020-05-31) [Article]
    The accurate estimation of CO2 sequestration potential in deep saline aquifers requires the knowledge of CO2 solubility in brine, thus placing importance on reliable thermodynamic models that account for the effect of different salts and their mixtures over wide ranges of pressure, temperature and salt concentration. Most literature investigated CO2 solubility in a single-salt solution as a replacement of real saline water, which may significantly overestimate CO2 sequestration potential through solubility trapping. In order to accurately estimate CO2 sequestration potential over geological conditions, the Peng-Robinson Cubic-Plus-Association (PR-CPA) equation of state (EOS) is used in this study to model both aqueous and nonaqueous phases. A promising flash technique at given moles, volume and temperature, known as NVT flash, is employed and the salting-out effect is reproduced by correcting the chemical potential of aqueous nonelectrolyte components. To represent real saline environments, five salts are considered, including sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl2), magnesium chloride (MgCl2) and sodium sulfate (Na2SO4). With taking into account the electrostatic contribution caused by salts, the combination of the salt-based PR-CPA EOS and NVT flash accurately models the solubility behavior of CO2 in mixed-salt solutions and the numerical results agree with experimental data very well. Moreover, the proposed CPA model exhibits neck-to-neck accuracy to the more sophisticated electrolyte CPA EOS, thus making it promising to accurately estimate carbon sequestration potential in saline aquifers through solubility trapping.
  • Variability of water exchanges through the Strait of Hormuz

    Vasou, Panagiotis; Vervatis, Vassilios; Krokos, Georgios; Hoteit, Ibrahim; Sofianos, Sarantis. S. (Ocean Dynamics, Springer Science and Business Media LLC, 2020-05-30) [Article]
    The variability of the water mass exchange between the Arabian Gulf and the Indian Ocean is investigated using a high-resolution (1/36°) ocean model. We focus on the period from December 1996 to March 1998, having as reference in situ measurements at the Strait of Hormuz. Previous studies, based on models and observations, suggested a perpetual deep outflow, mainly in the southern part of the Strait, and a variable flow in the upper layers. In the present study, we confirm that there is a permanent core of a deep outflow in the Strait at depths greater than 40 m, characterised by high-salinity waters. In addition, we show that there is a seasonal signal in the upper layers net flow in the southern part of the Strait, altering from net inflow during winter/spring to net outflow during summer/fall. The mean annual inflow through the Strait is estimated at 0.22 ± 0.01 Sv and the deep outflow at 0.147 ± 0.01 Sv. The water mass exchange through the Strait is controlled by synoptic processes with high variability net transport fields. These processes characterise the structure and the intensity of the transport patterns, exhibiting 2- to 5-day period. On synoptic time scales, winds drive an immediate baroclinic flow at the Strait of Hormuz, affecting mostly the upper layers, and a quasi-barotropic flow that peaks approximately 2 days later.
  • Nonlinearly preconditioned constraint-preserving algorithms for subsurface three-phase flow with capillarity

    Yang, Haijian; Li, Yiteng; Sun, Shuyu (Computer Methods in Applied Mechanics and Engineering, Elsevier BV, 2020-05-29) [Article]
    The multiphase flow model has been extensively used to describe complicated flow behaviors in subsurface formations, together with sophisticated reservoir models and well-defined fluid property. In this study, the fully implicit method, as one of most promising schemes for subsurface flow modeling, is employed to solve multiphase flow problems. In contrast to the conventional approach where mathematical models often include a pressure equation, the multiphase flow problems are modeled by up to three continuity equations so that mass conservation holds for all present phases. Another challenge that frequently shows up is the computed solution may sit outside its physically meaningful range, thereby leading to inaccurate predictions or even a failure of the simulation process. A simple remedy is to apply a cutting-off operation to the out-of-bound solution but such an action could ruin both local and global mass conservation. Instead, we replace the original model by a variational inequality formulation with box inequality constraints to protect the boundedness requirement on pressure and saturations from being violated. The variational inequality problem is then solved by a well-designed nonlinear solver consisting of the active-set reduced-space method and the nonlinear elimination preconditioning technique. A number of examples are presented to demonstrate that the proposed formulation is bound-preserving and mass-conservative for each of the present phases/components.
  • Air-quality assessment over the world’s Most Ambitious Project, NEOM in Kingdom of Saudi Arabia

    Dasari, Hari Prasad; Desamsetti, Srinivas; Langodan, Sabique; Rama Krishna, L.N.K; Singh, Shyamcharan; Hoteit, Ibrahim (IOP Conference Series: Earth and Environmental Science, IOP Publishing, 2020-05-28) [Article]
    NEOM is an under-development transnational city and economic zone spreading over an area of 26,500 along the northern Red Sea coast of Saudi Arabia, bordering Jordan and Egypt. This work analyzes the meteorological parameters and air pollution dispersion over the NEOM region based on observations and air-quality dispersion modeling. The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model was implemented to simulate air parcel trajectories, as well as transport, dispersion, chemical transformation, and deposition. To drive HYSPLIT, high-resolution meteorological data generated at 600 m resolution by downscaling ECMWF global reanalysis using the Weather Research and Forecasting (WRF) were used. The United Sates Environmental Protection Agency Air Pollutant Emission Factors 42 emission inventory was used to initialize HYSPLIT. A continuous three-year meteorological and air-quality data from WRF-HYSPLIT model is used to analyze the spatial and temporal distributions of air pollutants’ concentration over the NEOM region. Strong land and sea breezes resulting from the differential heating dominates the diurnal dispersion and distribution of pollutants in the NEOM region. The spatial distributions of the mean seasonal ambient air pollution dispersion show similar patterns, with relatively higher concentrations in spring and winter. This is more pronounced in the spatial distributions of the maximum concentrations of different pollutants, which show the maximum concentrations in the spring and winter due to lower boundary layer heights. The predicted maximum concentrations of NOX (~40 mg/m3), SO2 (~25 mg/m3), CO (~10 mg/m3), VOC (~0.05 mg/m3), and PMT (~4 mg/m3) over the study region remain well below the National Air Quality standards recommended by the Saudi General Authority for Meteorology and Environment Protection and the Royal commission. Our analysis provides needed information to understand the state of the air quality over the NEOM region, providing fundamental contribution to the environment impact assessment and planning in the region.
  • Non-Volcanic Earthquake Swarm Near the Harrat Lunayyir Volcanic Field, Saudi Arabia

    Youssof, Mohammad; Mai, Paul Martin; Nobile, Adriano; Jonsson, Sigurjon (Wiley, 2020-05-21) [Preprint]
    Understanding the origin of seismic swarms can be controversial, especially when they occur near volcanic areas. Here, we investigate a seismic sequence which is steadily active in a non-volcanic area close by the volcanic field of Harrat Lunayyir in the western shield of Arabia. Our results unveil a planar zone of seismicity with ~5 km long E-W, sub-vertically ~9 km south-dipping structure, which is characterized by a dominant tensional focal mechanism. Independent evidence for the tectonic style dominance came from assessing the ground deformation images using the InSAR technique. This local seismicity might be attributed to a reactivated structure along a regional weakness zone of the Najd Fault System, which dominates the Precambrian structure of our area. Comparing the effects of high- and low-frequency datasets for the moment tensor inversion conclude a consistency of our solution. The frequency index analysis for P- and S- waves spectral datasets, does not suggest fluid-driven processes. We observe average stress drop of ~5.40 MPa with corner frequency of ~2.75 Hz. Our study confirms a localized reactivation of a brittle crustal seismogenic zone in the area of interest. This interpretation relies on the integration of several analysis methods, including spatial and magnitude-frequency distributions statistics.
  • Improving dust simulations in WRF-Chem model v4.1.3 coupled with GOCART aerosol module

    Ukhov, Alexander; Ahmadov, Ravan; Grell, Georg; Stenchikov, Georgiy L. (Copernicus GmbH, 2020-05-19) [Preprint]
    <jats:p><p><strong>Abstract.</strong> In this paper, we rectify inconsistencies that emerge in the WRF-Chem code when using the Goddard Chemistry Aerosol Radiation and Transport (GOCART) aerosol module. These inconsistencies have been reported and corrections have been implemented in WRF-Chem v4.1.3. Here, we use a WRF-Chem experimental setup configured over the Middle East (ME) to estimate the effects of these inconsistencies. Firstly, we show that the diagnostic output of PM<sub>2.5</sub> surface concentration was underestimated by 7% and PM10 was overestimated by 5&#8201;%. Secondly, we demonstrate that the contribution of sub-micron dust particles was underestimated in the calculation of optical properties and thus, Aerosol Optical Depth (AOD) was consequently underestimated by 25&#8211;30&#8201;%. Thirdly, we show that an inconsistency in the process of gravitational settling led to the overestimation of the dust column loadings by 4&#8211;6&#8201;%, PM10 surface concentrations by 2&#8211;4&#8201;%, and the rate of dust gravitational settling by 5&#8211;10&#8201;%. We present a methodology to calculate diagnostics that can be used to estimate the effects of these applied changes. Our corrections also help to explain the overestimation of PM<sub>10</sub> surface concentrations encountered in many WRF-Chem simulations. We also share the developed <i>Merra2BC</i> interpolator, which allows constructing initial and boundary conditions for chemical species and aerosols based on MERRA-2 reanalysis. The results of this work can be useful for those who simulate the dust cycle using the WRF-Chem model coupled with the GOCART aerosol module.</p> </jats:p>
  • Increasing heavy rainfall events in South India due to changing land use land cover

    Boyaj, Alugula; Dasari, Hari Prasad; Hoteit, Ibrahim; Ashok, Karumuri (Quarterly Journal of the Royal Meteorological Society, Wiley, 2020-05-18) [Article]
    Through an analysis of land use land cover (LULC) data for the years 2005 and 2017 from the Advanced Wide Field Sensor onboard the Indian Remote Sensing satellite, we find considerable changes in the LULC in three major states of South India, namely, Tamil Nadu, Telangana, and Kerala. This change is mainly due to increasing urbanization, in addition to the change of prevalent mixed forest into deciduous needle/leaf forest in Kerala. Motivated by this finding, we study the impact of these LULC changes over a decade on the extremity of twelve heavy rainfall events in these states through several sensitivity experiments with a convection-permitting Weather Research and Forecasting model, by changing the LULC boundary conditions. We particularly focus on three representative heavy rainfall events, specifically, over (i) Chennai (December 01, 2015), (ii) Telangana (September 24, 2016), and (iii) Kerala (August 15, 2018). The simulated rainfall patterns of the three heavy rainfall events are found to be relatively better with the use of the 2017 LULC boundary conditions. The improvement is statistically significant in the case of the Chennai and Kerala events. On analysis of these simulations, and outputs from additional simulations we have conducted for nine other heavy rainfall events, we suggest that the recent LULC changes result in higher surface temperatures, sensible heat fluxes, and a deeper and moist boundary layer. This causes a relatively higher convective available potential energy and, consequently, heavier rainfall. We find the LULC changes in the three states, mainly dominated by the increasing urbanization in Telangana and Tamil Nadu, enhance the rainfall during the heavy rainfall events by 20% - 25%. This is the first extensive investigation of multiple and multi-regional cases over the Indian region.
  • A POD-DEIM Reduced Model for Compressible Gas Reservoir Flow Based on the Peng-Robinson Equation of State

    Li, Jingfa; Fan, Xiaolin; Wang, Yi; Yu, Bo; Sun, Shuyu; Sun, Dongliang (Journal of Natural Gas Science and Engineering, Elsevier BV, 2020-05-16) [Article]
    The efficient simulation of gas flow in porous media is highly required in petroleum engineering, CO2 sequestration, etc. However, it still remains a great challenge attributing to the gas compressibility compared with incompressible fluid flows. The commonly-used equation of states (EOS) of the gas are cubic equation and need to be updated in each iteration, and it also leads to the nonlinearity in flow equations, thus the computational cost mainly stems from the solution of gas EOS. In this paper, a hybrid reduced order model (ROM) coupling the proper orthogonal decomposition (POD) and the discrete empirical interpolation method (DEIM) is presented to accelerate the calculation of compressible single-phase gas reservoir flow, in which the Peng-Robinson EOS (P-R EOS) is considered to describe the gas states. To this end, in the hybrid ROM framework POD is applied to solve the flow equation and DEIM is used to solve the P-R EOS, respectively. The selection of POD modes and DEIM interpolation points, which plays a crucial role in the hybrid ROM, is discussed and carried out carefully. Performances of the proposed POD-DEIM-ROM are evaluated and demonstrated by two numerical cases. Simulation results illustrate that the proposed hybrid ROM displays a satisfactory computational speed-up (two orders of magnitudes faster) without sacrificing numerical accuracy significantly compared with the standard finite difference method. In addition, DEIM shows excellent acceleration and it is a perfect choice for solving the cubic gas EOS.
  • Multiparameter reflection waveform inversion for acoustic VTI media

    Li, Yuanyuan; Alkhalifah, Tariq Ali (Geophysical Prospecting, Wiley, 2020-05-13) [Article]
    Full waveform inversion (FWI) in transversely isotropic media with a vertical symmetry axis (VTI) provides an opportunity to better match the data at the near and far offsets. However, multiparameter FWI, in general, suffers from serious cycle-skipping and trade-off problems. Reflection waveform inversion (RWI) can help us recover a background model by projecting the residuals of reflections along the reflection wavepath. Thus, we extend RWI to acoustic VTI media utilizing the proper parameterization for reduced parameter tradeoff. From a radiation patterns analysis, an acoustic VTI medium is better described by a combination of the normal-moveout velocity v n and the anisotropic parameters η and δ for RWI applications. We design a three-stage inversion strategy to construct the optimal VTI model. In the first stage, we only invert for the background v n by matching the simulated reflections from the perturbations of v n and δ with the observed reflections. In the second stage, the background v n and η are optimized simultaneously and the far-offset reflections mainly contribute to their updates. We perform Born modelling to compute the reflections for the two stages of RWI. In the third stage, we perform FWI for the acoustic VTI medium to delineate the high-wavenumber structures. For this stage, the VTI medium is described by a combination of the horizontal velocity v h , η and ε instead of v n , η and δ. The acoustic VTI FWI utilizes the diving waves to improve the background, as well as utilizes the reflections for high-resolution information. Finally, we test our inversion algorithm on the modified VTI Sigsbee 2A model (a salt free part) and a 2D line from a 3D Ocean Bottom Cable dataset. The results demonstrate that the proposed VTI RWI approach can recover the background model for acoustic VTI media starting from an isotropic model. This background VTI model can mitigate the cycle skipping of FWI and help the inversion recover higher-resolution structures.

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