Variational interpolation of high-frequency radar surface currents using DIVAnd

Ocean Dynamics - DIVAnd (Data-Interpolating Variational Analysis, in n-dimensions) is a tool to interpolate observations on a regular grid using the variational inverse method. We have extended...     

Correcting surface winds by assimilating high-frequency radar surface currents in the German Bight

Surface winds are crucial for accurately modeling the surface circulation in the coastal ocean. In the present work, high-frequency radar surface currents are assimilated using an ensemble scheme which aims to obtain improved surface winds taking into account European Centre for Medium-Range Weather Forecasts winds as a first guess and surface current measurements. The objective of this study is to show that wind forcing can be improved using an approach similar to parameter estimation in ensemble data assimilation. Like variational assimilation schemes, the method provides an improved wind field based on surface current measurements. However, the technique does not require an adjoint, and it is thus easier to implement. In addition, it does not rely on a linearization of the model dynamics. The method is validated directly by comparing the analyzed wind speed to independent in situ measurements and indirectly by assessing the impact of the corrected winds on model sea surface temperature (SST) relative to satellite SST.     

Backtracking drifting objects using surface currents from high-frequency (HF) radar technology

In this work, the benefits of high-frequency (HF) radar ocean observation technology for backtracking drifting objects are analysed. The HF radar performance is evaluated by comparison of trajectories between drifter buoys versus numerical simulations using a Lagrangian trajectory model. High-resolution currents measured by a coastal HF radar network combined with atmospheric fields provided by numerical models are used to backtrack the trajectory of two dataset of surface-drifting buoys: group I (with drogue) and group II (without drogue). A methodology based on optimization methods is applied to estimate the uncertainty in the trajectory simulations and to optimize the search area of the backtracked positions. The results show that, to backtrack the trajectory of the buoys in group II, both currents and wind fields were required. However, wind fields could be practically discarded when simulating the trajectories of group I. In this case, the optimal backtracked trajectories were obtained using only HF radar currents as forcing. Based on the radar availability data, two periods ranging between 8 and 10?h were selected to backtrack the buoy trajectories. The root mean squared error (RMSE) was found to be 1.01?km for group I and 0.82?km for group II. Taking into account these values, a search area was calculated using circles of RMSE radii, obtaining 3.2 and 2.11?km² for groups I and II, respectively. These results show the positive contribution of HF radar currents for backtracking drifting objects and demonstrate that these data combined with atmospheric models are of value to perform backtracking analysis of drifting objects.     

Use of surface currents measured by HF radar in planning coastal discharges

Surface currents measured by HF Radar (OSCR) are analysed to determine near-shore circulation in connection with planning optimum locations for sewage discharges. Dual radar units, each measuring a radial velocity component, were deployed at a total of five sites for three separate periods each of one week's duration (one site being common to two deployments). Observed data from 20 radial ‘bins’ each 1.2 km long were obtained for six beams each 6° wide at all five sites. These data were analysed separately to determine 1. the tidal constituents, 2. the wind driven component and 3. the steady residual component. In deriving the wind-driven response a relationship

was assumed where R(t) is the non-tidal velocity, W_E and W_N east and west components of wind speed, Δt a time lag and α and β empirical coefficients determined by least squares fitting techniques. At positions where beams from any two sites crossed, their separate current components were combined, producing the following spatial distributions of surface current vectors: 1. tidal ellipses, 2. separate responses to unit wind forcing from a. west and b. south and 3. steady residual drifts. The validity of these current distributions is examined by 1. comparison with other observational data, 2. by consideration of their self coherence and 3. in terms of simple theory.     

Ekman portion of surface currents,as measured by radar in different areas

Summary Surface currents, as measured by the HF-radar CODAR (COstal raDAR), are investigated with respect to their dependence on wind. Time series of wind are available from single weather stations, while CODAR yields current velocities on a grid with a resolution of some 3 km. As part of various research programs experiments have been carried out by the University of Hamburg (Germany) in different areas. Two of the areas under consideration are located in the Baltic Sea, two in the North Sea, and one covering the northern part of the Dead Sea. Time series of about two weeks with 2-hourly sampling are available for some 50 grid points of each area.Vector-correlation techniques are used to determine the linear relation of surface currents on wind velocity and also windstress. In both cases, significant correlation of about the same order has been found. 35% to 60% of the variance in surface currents may be explained by linear forcing from the vectors of wind velocity or windstress. Current-to-wind ratios range from 0.015 to 0.025, and a veering to the right of currents against wind has been observed. The decomposition of horizontal two-dimensional current fields into empirical orthogonal eigenfunctions (EOF) yields higher amounts of variance in the 1. EOF as may be explained by linear windforcing.Two possible mechanisms for wind-driven currents are discussed, Ekman circulation and Stokes drift. Assuming the vertical eddy viscosity to be independent on wind velocity and water depth, it may be estimated. Values of about 5×10^–4 m²s^–1 are found in the Baltic and 20×10^–4 m²s^–1 in the North Sea, which are in reasonable agreement with those used in numerical models.

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Ekman-Anteil an den durch Radar gemessenen Oberflächenströmungen in verschiedenen Seegebieten

Zusammenfassung Oberflächenströmungen, gemessen mit dem Hochfrequenzradar CODAR (COastal RaDAR), werden hinsichtlich ihrer Windabhängigkeit untersucht. CODAR mißt Strömungsgeschwindigkeiten in einem Gitter mit 3 km Auflösung. Zeitserien des Windes stammen von naheliegenden Wetterstationen. Messungen wurden von der Universität Hamburg im Rahmen von Forschungsprojekten in verschiedenen Meeresgebieten durchgeführt. Zwei dieser Gebiete liegen in der Ostsee, zwei in der Nordsee und eines umfaßt den nördlichen Teil des Toten Meeres. Von allen Gebieten existieren Meßserien an etwa 50 Gitterpunkten mit zweistündiger Abtastung und etwa 2 Wochen Dauer.Zur Bestimmung der linearen Abhängigkeit der Strömung sowohl vom Wind als auch vom Windschub werden Vektorkorrelationen berechnet. In beiden Fällen ergibt sich eine signifikante Korrelation von etwa gleichem Betrag. 35% bis 60% der Varianz der Oberflächenströmungen kann durch linearen Antrieb des Windes oder Windschubes erklärt werden. Der Quotient von Strömungs- zu Windgeschwindigkeit liegt im Bereich 0,015 bis 0,025, und es ist eine Drehung der Strömung nach rechts gegenüber dem Wind zu beobachten. Die Zerlegung des zweidimensionalen Strömungsfeldes in EOFs (empirical orthogonal eigenfunctions) ergibt für die 1. EOF einen höheren Varianzanteil als durch linearen Windantrieb erklärt werden kann.Zwei unterschiedliche Antriebsmechanismen für winderzeugte Strömungen werden diskutiert, Ekman Zirkulation und Stokes Drift. Der vertikale Austauschkoeffizient kann aus Daten bestimmt werden, vorausgesetzt daß er nicht vom Wind und der Wassertiefe abhängt. Werte von etwa 5×10^–4 m²s^–1 werden für die Ostsee und 20×10^–4 m²s^–1 für die Nordsee gefunden. Diese Werte liegen in dem Bereich, der für numerische Modelle benutzt wird.

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Ekman-Part des courants de surface, mesurés par Radar dans différents endroits

Résumé Les courants de surface mesurés par le radar CODAR (COastal raDAR), sont analysés par rapport à leur dépendence vis-à-vis du vent. Les séries temporelles du vent sont disponibles á chaque station météorologique alors que CODAR donne les vitesses des courants sur une grille de 3 km de résolution. Dans le cadre de plusieurs programmes de recherche, des études ont été faites par l'Université de Hamburg (Allemagne) dans différents endroits. Deux des zones considérées sont situées dans la Mer Baltique, deux dans la Mer du Nord et une couvre la partie Nord de la Mer Morte. Des séries temporelles d'environ 2 semaines avec un échantillonnage de 2-heures sont disponibles sur 50 points pour chaque zone.Des techniques de correlation vectorielle sont employées pour déterminer la relation linéaire entre courants de surface et d'une part la vitesse et d'autre part la pousée du vent. Dans les deux cas nous avons trouvé une correlation significative du même ordre. Le modèle linéaire forcé par les vecteurs de vitesse et pousée de vent représente de 35%–60% de la variance des courants de surface. Les rapports courant-vent varient de 0.015–0.025 et la rotation vers la droite des courants par rapport au vent a été observée. La décomposition des champs bi-dimensionnels de courant sur des fonctions propres empiriques orthogonales (EOF) donnent des taux de variance plus importants dans la lère EOF, comme expliqué par le forçage linéaire du vent.Deux mécanismes possibles de courants induits par les vents sont discutés, la circulation d'Ekman et la dérive de Stokes. Supposant une viscosité de tourbillon verticale indépendente de la vitesse du vent et de la profondeur, la viscosité est estimée. Des valeurs d'environ 5×10^–4 m²s^–1 et de 20×10^–4 m²s^–1 sont trouvées respectivement pour la Mer Baltique et la Mer du Nord, ce qui est en accord raisonable avec les valeurs utilisées dans les modèles numériques.

     

VHF radar observations of surface currents off the northern Opal coast in the eastern English Channel

Two very high-frequency radars (VHFR) operating on the Opal coast of eastern English Channel provided a nearly continuous 35-day long dataset of surface currents over a 500 km² area at 0.6–1.8 km resolution. Argo drifter tracking and CTD soundings complemented the VHFR observations, which extended approximately 25 km offshore. The radar data resolve three basic modes of the surface velocity variation in the area, that are driven by tides, winds and freshwater fluxes associated with seasonal river discharge. The first mode, accounting for 90% of variability, is characterized by an along-shore flow pattern, whereas the second and third modes exhibit cross-shore, and eddy-like structures in the current velocity field. All the three modes show the dominant semi-diurnal variability and low-frequency modulation by the neap-spring tidal cycle. Although tidal forcing provides the major contribution to variability of local currents, baroclinicity plays an important role in shaping the 3D velocity field averaged over the tidal cycle and may strongly affect tracer dynamics on larger time scales. An empirical orthogonal function (EOF) decomposition and a spectral rotary analysis of the VHFR data reveal a discontinuity in the velocity field occurring approximately 10 km offshore which was caused by the reversal in the sign of rotation of the current vector. This feature of local circulation is responsible for surface current convergence on ebb, divergence on flood and strong oscillatory vertical motion. Spectral analysis of the observed currents and the results of the Agro drifter tracking indicate that the line of convergence approximately follows the 30-m isobath. The most pronounced feature of the radar-derived residual circulation is the along-coast intensification of surface currents with velocity magnitude of 0.25 m/s typical for the Regions of Freshwater Influence (ROFI). The analysis has provided a useful, exploratory examination of surface currents, suggesting that the circulation off the Opal coast is governed by ROFI dynamics on the hypertidal background.     

Simulation and detection of tsunami signatures in ocean surface currents measured by HF radar

High-frequency (HF) surface wave radars provide the unique capability to continuously monitor the coastal environment far beyond the range of conventional microwave radars. Bragg-resonant backscattering by ocean waves with half the electromagnetic radar wavelength allows ocean surface currents to be measured at distances up to 200 km. When a tsunami propagates from the deep ocean to shallow water, a specific ocean current signature is generated throughout the water column. Due to the long range of an HF radar, it is possible to detect this current signature at the shelf edge. When the shelf edge is about 100 km in front of the coastline, the radar can detect the tsunami about 45 min before it hits the coast, leaving enough time to issue an early warning. As up to now no HF radar measurements of an approaching tsunami exist, a simulation study has been done to fix parameters like the required spatial resolution or the maximum coherent integration time allowed. The simulation involves several steps, starting with the Hamburg Shelf Ocean Model (HAMSOM) which is used to estimate the tsunami-induced current velocity at 1 km spatial resolution and 1 s time step. This ocean current signal is then superimposed to modelled and measured HF radar backscatter signals using a new modulation technique. After applying conventional HF radar signal processing techniques, the surface current maps contain the rapidly changing tsunami-induced current features, which can be compared to the HAMSOM data. The specific radial tsunami current signatures can clearly be observed in these maps, if appropriate spatial and temporal resolution is used. Based on the entropy of the ocean current maps, a tsunami detection algorithm is described which can be used to issue an automated tsunami warning message.     

Multi-scale variability of circulation in the Gulf of Tonkin from remote sensing of surface currents by high-frequency radars

Ocean Dynamics - The sea surface velocity measurements obtained during the period of 2014–2016 using two high-frequency radars (HFR), which were installed on the northern coast of Vietnam,...     

Near-inertial currents in the DeSoto Canyon region

Near-inertial currents in the DeSoto Canyon region are described using current and wind observations taken between April 1997 and March 1998 for the “DeSoto Canyon Eddy Intrusion Study”. Distinct energy peaks are present at near-inertial frequencies for the clockwise spectrum and there is little energy at the same frequencies for the counterclockwise current spectrum. In this region, amplitudes of the near-inertial currents can be as high as 40 cm s⁻¹. These currents are surface-intensified and display an increase in amplitude from the shelf break to offshore. Between November 1997 and March 1998, they were effectively generated by shifting winds accompanying passages of cold fronts. For this time period, near-inertial currents are reasonably well-simulated by a mixed-layer model forced by observed winds. During summer 1997, however, enhanced near-inertial motions often resulted from resonance between winds and existing currents.     

A short-term predictive system for surface currents from a rapidly deployed coastal HF radar network

In order to address the need for surface trajectory forecasts following deployment of coastal HF radar systems during emergency-response situations (e.g., search and rescue, oil spill), a short-term predictive system (STPS) based on only a few hours data background is presented. First, open-modal analysis (OMA) coefficients are fitted to 1-D surface currents from all available radar stations at each time interval. OMA has the effect of applying a spatial low-pass filter to the data, fills gaps, and can extend coverage to areas where radial vectors are available from a single radar only. Then, a set of temporal modes is fitted to the time series of OMA coefficients, typically over a short 12-h trailing period. These modes include tidal and inertial harmonics, as well as constant and linear trends. This temporal model is the STPS basis for producing up to a 12-h current vector forecast from which a trajectory forecast can be derived. We show results of this method applied to data gathered during the September 2010 rapid-response demonstration in northern Norway. Forecasted coefficients, currents, and trajectories are compared with the same measured quantities, and statistics of skill are assessed employing 16 24-h data sets. Forecasted and measured kinetic variances of the OMA coefficients typically agreed to within 10–15%. In one case where errors were larger, strong wind changes are suspected and examined as the cause. Sudden wind variability is not included properly within the STPS attack we presently employ and will be a subject for future improvement.     

Analytical description of electric currents in the magnetosheath region

Volume currents in the magnetosheath region are calculated within the framework of a new analytical model. Magnetic field structure in the region is found, satisfying boundary conditions on the bow shock and the magnetopause, and then volume currents are calculated using the Maxwell equation. Surface bow shock and magnetopause currents are calculated, too. Free parameters of the model are interplanetary magnetic field, Mach number of the solar wind flow, distances to the bow shock and to the magnetopause, and field compression at the magnetopause.     

Assessing the fidelity of surface currents from a coastal ocean model and HF radar using drifting buoys in the Middle Atlantic Bight

The rapid expansion of urbanization along the world’s coastal areas requires a more comprehensive and accurate understanding of the coastal ocean. Over the past several decades, numerical ocean circulation models have tried to provide such insight, based on our developing understanding of physical ocean processes. The systematic establishment of coastal ocean observation systems adopting cutting-edge technology, such as high frequency (HF) radar, satellite sensing, and gliders, has put such ocean model predictions to the test, by providing comprehensive observational datasets for the validation of numerical model forecasts. The New York Harbor Observing and Prediction System (NYHOPS) is a comprehensive system for understanding coastal ocean processes on the continental shelf waters of New York and New Jersey. To increase confidence in the system’s ocean circulation predictions in that area, a detailed validation exercise was carried out using HF radar and Lagrangian drifter-derived surface currents from three drifters obtained between March and October 2010. During that period, the root mean square (RMS) differences of both the east–west and north–south currents between NYHOPS and HF radar were approximately 15 cm s^?1. Harmonic analysis of NYHOPS and HF radar surface currents shows similar tidal ellipse parameters for the dominant M₂ tide, with a mean difference of 2.4 cm s^?1 in the semi-major axis and 1.4 cm s^?1 in the semi-minor axis and 3° in orientation and 10° in phase. Surface currents derived independently from drifters along their trajectories showed that NYHOPS and HF radar yielded similarly accurate results. RMS errors when compared to currents derived along the trajectory of the three drifters were approximately 10 cm s^?1. Overall, the analysis suggests that NYHOPS and HF radar had similar skill in estimating the currents over the continental shelf waters of the Middle Atlantic Bight during this time period. An ensemble-based set of particle tracking simulations using one drifter which was tracked for 11 days showed that the ensemble mean separation generally increases with time in a linear fashion. The separation distance is not dominated by high frequency or short spatial scale wavelengths suggesting that both the NYHOPS and HF radar currents are representing tidal and inertial time scales correctly and resolving some of the smaller scale eddies. The growing ensemble mean separation distance is dominated by errors in the mean flow causing the drifters to slowly diverge from their observed positions. The separation distance for both HF radar and NYHOPS stays below 30 km after 5 days, and the two technologies have similar tracking skill at the 95 % level. For comparison, the ensemble mean distance of a drifter from its initial release location (persistence assumption) is estimated to be greater than 70 km in 5 days.     

高频面波方法的若干新进展

面波多道分析方法(MASW)通过分析高频瑞雷波确定浅地表剪切波速度.在过去的20年中, 由于该方法具有非侵入性、无损、高效及价格低的特点, 越来越受到浅地表地球物理和地质工程学界的重视, 视为未来最有希望的技术之一.这篇综述论文将介绍中国地质大学(武汉)浅地表地球物理团队近年来在研究高频面波的传播理论和应用中取得的部分成果.非几何波是一种仅存在于浅地表介质, 尤其是未固结的沉积物中的独特的地震波.它的存在对快速而准确地获得表层S波速度有一定价值.我们的研究表明非几何波是一种具有频散特性的泄漏波.泄漏波的存在可能导致将其误认为瑞雷波的基阶或高阶能量, 从而造成模式误判.这种模式误判会导致错误的反演结果.我们通过求取高基阶分离后的瑞雷波格林函数证明虚震源法瑞雷波勘探的可行性.这个结果将极大地降低野外瑞雷波勘探成本.勒夫波多道分析方法(MALW)中未知参数比瑞雷波的少, 这使得勒夫波的频散曲线比瑞雷波的简单.因此, 勒夫波反演更稳定, 非唯一性更低.勒夫波数据生成的能量图像通常比瑞雷波的清晰, 并具有更高的分辨率, 从而可以更容易地拾取精确的勒夫波的相速度.利用雅克比矩阵分析波长与探测深度的关系表明对相同波长的基阶模式而言, 瑞雷波的探测深度是勒夫波的1.3~1.4倍; 而两种波的相同波长的高阶模式波的探测深度相同.我们也尝试了时间域勒夫波反演.按照勒夫波分辨率将地球模型剖分成了不同尺寸的块体, 利用反卷积消除了地震子波对勒夫波波形的影响, 通过更新每个块体的S波速度来拟合勒夫波波形, 从而获得地下S波速度模型.该方法不基于水平层状模型假设, 适用于任意二维介质模型.     

Application of HF radar currents to oil spill modelling

In this work, the benefits of high-frequency (HF) radar currents for oil spill modeling and trajectory analysis of floating objects are analyzed. The HF radar performance is evaluated by means of comparison between a drifter buoy trajectory and the one simulated using a Lagrangian trajectory model. A methodology to optimize the transport model performance and to calculate the search area of the predicted positions is proposed. This method is applied to data collected during the Galicia HF Radar Experience. This experiment was carried out to explore the capabilities of this technology for operational monitoring along the Spanish coast. Two long-range HF radar stations were installed and operated between November 2005 and February 2006 on the Galician coast. In addition, a drifter buoy was released inside the coverage area of the radar. The HF radar currents, as well as numerical wind data were used to simulate the buoy trajectory using the TESEO oil spill transport model. In order to evaluate the contribution of HF radar currents to trajectory analysis, two simulation alternatives were carried out. In the first one, wind data were used to simulate the motion of the buoy. In the second alternative, surface currents from the HF radar were also taken into account. For each alternative, the model was calibrated by means of the global optimization algorithm SCEM-UA (Shuffled Complex Evolution Metropolis) in order to obtain the probability density function of the model parameters. The buoy trajectory was computed for 24 h intervals using a Monte Carlo approach based on the results provided in the calibration process. A bivariate kernel estimator was applied to determine the 95% confidence areas. The analysis performed showed that simulated trajectories integrating HF radar currents are more accurate than those obtained considering only wind numerical data. After a 24 h period, the error in the final simulated position improves using HF radar currents. Averaging the information from all the simulated daily periods, the mean search and rescue area calculated using HF radar currents, is reduced by approximately a 62% in comparison with the search area calculated without these data. These results show the positive contribution of HF radar currents for trajectory analysis, and demonstrate that these data combined with atmospheric forecast models, are of value for trajectory analysis of oil spills or floating objects.     

Magnetospheric currents and estimation of theSq focal latitude

The average latitude of theSq(X) focus along the longitude 75°E has been estimated using the Tsyganenko model (Tsyganenko, 1981) of the external magnetic field for storm time conditions of the magnetosphere. It is observed that the shift of the focal latitudes, due to magnetospheric currents, is only about 1 to 2° even during strong storms. It is also shown that the shift is asymmetric about the equator and longitude dependent. The day to day changes in observed focal position are much larger and the magnetospheric currents cannot, therefore, be regarded as a dominant mechanism of focal movement.The paper was presented at the IAGA General Assembly meeting held in Prague, Czechoslovakia, during August 1985.