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Shapiro, Alan, Joshua G. Gebauer, Nathan A. Dahl, David J. Bodine, Andrew Mahre i Corey K. Potvin. "Spatially Variable Advection Correction of Doppler Radial Velocity Data". Journal of the Atmospheric Sciences 78, nr 1 (styczeń 2021): 167–88. http://dx.doi.org/10.1175/jas-d-20-0048.1.
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AbstractTechniques to mitigate analysis errors arising from the nonsimultaneity of data collections typically use advection-correction procedures based on the hypothesis (frozen turbulence) that the analyzed field can be represented as a pattern of unchanging form in horizontal translation. It is more difficult to advection correct the radial velocity than the reflectivity because even if the vector velocity field satisfies this hypothesis, its radial component does not—but that component does satisfy a second-derivative condition. We treat the advection correction of the radial velocity (υr) as a variational problem in which errors in that second-derivative condition are minimized subject to smoothness constraints on spatially variable pattern-translation components (U, V). The Euler–Lagrange equations are derived, and an iterative trajectory-based solution is developed in which U, V, and υr are analyzed together. The analysis code is first verified using analytical data, and then tested using Atmospheric Imaging Radar (AIR) data from a band of heavy rainfall on 4 September 2018 near El Reno, Oklahoma, and a decaying tornado on 27 May 2015 near Canadian, Texas. In both cases, the analyzed υr field has smaller root-mean-square errors and larger correlation coefficients than in analyses based on persistence, linear time interpolation, or advection correction using constant U and V. As some experimentation is needed to obtain appropriate parameter values, the procedure is more suitable for non-real-time applications than use in an operational setting. In particular, the degree of spatial variability in U and V, and the associated errors in the analyzed υr field are strongly dependent on a smoothness parameter.
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Shapiro, Alan, Stefan Rahimi, Corey K. Potvin i Leigh Orf. "On the Use of Advection Correction in Trajectory Calculations". Journal of the Atmospheric Sciences 72, nr 11 (1.11.2015): 4261–80. http://dx.doi.org/10.1175/jas-d-15-0095.1.
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Abstract An advection correction procedure is used to mitigate temporal interpolation errors in trajectory analyses constructed from gridded (in space and time) velocity data. The procedure is based on a technique introduced by Gal-Chen to reduce radar data analysis errors arising for the nonsimultaneity of the data collection. Experiments are conducted using data from a high-resolution Cloud Model 1 (CM1) numerical model simulation of a supercell storm initialized within an environment representative of the 24 May 2011 El Reno, Oklahoma, tornadic supercell storm. Trajectory analyses using advection correction are compared to traditional trajectory analyses using linear time interpolation. Backward trajectories are integrated over a 5-min period for a range of data input time intervals and for velocity-pattern-translation estimates obtained from different analysis subdomain sizes (box widths) and first-guess options. The use of advection correction reduces trajectory end-point position errors for a large majority of the trajectories in the analysis domain, with substantial improvements for trajectories launched in the vicinity of the model storm’s gust front and in bands within the rear-flank downdraft. However, the pattern-translation components retrieved by this procedure may be nonunique if the data input time intervals are too large.
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Shapiro, Alan, Katherine M. Willingham i Corey K. Potvin. "Spatially Variable Advection Correction of Radar Data. Part I: Theoretical Considerations". Journal of the Atmospheric Sciences 67, nr 11 (1.11.2010): 3445–56. http://dx.doi.org/10.1175/2010jas3465.1.
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Abstract Radar data–based analysis products, such as accumulated rainfall maps, dual-Doppler wind syntheses, and thermodynamic retrievals, are prone to substantial error if the temporal sampling interval is too coarse. Techniques to mitigate these errors typically make use of advection-correction procedures (space-to-time conversions) in which the analyzed radial velocity or reflectivity field is idealized as a pattern of unchanging form that translates horizontally at constant speed. The present study is concerned with an advection-correction procedure for the reflectivity field in which the pattern-advection components vary spatially. The analysis is phrased as a variational problem in which errors in the frozen-turbulence constraint are minimized subject to smoothness constraints. The Euler–Lagrange equations for this problem are derived and a solution is proposed in which the trajectories, pattern-advection fields, and reflectivity field are analyzed simultaneously using a combined analytical and numerical procedure. The potential for solution nonuniqueness is explored.
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Miller, William, i Da-Lin Zhang. "A Three-Dimensional Trajectory Model with Advection Correction for Tropical Cyclones: Algorithm Description and Tests for Accuracy". Monthly Weather Review 147, nr 9 (12.08.2019): 3145–67. http://dx.doi.org/10.1175/mwr-d-18-0434.1.
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Abstract When computing trajectories from model output, gridded winds are often temporally interpolated to a time step shorter than model output intervals to satisfy computational stability constraints. This study investigates whether trajectory accuracy may be improved for tropical cyclone (TC) applications by interpolating the model winds using advection correction (AC) instead of the traditional linear interpolation in time (LI) method. Originally developed for Doppler radar processing, AC algorithms interpolate data in a reference frame that moves with the pattern translation, or advective flow velocity. A previously developed trajectory AC implementation is modified here by extending it to three-dimensional (3D) flows, and the advective flows are defined in cylindrical rather than Cartesian coordinates. This AC algorithm is tested on two model-simulated TC cases, Hurricanes Joaquin (2015) and Wilma (2005). Several variations of the AC algorithm are compared to LI on a sample of 10 201 backward trajectories computed from the modeled 5-min output data, using reference trajectories computed from 1-min output to quantify position errors. Results show that AC of 3D wind vectors using advective flows defined as local gridpoint averages improves the accuracy of most trajectories, with more substantial improvements being found in the inner eyewall where the horizontal flows are dominated by rotating cyclonic wind perturbations. Furthermore, AC eliminates oscillations in vertical velocity along LI backward trajectories run through deep convective updrafts, leading to a ~2.5-km correction in parcel height after 20 min of integration.
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Wildmann, Norman, Eileen Päschke, Anke Roiger i Christian Mallaun. "Towards improved turbulence estimation with Doppler wind lidar velocity-azimuth display (VAD) scans". Atmospheric Measurement Techniques 13, nr 8 (4.08.2020): 4141–58. http://dx.doi.org/10.5194/amt-13-4141-2020.
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Abstract. The retrieval of turbulence parameters with profiling Doppler wind lidars (DWLs) is of high interest for boundary layer meteorology and its applications. DWLs provide wind measurements above the level of meteorological masts while being easier and less expensive to deploy. Velocity-azimuth display (VAD) scans can be used to retrieve the turbulence kinetic energy (TKE) dissipation rate through a fit of measured azimuth structure functions to a theoretical model. At the elevation angle of 35.3∘ it is also possible to derive TKE. Modifications to existing retrieval methods are introduced in this study to reduce errors due to advection and enable retrievals with a low number of scans. Data from two experiments are utilized for validation: first, measurements at the Meteorological Observatory Lindenberg–Richard-Aßmann Observatory (MOL-RAO) are used for the validation of the DWL retrieval with sonic anemometers on a meteorological mast. Second, distributed measurements of three DWLs during the CoMet campaign with two different elevation angles are analyzed. For the first time, the ground-based DWL VAD retrievals of TKE and its dissipation rate are compared to in situ measurements of a research aircraft (here: DLR Cessna Grand Caravan 208B), which allows for measurements of turbulence above the altitudes that are in range for sonic anemometers. From the validation against the sonic anemometers we confirm that lidar measurements can be significantly improved by the introduction of the volume-averaging effect into the retrieval. We introduce a correction for advection in the retrieval that only shows minor reductions in the TKE error for 35.3∘ VAD scans. A significant bias reduction can be achieved with this advection correction for the TKE dissipation rate retrieval from 75∘ VAD scans at the lowest measurement heights. Successive scans at 35.3 and 75∘ from the CoMet campaign are shown to provide TKE dissipation rates with a good correlation of R>0.8 if all corrections are applied. The validation against the research aircraft encourages more targeted validation experiments to better understand and quantify the underestimation of lidar measurements in low-turbulence regimes and altitudes above tower heights.
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Satheesh, Kiran, Gowtham Srinivas i Santosh Hemchandra. "An advection velocity correction scheme for interface tracking using the level-set method". Computers & Fluids 168 (maj 2018): 232–44. http://dx.doi.org/10.1016/j.compfluid.2018.04.010.
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Oue, Mariko, Pavlos Kollias, Alan Shapiro, Aleksandra Tatarevic i Toshihisa Matsui. "Investigation of observational error sources in multi-Doppler-radar three-dimensional variational vertical air motion retrievals". Atmospheric Measurement Techniques 12, nr 3 (29.03.2019): 1999–2018. http://dx.doi.org/10.5194/amt-12-1999-2019.
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Abstract. Multi-Doppler-radar network observations have been used in different configurations over the last several decades to conduct three-dimensional wind retrievals in mesoscale convective systems. Here, the impacts of the selected radar volume coverage pattern (VCP), the sampling time for the VCP, the number of radars used, and the added value of advection correction on the retrieval of the vertical air motion in the upper part of convective clouds are examined using the Weather Research and Forecasting (WRF) model simulation, the Cloud Resolving Model Radar SIMulator (CR-SIM), and a three-dimensional variational multi-Doppler-radar retrieval technique. Comparisons between the model truth (i.e., WRF kinematic fields) and updraft properties (updraft fraction, updraft magnitude, and mass flux) retrieved from the CR-SIM-generated multi-Doppler-radar field are used to investigate these impacts. The findings are that (1) the VCP elevation strategy and sampling time have a significant effect on the retrieved updraft properties above 6 km in altitude; (2) 2 min or shorter VCPs have small impacts on the retrievals, and the errors are comparable to retrievals using a snapshot cloud field; (3) increasing the density of elevation angles in the VCP appears to be more effective to reduce the uncertainty than an addition of data from one more radar, if the VCP is performed in 2 min; and (4) the use of dense elevation angles combined with an advection correction applied to the 2 min VCPs can effectively improve the updraft retrievals, but for longer VCP sampling periods (5 min) the value of advection correction is challenging. This study highlights several limiting factors in the retrieval of upper-level vertical velocity from multi-Doppler-radar networks and suggests that the use of rapid-scan radars can substantially improve the quality of wind retrievals if conducted in a limited spatial domain.
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Al-Ibadi, Hasan, Karl Stephen i Eric Mackay. "Novel Observations of Salinity Transport in Low-Salinity Waterflooding". SPE Journal 24, nr 03 (9.01.2019): 1108–22. http://dx.doi.org/10.2118/190068-pa.
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Summary Low-salinity waterflooding (LSWF) is a promising process that could lead to increased oil recovery. To date, the greatest attention has been paid to the complex oil/water/rock chemical reactions that might explain the mechanisms of LSWF, and it is generally accepted that these result in behavior equivalent to changing oil and water mobility. This behavior is modeled using an effective salinity range and weighting function to gradually switch from high- to low-salinity relative permeability curves. There has been limited attention on physical transport of fluids during LSWF, particularly at large scale. We focus on how the salinity profile interacts with water fronts through the effective salinity range and dispersion to alter the transport behavior and change the flow velocities, particularly for the salinity profile. We examined a numerical simulation of LSWF at the reservoir scale. Various representations of the effective salinity range and weighting function were also examined. The dispersion of salinity was compared with a theoretical form of numerical dispersion based on input parameters. We also compared salinity movement with the analytical solution of the conventional dispersion/advection equation. From simulations we observed that salinity is dispersed as analytically predicted, although the advection velocity might be changed. In advection-dominated flow, the salinity profile moves at the speed of the injected water. However, as dispersion increases, the mixing zone falls under the influence of the faster-moving formation water and, thus, speeds up. To predict the salinity profile theoretically, we have modified the advection term of the analytical solution as a function of the formation- and injected-water velocities, Péclet number, and effective salinity range. This important result enables prediction of the salinity transport by this newly derived modification of the analytical solution for 1D flow. We can understand the correction to the flow behavior and quantify it from the model input parameters. At the reservoir scale, we typically simulate flow on coarse grids, which introduces numerical dispersion or must include physical dispersion from underlying heterogeneity. Corrections to the equations can contribute to improving the precision of the coarse-scale models, and, more generally, the suggested form of the correction can also be used to calculate the movement of any solute that transports across an interface between two mobile fluids. We can also better understand the relative behaviors of passive tracers and those that are adsorbed.
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Maneke-Fiegenbaum, Falk, Otto Klemm, Yen-Jen Lai, Chih-Yuan Hung i Jui-Chu Yu. "Carbon Exchange between the Atmosphere and a Subtropical Evergreen Mountain Forest in Taiwan". Advances in Meteorology 2018 (21.11.2018): 1–12. http://dx.doi.org/10.1155/2018/9287249.
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Tropical, temperate, and boreal forests are the subject of various eddy covariance studies, but less is known about the subtropical region. As there are large areas of subtropical forests in the East Asian monsoon region with possibly high carbon uptake, we used three years (2011–2013) of eddy covariance data to estimate the carbon balance of a subtropical mountain forest in Taiwan. Two techniques of flux partitioning are applied to evaluate ecosystem respiration, thoroughly evaluate the validity of the estimated fluxes, and arrive at an estimate of the yearly net ecosystem exchange (NEE). We found that advection is a strong player at our site. Further, when used alone, the nighttime flux correction with the so-called u∗ method (u∗ = friction velocity) cannot avoid underestimating the nighttime respiration. By using a two-technique method employing both nighttime and daytime parameterizations for flux corrections, we arrive at an estimate of the three-year mean NEE of −561 (±standard deviation 114) g·C·m−2·yr−1. The corrected flux estimate represents a rather large uptake of CO2 for this mountain cloud forest, but the value is in good agreement with the few existing comparable estimates for other subtropical forests.
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Potvin, Corey K., Alan Shapiro i Ming Xue. "Impact of a Vertical Vorticity Constraint in Variational Dual-Doppler Wind Analysis: Tests with Real and Simulated Supercell Data". Journal of Atmospheric and Oceanic Technology 29, nr 1 (1.01.2012): 32–49. http://dx.doi.org/10.1175/jtech-d-11-00019.1.
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Abstract One of the greatest challenges to dual-Doppler retrieval of the vertical wind is the lack of low-level divergence information available to the mass conservation constraint. This study examines the impact of a vertical vorticity equation constraint on vertical velocity retrievals when radar observations are lacking near the ground. The analysis proceeds in a three-dimensional variational data assimilation (3DVAR) framework with the anelastic form of the vertical vorticity equation imposed along with traditional data, mass conservation, and smoothness constraints. The technique is tested using emulated radial wind observations of a supercell storm simulated by the Advanced Regional Prediction System (ARPS), as well as real dual-Doppler observations of a supercell storm that occurred in Oklahoma on 8 May 2003. Special attention is given to procedures to evaluate the vorticity tendency term, including spatially variable advection correction and estimation of the intrinsic evolution. Volume scan times ranging from 5 min, typical of operational radar networks, down to 30 s, achievable by rapid-scan mobile radars, are considered. The vorticity constraint substantially improves the vertical velocity retrievals in our experiments, particularly for volume scan times smaller than 2 min.
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Shapiro, Carl R., Johan Meyers, Charles Meneveau i Dennice F. Gayme. "Wind farms providing secondary frequency regulation: evaluating the performance of model-based receding horizon control". Wind Energy Science 3, nr 1 (22.01.2018): 11–24. http://dx.doi.org/10.5194/wes-3-11-2018.
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Abstract. This paper is an extended version of our paper presented at the 2016 TORQUE conference (Shapiro et al., 2016). We investigate the use of wind farms to provide secondary frequency regulation for a power grid using a model-based receding horizon control framework. In order to enable real-time implementation, the control actions are computed based on a time-varying one-dimensional wake model. This model describes wake advection and wake interactions, both of which play an important role in wind farm power production. In order to test the control strategy, it is implemented in a large-eddy simulation (LES) model of an 84-turbine wind farm using the actuator disk turbine representation. Rotor-averaged velocity measurements at each turbine are used to provide feedback for error correction. The importance of including the dynamics of wake advection in the underlying wake model is tested by comparing the performance of this dynamic-model control approach to a comparable static-model control approach that relies on a modified Jensen model. We compare the performance of both control approaches using two types of regulation signals, “RegA” and “RegD”, which are used by PJM, an independent system operator in the eastern United States. The poor performance of the static-model control relative to the dynamic-model control demonstrates that modeling the dynamics of wake advection is key to providing the proposed type of model-based coordinated control of large wind farms. We further explore the performance of the dynamic-model control via composite performance scores used by PJM to qualify plants for regulation services or markets. Our results demonstrate that the dynamic-model-controlled wind farm consistently performs well, passing the qualification threshold for all fast-acting RegD signals. For the RegA signal, which changes over slower timescales, the dynamic-model control leads to average performance that surpasses the qualification threshold, but further work is needed to enable this controlled wind farm to achieve qualifying performance for all regulation signals.
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Dahl, Nathan A., Alan Shapiro, Corey K. Potvin, Adam Theisen, Joshua G. Gebauer, Alexander D. Schenkman i Ming Xue. "High-Resolution, Rapid-Scan Dual-Doppler Retrievals of Vertical Velocity in a Simulated Supercell". Journal of Atmospheric and Oceanic Technology 36, nr 8 (sierpień 2019): 1477–500. http://dx.doi.org/10.1175/jtech-d-18-0211.1.
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AbstractObservation system simulation experiments are used to evaluate different dual-Doppler analysis (DDA) methods for retrieving vertical velocity w at grid spacings on the order of 100 m within a simulated tornadic supercell. Variational approaches with and without a vertical vorticity equation constraint are tested, along with a typical (traditional) method involving vertical integration of the mass conservation equation. The analyses employ emulated radar data from dual-Doppler placements 15, 30, and 45 km east of the mesocyclone, with volume scan intervals ranging from 10 to 150 s. The effect of near-surface data loss is examined by denying observations below 1 km in some of the analyses. At the longer radar ranges and when no data denial is imposed, the “traditional” method produces results similar to those of the variational method and is much less expensive to implement. However, at close range and/or with data denial, the variational method is much more accurate, confirming results from previous studies. The vorticity constraint shows the potential to improve the variational analysis substantially, reducing errors in the w retrieval by up to 30% for rapid-scan observations (≤30 s) at close range when the local vorticity tendency is estimated using spatially variable advection correction. However, the vorticity constraint also degrades the analysis for longer scan intervals, and the impact diminishes with increased range. Furthermore, analyses using 30-s data also frequently outperform analyses using 10-s data, suggesting a limit to the benefit of increasing the radar scan rate for variational DDA employing the vorticity constraint.
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Dellwik, E., J. Mann, F. Bingöl i K. S. Larsen. "Mean vertical velocities and flow tilt angles at a fetch-limited forest site in the context of carbon dioxide vertical advection". Biogeosciences Discussions 6, nr 4 (12.08.2009): 8167–213. http://dx.doi.org/10.5194/bgd-6-8167-2009.
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Abstract. An analysis of flow angles from a fetch-limited beech forest site with clearings is presented. Flow angles and vertical velocities from two types of sonic anemometers as well as a ground based remote sensing lidar were analysed. Instead of using rotations, where zero-flow angles were assumed for neutral flow, the data from the instruments were interpreted in relation to the terrain. Uncertainties regarding flow distortion and limited sampling time (statistical uncertainty) were evaluated and found to be significant. Especially for one of the sonic anemometers, relatively small changes in the flow distortion correction could change the sign of mean vertical velocities taken during stable atmospheric stratification relative to the neutral flow. Despite the uncertainties, it was possible to some extent to relate both positive and negative mean flow angles to features in the terrain. Conical and linear scans with a remote sensing lidar were evaluated for estimation of vertical velocities and flow angles. The results of the vertical conical scans were promising, and yielded negative flow angles for a sector where the forest is fetch-limited. However, more data and analysis is needed for a complete evaluation of the technique. The horizontal linear scans showed the variability of the mean wind speed field. A vertical velocity was calculated from different focusing distances, but this estimate yielded unrealistically high vertical velocities, due to neglect of the transversal wind component. The vertical advection term was calculated using the measured mean flow angles at the mast and profile measurements of carbon dioxide, but it is not recommended to use in relation with the flux measurement as the vertical velocity measured at the mast is most likely not representative for the whole forest.
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Dellwik, E., J. Mann i K. S. Larsen. "Flow tilt angles near forest edges – Part 1: Sonic anemometry". Biogeosciences 7, nr 5 (26.05.2010): 1745–57. http://dx.doi.org/10.5194/bg-7-1745-2010.
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Abstract. An analysis of flow tilt angles from a fetch-limited beech forest site with clearings is presented in the context of vertical advection of carbon dioxide. Flow angles and vertical velocities from two sonic anemometers by different manufacturers were analyzed. Instead of using rotations, where zero-flow angles were assumed for neutral flow, the data was interpreted in relation to upstream and downstream forest edges. Uncertainties caused by flow distortion, vertical misalignment and limited sampling time (statistical uncertainty) were evaluated and found to be highly significant. Since the attack angle distribution of the wind on the sonic anemometer is a function of atmospheric stratification, an instrumental error caused by imperfect flow distortion correction is also a function of the atmospheric stratification. In addition, it is discussed that the sonic anemometers have temperature dependent off-sets. These features of the investigated sonic anemometers make them unsuitable for measuring vertical velocities over highly turbulent forested terrain. By comparing the sonic anemometer results to that of a conically scanning Doppler lidar (Dellwik et al., 2010b), sonic anemometer accuracy for measuring mean flow tilt angles was estimated to between 2° and 3°. Use of planar fit algorithms, where the mean vertical velocity is calculated as the difference between the neutral and non-neutral flow, does not solve this problem of low accuracy and is not recommended. Because of the large uncertainties caused by flow distortion and vertical alignment, it was only possible to a limited extent to relate sonic anemometer flow tilt angles to upwind forest edges, but the results by the lidar indicated that an internal boundary layer affect flow tilt angles at 21m above the forest. This is in accordance with earlier studies at the site. Since the mean flow tilt angles do not follow the terrain, an estimate of the vertical advection term for near-neutral conditions was calculated using profile measurements of carbon dioxide. The estimated advection term is large, but it is not recommended to include it in the surface carbon balance, unless all terms in the carbon dioxide conservation equation can be precisely estimated.
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Mut, Fernando, Gustavo C. Buscaglia i Enzo A. Dari. "New Mass-Conserving Algorithm for Level Set Redistancing on Unstructured Meshes". Journal of Applied Mechanics 73, nr 6 (1.02.2006): 1011–16. http://dx.doi.org/10.1115/1.2198244.
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The level set method is becoming increasingly popular for the simulation of several problems that involve interfaces. The level set function is advected by some velocity field, with the zero-level set of the function defining the position of the interface. The advection distorts the initial shape of the level set function, which needs to be re-initialized to a smooth function preserving the position of the zero-level set. Many algorithms re-initialize the level set function to (some approximation of) the signed distance from the interface. Efficient algorithms for level set redistancing on Cartesian meshes have become available over the last years, but unstructured meshes have received little attention. This presentation concerns algorithms for construction of a distance function from the zero-level set, in such a way that mass is conserved on arbitrary unstructured meshes. The algorithm is consistent with the hyperbolic character of the distance equation (‖∇d‖=1) and can be localized on a narrow band close to the interface, saving computing effort. The mass-correction step is weighted according to local mass differences, an improvement over usual global rebalancing techniques.
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Larsén, Xiaoli G., i Jana Fischereit. "A case study of wind farm effects using two wake parameterizations in the Weather Research and Forecasting (WRF) model (V3.7.1) in the presence of low-level jets". Geoscientific Model Development 14, nr 6 (2.06.2021): 3141–58. http://dx.doi.org/10.5194/gmd-14-3141-2021.
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Abstract. While the wind farm parameterization by Fitch et al. (2012) in the Weather Research and Forecasting (WRF) model has been used and evaluated frequently, the explicit wake parameterization (EWP) by Volker et al. (2015) is less well explored. The openly available high-frequency flight measurements from Bärfuss et al. (2019a) provide an opportunity to directly compare the simulation results from the EWP and Fitch scheme with in situ measurements. In doing so, this study aims to complement the recent study by Siedersleben et al. (2020) by (1) comparing the EWP and Fitch schemes in terms of turbulent kinetic energy (TKE) and velocity deficit, together with FINO 1 measurements and synthetic aperture radar (SAR) data, and (2) exploring the interactions of the wind farm with low-level jets (LLJs). This is done using a bug-fixed WRF version that includes the correct TKE advection, following Archer et al. (2020). Both the Fitch and the EWP schemes can capture the mean wind field in the presence of the wind farm consistently and well. TKE in the EWP scheme is significantly underestimated, suggesting that an explicit turbine-induced TKE source should be included in addition to the implicit source from shear. The value of the correction factor for turbine-induced TKE generation in the Fitch scheme has a significant impact on the simulation results. The position of the LLJ nose and the shear beneath the jet nose are modified by the presence of wind farms.
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Du, Yunfei, Huiyi Lin, Shuangyan He, Daosheng Wang, Ya Ping Wang i Jicai Zhang. "Tide-Induced Variability and Mechanisms of Surface Suspended Sediment in the Zhoushan Archipelago along the Southeastern Coast of China Based on GOCI Data". Remote Sensing 13, nr 5 (2.03.2021): 929. http://dx.doi.org/10.3390/rs13050929.
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The variations and dynamics of suspended sediment in the coastal waters around the Zhoushan Archipelago are complex due to strong tidal dynamics. This study aims to delineate the tide-induced variability of surface suspended sediment and reveal its mechanisms in the Zhoushan Archipelago based on Geostationary Ocean Color Imager (GOCI) data. Suspended sediment concentration (SSC) estimations through the ultraviolet wavelength atmospheric correction (UV-AC) algorithm and a tuned empirical inversion algorithm using extensive in situ measurements are presented. GOCI-processed remote sensing reflectance (Rrs) is validated using field data and is cross-validated with Landsat 8 Operational Land Imager (L8/OLI) measurements, respectively. The estimated SSC is validated with independent datasets. The validations reveal that GOCI-processed Rrs is reasonable and that the GOCI-retrieved SSC is accurate and can be used to quantify SSC distributions and variations in the Zhoushan Archipelago. The variations of the SSC in the study area are closely related to the temporal variations of the tidal level. High SSCs often occur in the middle of ebb or flood tides due to large tidal discharge and high tidal velocity. Significant hourly variations of the SSC are mainly controlled by tidal forces. Dynamic mechanism analysis indicates that during neap and middle tides, the local high SSC is mainly attributed to the sediment resuspension process driven by tidal currents; in addition, during the spring tide, the variations of the SSC are simultaneously modulated by sediment resuspension and horizontal advection processes.
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Pellegrino, J., S. Wright, J. Ranvill i G. Amy. "Predicting membrane flux decline from complex mixtures using flow-field flow fractionation measurements and semi-empirical theory". Water Science and Technology 51, nr 6-7 (1.03.2005): 85–92. http://dx.doi.org/10.2166/wst.2005.0625.
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Flow-Field Flow Fractionation (Fl-FFF) is an idealization of the cross flow membrane filtration process in that, (1) the filtration flux and crossflow velocity are constant from beginning to end of the device, (2) the process is a relatively well-defined laminar-flow hydrodynamic condition, and (3) the solutes are introduced as a pulse-input that spreads due to interactions with each other and the membrane in the dilute-solution limit. We have investigated the potential for relating Fl-FFF measurements to membrane fouling. An advection-dispersion transport model was used to provide ‘ideal’ (defined as spherical, non-interacting solutes) solute residence time distributions (RTDs) for comparison with ‘real’ RTDs obtained experimentally at different cross-field velocities and solution ionic strength. An RTD moment analysis based on a particle diameter probability density function was used to extract “effective” characteristic properties, rather than uniquely defined characteristics, of the standard solute mixture. A semi-empirical unsteady-state, flux decline model was developed that uses solute property parameters. Three modes of flux decline are included: (1) concentration polarization, (2) cake buildup, and (3) adsorption on/in pores, We have used this model to test the hypothesis—that an analysis of a residence time distribution using Fl-FFF can describe ‘effective’ solute properties or indices that can be related to membrane flux decline in crossflow membrane filtration. Constant flux filtration studies included the changes of transport hydrodynamics (solvent flux to solute back diffusion (J/k) ratios), solution ionic strength, and feed water composition for filtration using a regenerated cellulose ultrafiltration membrane. Tests of the modeling hypothesis were compared with experimental results from the filtration measurements using several correction parameters based on the mean and variance of the solute RTDs. The corrections used to modify the boundary layer mass transfer coefficient and the specific resistance of cake or adsorption layers demonstrated that RTD analysis is potentially useful technique to describe colloid properties but requires improvements.
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Chang, Na, Maoshan Li, Ming Gong, Pei Xu, Yaoming Ma, Fanglin Sun i Yaoxian Yang. "Study on Surface Characteristic Parameters and Surface Energy Exchange in Eastern Edge of the Tibetan Plateau". Atmosphere 13, nr 11 (24.10.2022): 1749. http://dx.doi.org/10.3390/atmos13111749.
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Mount Emei is located on the eastern edge of the Tibetan Plateau, on the transition zone between the main body of the Tibetan Plateau and the Sichuan Basin in China. It is not only the necessary place for the eastward movement of the plateau system but also the place where the southwest vortex begins to develop. Its special geographical location makes it particularly important to understand the turbulence characteristics and surface energy balance of this place. Based on the Atmospheric Boundary Layer (ABL) tower data, radiation observation data and surface flux data of Mount Emei station on the eastern edge of the Tibetan Plateau from December 2019 to February 2022, the components of surface equilibrium are estimated by the eddy correlation method and Thermal Diffusion Equation and Correction (TDEC) method, the characteristics of surface energy exchange in the Mount Emei area are analyzed, and the aerodynamic and thermodynamic parameters are estimated. The results show that the annual average value of zero-plane displacement d is 10.45 m, the annual average values of aerodynamic roughness Z0m and aerothermal roughness Z0h are 1.61 and 1.67 m, respectively, and the annual average values of momentum flux transport coefficient CD and sensible heat flux transport coefficient CH are 1.58 × 10−2 and 3.79 × 10−3, respectively. The dimensionless vertical wind fluctuation variance in the Mount Emei area under unstable conditions can better conform to the 1/3rd power law of the Monin–Obukhov similarity theory, while the dimensionless horizontal wind fluctuation variance under unstable lamination and the dimensionless 3D wind fluctuation variance under stable condition does not conform to this law. In the near-neutral case, the dimensionless velocity variance in the vertical direction in this area is 1.314. The daytime dominance of sensible and latent heat fluxes varied seasonally, with latent heat fluxes dominating in summer and sensible heat transport dominating in winter. he surface albedo of Mount Emei in four seasons is between 0.04 and 0.08. The surface albedo in summer and autumn is higher than that in Mount Emei. The influence of the underlying surface on surface reflectance is much greater than other factors, such as altitude, longitude and latitude. The non-closure phenomenon is significant in the Mount Emei area. The energy closure rates before and after considering canopy thermal storage are 46% and 48%, respectively. The possible reason for the energy non-closure in this area is that the influence of horizontal advection and vertical advection on the energy closure is not considered.
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Morris, Jeffrey F., i John F. Brady. "Self-diffusion in sheared suspensions". Journal of Fluid Mechanics 312 (10.04.1996): 223–52. http://dx.doi.org/10.1017/s002211209600198x.
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Self-diffusion in a suspension of spherical particles in steady linear shear flow is investigated by following the time evolution of the correlation of number density fluctuations. Expressions are presented for the evaluation of the self-diffusivity in a suspension which is either raacroscopically quiescent or in linear flow at arbitrary Peclet number $Pe = \dot{\gamma}a^2/2D$, where $\dot{\gamma}$ is the shear rate, a is the particle radius, and D = kBT/6πa is the diffusion coefficient of an isolated particle. Here, kB is Boltzmann's constant, T is the absolute temperature, and η is the viscosity of the suspending fluid. The short-time self-diffusion tensor is given by kBT times the microstructural average of the hydrodynamic mobility of a particle, and depends on the volume fraction $\phi = \frac{4}{3}\pi a^3n$ and Pe only when hydrodynamic interactions are considered. As a tagged particle moves through the suspension, it perturbs the average microstructure, and the long-time self-diffusion tensor, D∞s, is given by the sum of D0s and the correlation of the flux of a tagged particle with this perturbation. In a flowing suspension both D0s and D∞ are anisotropic, in general, with the anisotropy of D0s due solely to that of the steady microstructure. The influence of flow upon D∞s is more involved, having three parts: the first is due to the non-equilibrium microstructure, the second is due to the perturbation to the microstructure caused by the motion of a tagged particle, and the third is by providing a mechanism for diffusion that is absent in a quiescent suspension through correlation of hydrodynamic velocity fluctuations.The self-diffusivity in a simply sheared suspension of identical hard spheres is determined to O(øPe3/2) for Pe ≤ 1 and ø ≤ 1, both with and without hydro-dynamic interactions between the particles. The leading dependence upon flow of D0s is 0.22DøPeÊ, where Ê is the rate-of-strain tensor made dimensionless with $\dot{\gamma}$. Regardless of whether or not the particles interact hydrodynamically, flow influences D∞s at O(øPe) and O(øPe3/2). In the absence of hydrodynamics, the leading correction is proportional to øPeDÊ. The correction of O(øPe3/2), which results from a singular advection-diffusion problem, is proportional, in the absence of hydrodynamic interactions, to øPe3/2DI; when hydrodynamics are included, the correction is given by two terms, one proportional to Ê, and the second a non-isotropic tensor.At high ø a scaling theory based on the approach of Brady (1994) is used to approximate D∞s. For weak flows the long-time self-diffusivity factors into the product of the long-time self-diffusivity in the absence of flow and a non-dimensional function of $\bar{P}e = \dot{\gamma}a^2/2D^s_0(\phi)$. At small $\bar{P}e$ the dependence on $\bar{P}e$ is the same as at low ø.
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Ovtchinnikov, Mikhail, i Richard C. Easter. "Nonlinear Advection Algorithms Applied to Interrelated Tracers: Errors and Implications for Modeling Aerosol–Cloud Interactions". Monthly Weather Review 137, nr 2 (1.02.2009): 632–44. http://dx.doi.org/10.1175/2008mwr2626.1.
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Abstract Monotonicity constraints and gradient-preserving flux corrections employed by many advection algorithms used in atmospheric models make these algorithms nonlinear. Consequently, any relations among model variables transported separately are not necessarily preserved in such models. These errors cannot be revealed by traditional algorithm testing based on advection of a single tracer. New types of tests are developed and conducted to evaluate the monotonicity of a sum of several number mixing ratios advected independently of each other—as is the case, for example, in models using bin or sectional representations of aerosol or cloud particle size distributions. The tests show that when three tracers with an initially constant sum are advected separately in one-dimensional constant velocity flow, local errors in their sum can be on the order of 10%. When cloudlike interactions are allowed among the tracers in the idealized “cloud base” test, errors in the sum of three mixing ratios can reach 30%. Several approaches to eliminate the error are suggested, all based on advecting the sum as a separate variable and then using it to normalize the sum of the individual tracers’ mixing ratios or fluxes. A simple scalar normalization ensures the monotonicity of the total number mixing ratio and positive definiteness of the variables, but the monotonicity of individual tracers is no longer maintained. More involved flux normalization procedures are developed for the flux-based advection algorithms to maintain the monotonicity for individual scalars and their sum.
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PEDRIZZETTI, GIANNI, i J. C. VASSILICOS. "Interscale transfer in two-dimensional compact vortices". Journal of Fluid Mechanics 406 (10.03.2000): 109–29. http://dx.doi.org/10.1017/s0022112099007302.
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The property of transfer between different scales of motion in evolving two-dimensional compact vortices is studied here, and a general mathematical framework is developed to describe the transfer between scales inside compact structures. This new approach is applied to the case of an axisymmetric advection which represents the leading-order (large time) approximation for Lundgren's family of two-dimensional vortices. It is also generalized to passive scalar advection by non-axisymmetric velocity fields. It is shown that scale interactions generated by an axisymmetric advection are essentially local and dominated by distant triadic interactions: in the case of an evolving spiral vortex sheet this result is confirmed even when non-axisymmetric corrections are included. A physical interpretation of the results is given, which can be summarized by saying that locality of scale interactions is caused by the uniformity of shear at a given scale and is therefore increasingly natural at small lengthscales. Local interactions are shown to arise in axisymmetric advection but to be uncommon in non-axisymmetric advection.
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Ginzburg, Irina. "Truncation Errors, Exact And Heuristic Stability Analysis Of Two-Relaxation-Times Lattice Boltzmann Schemes For Anisotropic Advection-Diffusion Equation". Communications in Computational Physics 11, nr 5 (maj 2012): 1439–502. http://dx.doi.org/10.4208/cicp.211210.280611a.
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AbstractThis paper establishes relations between the stability and the high-order truncated corrections for modeling of the mass conservation equation with the two-relaxation-times (TRT) collision operator. First we propose a simple method to derive the truncation errors from the exact, central-difference type, recurrence equations of the TRT scheme. They also supply its equivalent three-time-level discretization form. Two different relationships of the two relaxation rates nullify the third (advection) and fourth (pure diffusion) truncation errors, for any linear equilibrium and any velocity set. However, the two relaxation times alone cannot remove the leading-order advection-d if fusion error, because of the intrinsic fourth-order numerical diffusion. The truncation analysis is carefully verified for the evolution of concentration waves with the anisotropic diffusion tensors. The anisotropic equilibrium functions are presented in a simple but general form, suitable for the minimal velocity sets and the d2Q9, d3Q13, d3Q15 and d3Q19 velocity sets. All anisotropic schemes are complemented by their exact necessary von Neumann stability conditions and equivalent finite-difference stencils. The sufficient stability conditions are proposed for the most stable (OTRT) family, which enables modeling at any Peclet numbers with the same velocity amplitude. The heuristic stability analysis of the fourth-order truncated corrections extends the optimal stability to larger relationships of the two relaxation rates, in agreement with the exact (one-dimensional) and numerical (multi-dimensional) stability analysis. A special attention is put on the choice of the equilibrium weights. By combining accuracy and stability predictions, several strategies for selecting the relaxation and free-tunable equilibrium parameters are suggested and applied to the evolution of the Gaussian hill.
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Resseguier, Valentin, Etienne Mémin, Dominique Heitz i Bertrand Chapron. "Stochastic modelling and diffusion modes for proper orthogonal decomposition models and small-scale flow analysis". Journal of Fluid Mechanics 826 (15.08.2017): 888–917. http://dx.doi.org/10.1017/jfm.2017.467.
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We present here a new stochastic modelling approach in the constitution of fluid flow reduced-order models. This framework introduces a spatially inhomogeneous random field to represent the unresolved small-scale velocity component. Such a decomposition of the velocity in terms of a smooth large-scale velocity component and a rough, highly oscillating component gives rise, without any supplementary assumption, to a large-scale flow dynamics that includes a modified advection term together with an inhomogeneous diffusion term. Both of those terms, related respectively to turbophoresis and mixing effects, depend on the variance of the unresolved small-scale velocity component. They bring an explicit subgrid term to the reduced system which enables us to take into account the action of the truncated modes. Besides, a decomposition of the variance tensor in terms of diffusion modes provides a meaningful statistical representation of the stationary or non-stationary structuration of the small-scale velocity and of its action on the resolved modes. This supplies a useful tool for turbulent fluid flow data analysis. We apply this methodology to circular cylinder wake flow at Reynolds numbers $Re=100$ and $Re=3900$. The finite-dimensional models of the wake flows reveal the energy and the anisotropy distributions of the small-scale diffusion modes. These distributions identify critical regions where corrective advection effects, as well as structured energy dissipation effects, take place. In providing rigorously derived subgrid terms, the proposed approach yields accurate and robust temporal reconstruction of the low-dimensional models.
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Esler, J. G. "Adaptive stochastic trajectory modelling in the chaotic advection regime". Journal of Fluid Mechanics 769 (13.03.2015): 1–25. http://dx.doi.org/10.1017/jfm.2015.75.
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Motivated by the goal of improving and augmenting stochastic Lagrangian models of particle dispersion in turbulent flows, techniques from the theory of stochastic processes are applied to a model transport problem. The aim is to find an efficient and accurate method to calculate the total tracer transport between a source and a receptor when the flow between the two locations is weak, rendering direct stochastic Lagrangian simulation prohibitively expensive. Importance sampling methods that combine information from stochastic forward and back trajectory calculations are proposed. The unifying feature of the new methods is that they are developed using the observation that a perfect strategy should distribute trajectories in proportion to the product of the forward and adjoint solutions of the transport problem, a quantity here termed the ‘density of trajectories’ $D(\boldsymbol{x},t)$. Two such methods are applied to a ‘hard’ model problem, in which the prescribed kinematic flow is in the large-Péclet-number chaotic advection regime, and the transport problem requires simulation of a complex distribution of well-separated trajectories. The first, Milstein’s measure transformation method, involves adding an artificial velocity to the trajectory equation and simultaneously correcting for the weighting given to each particle under the new flow. It is found that, although a ‘perfect’ artificial velocity $\boldsymbol{v}^{\ast }$ exists, which is shown to distribute the trajectories according to $D$, small errors in numerical estimates of $\boldsymbol{v}^{\ast }$ cumulatively lead to difficulties with the method. A second method is Grassberger’s ‘go-with-the-winners’ branching process, where trajectories found unlikely to contribute to the net transport (losers) are periodically removed, while those expected to contribute significantly (winners) are split. The main challenge of implementation, which is finding an algorithm to select the winners and losers, is solved by a choice that explicitly forces the distribution towards a numerical estimate of $D$ generated from a previous back trajectory calculation. The result is a robust and easily implemented algorithm with typical variance up to three orders of magnitude lower than the direct approach.
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Daniel, Pierre, Fabien Marty, Patrick Josse, Chafih Skandrani i Rachid Benshila. "Improvement of Drift Calculation in Mothy Operational Oil Spill Prediction System". International Oil Spill Conference Proceedings 2003, nr 1 (1.04.2003): 1067–72. http://dx.doi.org/10.7901/2169-3358-2003-1-1067.
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ABSTRACT MOTHY (Modèle Océanique de Transport d'Hydrocarbures) is a pollutant drift model, developed and operated by Météo-France. MOTHY includes hydrodynamic coastal ocean modelling and real time atmospheric forcing from a global meteorological model. Pollutants can be oil or floating objects. To improve forecasts on the Mediterranean Sea, several methods were tested to inject large scale currents (permanent part) into the MOTHY system. The best results were obtained with monthly means of currents at 5 meters (from Mercator system). The addition of altimetric corrections improved the results. In addition the impact of wave (or swell) current, which is usually neglected in such models, is investigated. The literature has surprisingly little to say on the topic of wave-driven surface oil slicks. Earlier review on oil spill transport modelling includes wave driven transport among potential advection mechanisms. The discussion of wave-induced advection (mass transport) adopts a Lagrangian framework, focusing on the analyses of Stokes and Longuet-Higgins for the vertical profile of the Lagrangian velocity beneath waves. In our work, the action of a vertical shear due to waves is accounted for by including the Stokes drift due to weakly non linear waves. We evaluate this term and compare with observations of Erika pollution incident.
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SANSÓN, L. ZAVALA, i G. J. F. van HEIJST. "Nonlinear Ekman effects in rotating barotropic flows". Journal of Fluid Mechanics 412 (10.06.2000): 75–91. http://dx.doi.org/10.1017/s0022112000008193.
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In the presence of background rotation, conventional two-dimensional models of geostrophic flow in a rotating system usually include Ekman friction – associated with the no-slip condition at the bottom – by adding a linear term in the vorticity evolution equation. This term is proportional to E1/2 (where E is the Ekman number), and arises from the linear Ekman theory, which yields an expression for the vertical velocity produced by the thin Ekman layer at the flat bottom. In this paper, a two- dimensional model with Ekman damping is proposed using again the linear Ekman theory, but now including nonlinear Ekman terms in the vorticity equation. These terms represent nonlinear advection of relative vorticity as well as stretching effects. It is shown that this modified two-dimensional model gives a better description of the spin-down of experimental barotropic vortices than conventional models. Therefore, it is proposed that these corrections should be included in studies of the evolution of quasi-two-dimensional flows, during times comparable to the Ekman period.
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Emad, Anas, i Lukas Siebicke. "True eddy accumulation – Part 1: Solutions to the problem of non-vanishing mean vertical wind velocity". Atmospheric Measurement Techniques 16, nr 1 (4.01.2023): 29–40. http://dx.doi.org/10.5194/amt-16-29-2023.
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Abstract. The true eddy accumulation (TEA) method provides direct measurements of ecosystem-level turbulent fluxes for a wide range of atmospheric constituents. TEA utilizes conditional sampling to overcome the requirement for a fast sensor response demanded by the state-of-the-art eddy covariance (EC) method. The TEA method is formulated under the assumption of ideal conditions with a zero mean vertical wind velocity during the averaging interval. However, this idealization is rarely met under field conditions. Additionally, unlike in EC, this assumption cannot be imposed in post-processing due to the real-time nature of sampling and the absence of high-frequency measurements of the scalar. Consequently, fluxes measured with the TEA method are biased with a non-turbulent advective term that scales with the scalar mean concentration. Here, we explore the magnitude of this biased advective term and potential ways to minimize or remove it. We propose a new formulation to calculate TEA fluxes that minimizes the bias term. The new formulation shows that the magnitude of the error is constrained to w‾/|w|‾ when the stationarity criterion is fulfilled. Here, w is the vertical wind velocity, and the overbar denotes time averaging. The error is shown to be dependent on the asymmetry of atmospheric transport, represented by the coefficient αc. Two methods of estimating the coefficient αc are proposed: a probabilistic treatment of turbulent transport and a method utilizing the assumption of scalar similarity. We show how other formulas for calculating the TEA flux are linked to the new formulation and explore the different corrections in a numerical simulation. The new formulation avoids the direct dependence of the bias term on the scalar background concentration. This result increases confidence in applying the TEA method to measuring fluxes of atmospheric constituents. This is particularly relevant to scalars with a large background concentration and a small flux. This paper is Part 1 of a two-part series on true eddy accumulation.
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Arons, Jonathan, i Richard I. Klein. "Polar Cap Accretion onto Magnetized Neutron Stars: An Analytic Solution". Symposium - International Astronomical Union 125 (1987): 245. http://dx.doi.org/10.1017/s0074180900160814.
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This abstract should be read in conjunction with the papers by Arons and by Klein and Arons in these proceedings. In the context of the accretion models described there, one can find an analytic solution for the flow down the polar field lines if a number of simplifying assumptions are made. These are (1) steady flow in the co–rotating frame; (2) radiation pressure large compared to gas pressure; (3) pure scattering for the Rosseland opacity, with the magnetic corrections set equal to constants instead of using the actual functions of temperature; (4) diffusion flux of radiative energy proportional to the gradient of the energy density alone, instead of the correct sum of terms proportional to the photon energy density and the number density gradients; and (5) below a radiative shock, subsonic flow in approximate hydrostatic equilibrium. We assumed dipole geometry, and also assume the mass flux is independent of distance from the magnetic axis. The essential trick is to use (1), (2) and (5) to write the advective contribution to the radiation transfer equation as Mg/area = rate at which gravity does work on a fluid element, and use (3) and (4) to write the nonlinear diffusion flux as the ratio of gradients in the energy density. Then the multidimensional diffusion equation can be cast in a separable, linear form by using the logarithmic radial gradient of the energy density as the basic variable (see also Kirk, J., 1985, Astron. and Astrophys., 142, 430). The result is exponential stratification of the energy density, velocity and mass density along B with scale height R*[L(EDeff)/4Lco-p]; the effective Eddington luminosity is discussed by Arons, these proceedings. This result can be understood as the result of almost exact balance between upward diffusion and downward advection of photons in the optically thick medium. The same fluid quantities are stratified in a Gaussian manner across B, with angular half width at half maximum Δθ = [L(EDeff)/Lcap](r/R*)3/2. These distributions agree well with more sophisticated computational results, during times when the flow is steady. When used as a basis for calculations of the radiative entropy, the calculated emergent spectra are not dissimilar to the spectra of high luminosity, accretion powered pulsars.
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Lien, F. S., i M. A. Leschziner. "A Pressure-Velocity Solution Strategy for Compressible Flow and Its Application to Shock/Boundary-Layer Interaction Using Second-Moment Turbulence Closure". Journal of Fluids Engineering 115, nr 4 (1.12.1993): 717–25. http://dx.doi.org/10.1115/1.2910204.
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A nonorthogonal, collocated finite-volume scheme, based on a pressure-correction strategy and originally devised for general-geometry incompressible turbulent recirculating flow, has been extended to compressible transonic conditions. The key elements of the extension are a solution for flux variables and the introduction of streamwise-directed density-retardation which is controlled by Mach-number-dependent monitor functions, and which is applied to all transported flow properties. Advective fluxes are approximated using the quadratic scheme QUICK or the second-order TVD scheme MUSCL, the latter applied to all transport equations, including those for turbulence properties. The procedure incorporates a number of turbulence models including a new low-Re k–ε eddy-viscosity variant and a Reynolds-stress-transport closure. The predictive capabilities of the algorithm are illustrated by reference to a number of inviscid and turbulent transonic applications, among them a normal shock in a Laval nozzle, combined oblique-shock reflection and shock-shock interaction over a bump in a channel and shock-induced boundary-layer separation over channel bumps. The last-named application was computed both with eddy-viscosity models and Reynolds-stress closure, leading to the conclusion that the latter yields a much greater sensitivity of the boundary layer to the shock and, arising therefrom, a more pronounced λ-shock structure, earlier separation and more extensive recirculation. On the other hand, the stress closure is found to return an insufficient rate of wake recovery following reattachment.
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Huybrechts, P., O. Rybak, F. Pattyn, U. Ruth i D. Steinhage. "Ice thinning, upstream advection, and non-climatic biases for the upper 89% of the EDML ice core from a nested model of the Antarctic ice sheet". Climate of the Past 3, nr 4 (2.10.2007): 577–89. http://dx.doi.org/10.5194/cp-3-577-2007.
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Abstract. A nested ice flow model was developed for eastern Dronning Maud Land to assist with the dating and interpretation of the EDML deep ice core. The model consists of a high-resolution higher-order ice dynamic flow model that was nested into a comprehensive 3-D thermomechanical model of the whole Antarctic ice sheet. As the drill site is on a flank position the calculations specifically take into account the effects of horizontal advection as deeper ice in the core originated from higher inland. First the regional velocity field and ice sheet geometry is obtained from a forward experiment over the last 8 glacial cycles. The result is subsequently employed in a Lagrangian backtracing algorithm to provide particle paths back to their time and place of deposition. The procedure directly yields the depth-age distribution, surface conditions at particle origin, and a suite of relevant parameters such as initial annual layer thickness. This paper discusses the method and the main results of the experiment, including the ice core chronology, the non-climatic corrections needed to extract the climatic part of the signal, and the thinning function. The focus is on the upper 89% of the ice core (appr. 170 kyears) as the dating below that is increasingly less robust owing to the unknown value of the geothermal heat flux. It is found that the temperature biases resulting from variations of surface elevation are up to half of the magnitude of the climatic changes themselves.
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Huybrechts, P., O. Rybak, F. Pattyn, U. Ruth i D. Steinhage. "Ice thinning, upstream advection, and non-climatic biases for the upper 89% of the EDML ice core from a nested model of the Antarctic ice sheet". Climate of the Past Discussions 3, nr 3 (7.05.2007): 693–727. http://dx.doi.org/10.5194/cpd-3-693-2007.
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Abstract. A nested ice flow model was developed for eastern Dronning Maud Land to assist with the dating and interpretation of the EDML deep ice core. The model consists of a high-resolution higher-order ice dynamic flow model that was nested into a comprehensive 3-D thermomechanical model of the whole Antarctic ice sheet. As the drill site is on a flank position the calculations specifically take into account the effects of horizontal advection as deeper ice in the core originated from higher inland. First the regional velocity field and ice sheet geometry is obtained from a forward experiment over the last 8 glacial cycles. The result is subsequently employed in a Lagrangian backtracing algorithm to provide particle paths back to their time and place of deposition. The procedure directly yields the depth-age distribution, surface conditions at particle origin, and a suite of relevant parameters such as initial annual layer thickness. This paper discusses the method and the main results of the experiment, including the ice core chronology, the non-climatic corrections needed to extract the climatic part of the signal, and the thinning function. The focus is on the upper 89% of the ice core (appr. 170 kyears) as the dating below that is increasingly less robust owing to the unknown value of the geothermal heat flux. It is found that the temperature biases resulting from variations of surface elevation are up to half of the magnitude of the climatic changes themselves.
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CELANI, ANTONIO, MARCO MARTINS AFONSO i ANDREA MAZZINO. "Point-source scalar turbulence". Journal of Fluid Mechanics 583 (4.07.2007): 189–98. http://dx.doi.org/10.1017/s0022112007006520.
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The statistics of a passive scalar randomly emitted from a point source is investigated analytically for the Kraichnan ensemble. Attention is focused on the two-point equal-time scalar correlation function, a statistical indicator widely used both in experiments and in numerical simulations. The only source of inhomogeneity/anisotropy is the injection mechanism, the advecting velocity being here statistically homogeneous and isotropic. The main question we address is on the possible existence of an inertial range of scales and a consequent scaling behaviour. The question arises from the observation that for a point source the injection scale is formally zero and the standard cascade mechanism cannot thus be taken for granted. We find from first principles that an intrinsic integral scale, whose value depends on the distance from the source, emerges as a result of sweeping effects. For separations smaller than this integral scale a standard forward cascade occurs. This is characterized by a Kolmogorov–Obukhov power-law behaviour as in the homogeneous case, except that the dissipation rate is also dependent on the distance from the source. Finally, we also find that the combined effect of a finite inertial-range extent and of inhomogeneities causes the emergence of subleading anisotropic corrections to the leading isotropic term, that are here quantified and discussed.
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REZNIK, G. M., R. GRIMSHAW i E. S. BENILOV. "On the long-term evolution of an intense localized divergent vortex on the beta-plane". Journal of Fluid Mechanics 422 (3.11.2000): 249–80. http://dx.doi.org/10.1017/s0022112000001750.
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The evolution of an intense barotropic vortex on the β-plane is analysed for the case of finite Rossby deformation radius. The analysis takes into account conservation of vortex energy and enstrophy, as well as some other quantities, and therefore makes it possible to gain insight into the vortex evolution for longer times than was done in previous studies on this subject. Three characteristic scales play an important role in the evolution: the advective time scale Ta (a typical time required for a fluid particle to move a distance of the order of the vortex size), the wave time scale Tw (the typical time it takes for the vortex to move through its own radius), and the distortion time scale Td (a typical time required for the change in relative vorticity of the vortex to become of the order of the relative vorticity itself). For an intense vortex these scales are well separated, Ta [Lt ] Tw [Lt ] Td, and therefore one can consider the vortex evolution as consisting of three different stages. The first one, t [les ] Tw, is dominated by the development of a near-field dipolar circulation (primary β-gyres) accelerating the vortex. During the second stage, Tw [les ] t [les ] Td, the quadrupole and secondary axisymmetric components are intensified; the vortex decelerates. During the last, third, stage the vortex decays and is destroyed. Our main attention is focused on exploration of the second stage, which has been studied much less than the first stage. To describe the second stage we develop an asymptotic theory for an intense vortex with initially piecewise-constant relative vorticity. The theory allows the calculation of the quadrupole and axisymmetric corrections, and the correction to the vortex translation speed. Using the conservation laws we estimate that the vortex lifetime is directly proportional to the vortex streamfunction amplitude and inversely proportional to the squared group velocity of Rossby waves. For open-ocean eddies a typical lifetime is about 130 days, and for oceanic rings up to 650 days. Analysis of the residual produced by the asymptotic solution explains why this solution is a good approximation for times much longer than the expected formal range of applicability. All our analytical results are in a good qualitative agreement with several numerical experiments carried out for various vortices.
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Gebauer, Joshua G., Alan Shapiro, Corey K. Potvin, Nathan A. Dahl, Michael I. Biggerstaff i A. Addison Alford. "Evaluating vertical velocity retrievals from vertical vorticity equation constrained dual-Doppler analysis of real, rapid-scan radar data". Journal of Atmospheric and Oceanic Technology, 21.07.2022. http://dx.doi.org/10.1175/jtech-d-21-0136.1.
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Abstract Accurate vertical velocity retrieval from dual-Doppler analysis (DDA) is a longstanding problem of radar meteorology. Typical radar scanning strategies poorly observe the vertical component of motion, leading to large uncertainty in vertical velocity estimates. Using a vertical vorticity equation constraint in addition to a mass conservation constraint in DDA has shown promise in improving vertical velocity retrievals. However, observation system simulation experiments (OSSEs) suggest this technique requires rapid radar volume scans to realize the improvements due to the vorticity tendency term in the vertical vorticity constraint. Here, the vertical vorticity constraint DDA is tested with real, rapid-scan radar data to validate prior OSSEs results. Generally, the vertical vorticity constraint DDA produced more accurate vertical velocities from DDAs than those that did not use the constraint. When the time between volume scans was greater than 30 seconds, the vertical velocity accuracy was significantly affected by the vorticity tendency estimation method. A technique that uses advection correction on provisional DDA wind fields to shorten the discretization interval for the vorticity tendency calculation improved the vertical velocity retrievals for longer times between volume scans. The skill of these DDAs was similar to those using a shorter time between volume scans. These improvements were due to increased accuracy of the vertical vorticity tendency using the advection correction technique. The real radar data tests also revealed that the vertical vorticity constraint DDAs are more forgiving to radar data errors. These results suggest that vertical vorticity constraint DDA with rapid-scan radars should be prioritized for kinematic analyses.
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Xin, Jianjian, Fulong Shi, Qiu Jin i Lin Ma. "Gradient-Augmented Level Set Two-Phase Flow Method With Pretreated Reinitialization for Three-Dimensional Violent Sloshing". Journal of Fluids Engineering 142, nr 1 (4.10.2019). http://dx.doi.org/10.1115/1.4044823.
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Abstract A three-dimensional (3D) gradient-augmented level set (GALS) two-phase flow model with a pretreated reinitialization procedure is developed to simulate violent sloshing in a cuboid tank. Based on a two-dimensional (2D) GALS method, 3D Hermite, and 3D Lagrange polynomial schemes are derived to interpolate the level set function and the velocity field at arbitrary positions over a cell, respectively. A reinitialization procedure is performed on a 3D narrow band to treat the strongly distorted interface and improve computational efficiency. In addition, an identification-correction technique is proposed and incorporated into the reinitialization procedure to treat the tiny droplet which can distort the free surface shape, even lead to computation failure. To validate the accuracy of the present GALS method and the effectiveness of the proposed identification-correction technique, a 3D velocity advection case is first simulated. The present method is validated to have better mass conservation property than the classical level set and original GALS methods. Also, distorted and thin interfaces are well captured on all grid resolutions by the present GALS method. Then, sloshing under coupled surge and sway excitation, sloshing under rotational excitation are simulated. Good agreements are obtained when the present wave and pressure results are compared with the experimental and numerical results. In addition, the highly nonlinear free surface is observed, and the relationship between the excitation frequency and the impulsive pressure is investigated.
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Park, Young-Gyu, Seongbong Seo, Dong Guk Kim, Joocheul Noh i Hyuk Min Park. "Coastal Observation Using a Vertical Profiling System at the Southern Coast of Korea". Frontiers in Marine Science 8 (21.04.2021). http://dx.doi.org/10.3389/fmars.2021.668733.
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At a coastal station near the southern coast of Korea, the vertical profiles of temperature salinity dissolved oxygen and velocity were obtained using a vertical profiler, Aqualog, every summer from 2016 to 2020. At the site, fishing activity was not allowed, and it was possible to maintain the profiler continuously and stably. It was set to travel every one or 2 h for two to 4 months. Thus, we were able to observe the variations of the water properties from hourly to monthly scales. The sensors were contaminated much less than we expected, and the data could be used without correction at least for our coastal applications. The main phenomena we observed are tides, coastal warming, fresh water, and responses to typhoons. On the daily time scale, the most prominent phenomenon is semi-diurnal tides, with which the thickness and temperature of coastal warm waters changed. The warm water also showed fluctuations between 10 and 15 days. The data also revealed that the tide showed strong seasonality. In summer, when the water is strongly stratified, the tidal current is baroclinic, while in winter, when the water is well mixed, the current is barotropic. Responses to typhoon induced winds were rather complicated. In one case, increase in the upper mixed layer was observed. The thick mixed layer disappeared in about a day due to advection. In another case the upper mixed layer became thinner, while the wind became stronger due the advection of the offshore water. Hydrographic observations conducted every 2 months, of course, or point measurement at a surface buoy could not show such continuous changes. More and more local fishermen are showing interest in oceanographic information, and data from the profiler could be of much use to them.
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Esters, Leonie, Anna Rutgersson, Erik Nilsson i Erik Sahlée. "Non-local Impacts on Eddy-Covariance Air–Lake $$\hbox {CO}_2$$ Fluxes". Boundary-Layer Meteorology, 15.09.2020. http://dx.doi.org/10.1007/s10546-020-00565-2.
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Abstract Inland freshwater bodies form the largest natural source of carbon to the atmosphere. To study this contribution to the atmospheric carbon cycle, eddy-covariance flux measurements at lake sites have become increasingly popular. The eddy-covariance method is derived for solely local processes from the surface (lake). Non-local processes, such as entrainment or advection, would add erroneous contributions to the eddy-covariance flux estimations. Here, we use four years of eddy-covariance measurements of carbon dioxide from Lake Erken, a freshwater lake in mid-Sweden. When the lake is covered with ice, unexpected lake fluxes were still observed. A statistical approach using only surface-layer data reveals that non-local processes produce these erroneous fluxes. The occurrence and strength of non-local processes depend on a combination of wind speed and distance between the instrumented tower and upwind shore (fetch), which we here define as the time over water. The greater the wind speed and the shorter the fetch, the higher the contribution of non-local processes to the eddy-covariance fluxes. A correction approach for the measured scalar fluxes due to the non-local processes is proposed and also applied to open-water time periods. The gas transfer velocity determined from the corrected fluxes is close to commonly used wind-speed based parametrizations.