FVCOM Research Articles

Modulation of Residual Current on Impacts of Shoreline Changes in the Yangshan Deepwater Harbor

Abstract Coastal land reclamation projects significantly alter tidal wave propagation and residual flow structures. This study focuses on the Yangshan Deepwater Harbor (YDH) in China, aiming to scrutinize the effects of construction activities on residual flow patterns employing the Finite Volume Community Ocean Model (FVCOM). By calculating Lagrangian residual currents across different tidal periods, we analyzed alterations in flow patterns and velocity distribution pre- and post-construction. The research findings reveal that, although the residual flow direction in the harbor and the southern sea area remains largely unchanged from the pre-construction phase, the northern sea area has experienced a shift in residual flow direction. It is noteworthy that after the implementation of the project, the residual flow at the East Entrance was greatly weakened. This study underscores that coastal constructions can lead to remarkable influence on residual currents and emphasizes the necessity for thorough hydrodynamic assessments in the planning and implementation stages of coastal constructions.

Simulating cross-scale flow dynamics around offshore monopile foundations with a GFD-CFD coupled model

Flow dynamics are complex and undergo significant impact from offshore engineering infrastructures, such as wind turbines, involving interactions across different temporal and spatial scales. Individual numerical models of geophysical fluid dynamics (GFD) or computational fluid dynamics (CFD) face challenges in accurately representing these dynamics. To effectively capture the three-dimensional cross-scale flow around offshore infrastructures, we propose a generalized coupling framework that integrates the unstructured-grid Finite-Volume Community Ocean Model (FVCOM) for GFD and the Open Field Operation and Manipulation (OpenFOAM) for CFD. With the validation against the flume experiment, the common-boundary nesting method was developed to allow an accurate representation of flow dynamics around offshore hydraulic infrastructures. Idealized monopile foundation experiments demonstrate the model's capability to capture the formation of Karman vortex street, rotating horseshoe vortex under realistic astronomical tidal conditions. Application of this coupling framework to the offshore wind farm in Yangjiang, China, reveals flow direction lagging of 15° in the wake of a monopile under diurnal tide. This coupling approach, leveraging the strengths of both CFD and GFD models, provides a universal downscaling methodology in geophysical conditions, for a better understanding of the impact of offshore hydraulic infrastructures on the surrounding environment and further planning offshore engineering projects.

Baroclinic responses of Qiongzhou Strait throughflow to different forcings

The Qiongzhou Strait (QS) throughflow is affected by the seasonal circulation in both the northern South China Sea (SCS) shelf and the Beibu Gulf. The water masses with distinct properties between these two regions signify the baroclinic processes in the QS. In this study, a high-resolution unstructured-grid Finite Volume Community Ocean Model (FVCOM) was used to investigate the features and dynamic mechanisms of baroclinic-induced currents (BCC) in the QS. Different forcings were considered such as tides, river discharges, winds, and remote forcings (low-frequency sea surface level and current variations). The model results indicate that the volume transport of the BCC contributes to 31.26% of the total in the QS, determining the transport direction in summer. The baroclinic response to low-frequency remote forcings is most pronounced during the summer-autumn season, but less than winds during the winter-spring season. This finding underscores the importance of low-frequency remote forcings on the QS throughflow, which has often been overlooked in previous studies. Momentum balance suggests that the BCCs are predominantly driven by both the barotropic and baroclinic pressure gradient forces. Decrease (increase) in either of these two forces will lead to an intensified eastward (westward) flow. The contribution of the horizontal advection is relatively weak but cannot be ignored. This study highlights the baroclinic responses of QS throughflow to different forcings, which is important when investigating the circulation in the northern SCS, especially in the Beibu Gulf.

Modelling water temperature dynamics for eelgrass (Zostera marina) areas in the nearshore Scotian Shelf

Water temperature is an important environmental factor for many ecological processes in coastal ecosystems. Here, we study water temperature dynamics at a set of study sites on the Atlantic coast of Nova Scotia where eelgrass beds are found. The central emphasis is to predict temperature on scales relevant to coastal ecosystem processes using a high-resolution nearshore oceanographic model based on the Finite Volume Community Ocean Model (FVCOM). The model predictions were evaluated against observed temperature time series at six sites for three years from 2017-2019; the evaluation indicates that the model was able to replicate the temperature variation on time scales from hours to seasonal. We also used various biologically tailored temperature metrics relevant to eelgrass condition, including mean seasonal values and variability, daily ranges, growing degree day (GDD), and warm events, to validate the model against time series observations to better understand the temperature regime at the study sites. Frequency resolved Willmott skill scores were >0.7, and the temperature metrics were well predicted with the exception of a bias in GDD at some of the shallow sites. The eelgrass sites have a wide range of temperature conditions. Mean water temperature in the summer differed by more than 7°C between the shallowest and the deepest sites, and the rate of heat accumulation was fastest at shallow sites which had ≥ 12 extreme warm events per year. While the amplitude of the temperature variations within the high frequency band (<48 hr) was greater in shallower sites, temperature changes on meteorological time scales (48 hr to 60 days) were coherent at all sites, suggesting the importance of coast-wide processes. The results of this study demonstrated that our high resolution numerical model captured biologically relevant temperature dynamics at different time scales and over a large spatial region, and yet still accurately predicted detailed temperature dynamics at specific nearshore sites. Thus, the model can provide important insights into coastal temperature dynamics that are potentially useful for conservation planning and understanding the implications of future change.

Modeling the water use associated with energy consumption changes on saltwater intrusion in the Pearl River estuary, China

The intrusion of saline tide into estuaries is one of the major environmental issues impedding the regional development of the Pearl River Estuary, which has been exacerbated by increasing water use associated with energy consumption in the estuary areas. However, the relationship among water use, energy consumption and saltwater intrusion remains ambiguous. Firstly, this paper established a k-nearest neighbor model that identified the correlation between inputs of precipitation and sectoral energy consumption and output of runoff in the Humen Channel as part of the Pearl River Estuary during 2019–2020. Then, the Finite–Volume Community Ocean Model (FVCOM) was employed to explore the process-based mechanism between runoff and salinity via three scenarios simulated for the periods 2030–2031 and 2060–2061 with different water use and precipitation levels. Finally, the impacts of sectoral energy consumption on saltwater intrusion under the different scenarios were analyzed. The results showed that the increase of the energy consumption was positively correlated to runoff, especially in the wet season of Humen Channel. The monthly average salinity exhibited remarkable seasonal variations with similar trends under three scenarios. Totally 654 days with salinity exceeding the standard were projected to take place in the downstream of Humen Channel during the simulation period.

Response of Sediment Dynamics to Tropical Cyclones under Various Scenarios in the Jiangsu Coast

The Jiangsu Coast (JC), China, is an area susceptible to the impact of tropical cyclones (TCs). However, due to the lack of available on-site observation data, nearshore sedimentary dynamic processes under the impact of TCs have not been fully explored. This study developed a 3D wave–current–sediment numerical model for the JC based on the Finite Volume Community Ocean Model (FVCOM) to investigate sediment dynamic responses to TCs under various scenarios, including different tracks, intensities of TCs and tidal conditions. The validation results demonstrated the model’s satisfactory performance. According to the simulation results, typhoons can significantly impact the hydrodynamics and sediment dynamics. During Typhoon Lekima in 2019, strong southeasterly winds substantially increased the current velocity, bottom stress, wave height, and suspended sediment concentration (SSC). Three typical landfall-type typhoons, with prevailing southeasterly winds, brought significant sediment flux from southeast to northwest along the coast, while the typhoon that moved northward in the Yellow Sea induced a relatively small sediment flux from north to south. Typhoons could also induce stripe-like erosion and deposition, which is closely related to seafloor topography, resulting in seabed thickness variations of up to ±0.3 m. Additionally, strengthening typhoon wind fields can lead to increased sediment flux and seabed morphological changes. Typhoon Winnie, particularly at spring tide, had a greater impact on sediment dynamics compared to other landfall typhoons. Numerical simulations showed that the typhoon-induced net sediment flux within the spring tidal cycle could increase by 80% to 100% compared to the neap tidal cycle, indicating the significant influence of tidal conditions on sediment transport during TC events.

Response of salt intrusion in a tidal estuary to regional climatic forcing

Salinity distribution in a large tidal estuary is subject to estuarine adjustment under the influences of multiple physical drivers such as freshwater pulses and sea level rise, and is crucial to upstream water quality, aquaculture, and ecosystem functions of the estuary. To better understand the estuarine salinity response to climate change, the unstructured-grid Finite Volume Community Ocean Model was implemented to simulate the salt intrusion in the Delaware Bay Estuary. The model was first validated by multiple observational data sets and subsequently applied in an idealized setting to examine the response of salt front to freshwater pulses in high flow conditions, followed by a long-term drought condition supported by a multi-decadal streamflow drought analysis in the estuary. The model results showed that after the freshwater pulses the salt front location moved further upstream with sea level rise. Under the simulated long-term drought condition, the adjustment timescale of salt intrusion varies nonlinearly with sea level rise. With a significant increase in sea level rise, the adjustment timescale starts to decrease. This shift suggests a transition into a different regime where the estuary becomes more stratified, as indicated by an increasing bulk Simpson number with rising sea levels.

Modeling attenuation of photosynthetically active radiation across the optical gradient in the Laurentian Great Lakes with application to Lake Erie

Representation of subsurface photosynthetically active radiation (PAR) in biophysical models of the Laurentian Great Lakes (LGL) is imperative to their utility as tools for research and management. Here we consolidated measured vertical profiles of subsurface PAR with concurrent water quality (WQ) data from four LGL. We estimated the diffuse attenuation coefficient of PAR (Kd(PAR)) by fitting an exponential function to measured PAR over depth, and evaluated 68 regressions predicting Kd(PAR) as a function of water quality variables (Kd-WQ regressions). We compare four of the top cross-lake calibrated regressions against two published regressions trained on western Lake Erie (WLE) data. Then, as a case study, we demonstrate the utility of our cross-lake calibrated Kd-WQ regressions with a simplified biophysical model of Lake Erie consisting of the Finite Volume Community Ocean Model with submodules for simulating suspended sediment and dissolved organic carbon (FVCOM-SS-DOC). Twenty-five Kd-WQ regressions were identified as candidates for use in biophysical models based on their skill determined via cross-validation. WLE-trained Kd-WQ regressions were less able to simulate Kd(PAR) and PAR in more transparent waters compared to cross-lake calibrated Kd-WQ regressions, which translated to considerable differences in primary production estimates for the central and eastern basins when using WQ data simulated by FVCOM-SS-DOC. A cross-lake calibrated Kd-WQ regression was installed into FVCOM-SS-DOC, which then simulated spatial patterns of suspended sediments and Kd(PAR). These calibrated Kd-WQ regressions can be used in a variety of biophysical models across optically-distinct waters of the LGL to support adaptive management of nutrient inputs and fisheries.

Modes of Weather System-Induced Flows through an Arctic Lagoon

With the increasing warming of the Arctic, the summertime ice-free period in the coastal Arctic becomes longer and the water exchange between arctic lagoons and coastal Beaufort Sea becomes more important for land–ocean interaction. This study examined the dynamics of water exchange between the arctic lagoons and the Arctic Ocean under the influence of weather systems (the transient arctic cyclones and hovering Beaufort High pressure system). We implemented rare observations, numerical modeling with the Finite Volume Community Ocean Model (FVCOM), and a forcing-response Empirical Orthogonal Function (fr-EOF) analysis to determine the weather-driven flow patterns and characteristics in the micro-tidal arctic lagoon (Elson Lagoon) with little freshwater discharge. The results were validated for both tidal and subtidal currents with in situ data. The inlets of the lagoon were significantly impacted by wind associated with the weather systems and the flows through the inlets were highly correlated with each other. The fr-EOF analysis for the 1.5-month FVCOM output indicated three significant modes of wind-driven flow. In the deepest (~16 m) northwestern-most inlet, a counter-wind flow occurred more than 96% of the time due to setup and set down of water level inside the lagoon and the vorticity balance related to the wind stress and water depth. For about 60–80% of the time, the exchange flow was out of the lagoon through the northwestern-most and deepest inlet due to the strong easterly winds dictated by the Beaufort High; this dominant flow is mainly caused by the persistent easterly wind as a limb of the Beaufort High pressure system, modified by the transient arctic cyclones with a westerly wind and inward flows at the westernmost inlet of Elson Lagoon. This study shows that the alternating influence from the cyclone-anticyclone weather systems produces a meteorological tide in the subtidal spectrum which dominates the water exchange in the region through the multiple inlets. With the observed increase in cyclone strength and frequency under the warming trend, this may imply a greater contribution from the westerly wind because of the increased cyclonic activities. If this is the case, the inward flow might increase and have an effect on sediment, larval, and nutrient transports through this system.

Quantitative assessment of building risks and loss ratios caused by storm surge disasters: A case study of Xiamen, China

China is severely affected by storm surge disasters, which result in substantial economic losses and casualties in coastal regions. Assessing the risk of storm surge disasters can provide valuable insights into the expected losses and severity of future impacts, offering critical foresight for disaster prevention and mitigation strategies. This study assesses the quantitative risk of storm surge disasters, focusing on coastal buildings since they are particularly susceptible to storm surges and frequently bear the brunt such disasters. Xiamen city, China, was used as case study. A high-precision numerical model, using Finite Volume Community Ocean Model (FVCOM), was developed to simulate inundation during storm surges. By referencing historical storm surge records, we defined key parameters for probable maximum typhoon-induced storm surge (PMTSS) in Xiamen. These parameters were used to calculate the corresponding inundation range and water depth distribution within the region. Subsequently, the results were integrated with vulnerability curves that represent the susceptibility of buildings in Xiamen to storm surge-induced damage, enabling the quantitative risk assessment for associated loss risks. The study findings offer valuable guidance for urban planning and functional layout design in coastal areas. Furthermore, the findings contribute to understanding storm surge disaster risks and facilitating informed decision-making processes, ultimately enhancing disaster preparedness and resilience in vulnerable coastal regions.

Investigating Multidecadal Trends in Ice Cover and Subsurface Temperatures in the Laurentian Great Lakes Using a Coupled Hydrodynamic–Ice Model

Abstract While changing lake surface conditions have received significant research scrutiny, changes in subsurface conditions, including stratification and heat content, remain largely unexplored. In this work, we highlight changes in thermal structure, stratification dynamics, and ice characteristics in the Laurentian Great Lakes (Lakes Superior, Huron, Michigan, Erie, and Ontario) as simulated between 1979 and 2021. Three-dimensional lake hydrodynamics and ice cover were modeled using the Finite Volume Community Ocean Model (FVCOM) coupled with the Los Alamos sea ice model (CICE). Analysis revealed significant increases in surface (0.4°–0.6°C decade−1) and subsurface (0.1°–0.4°C decade−1) temperatures as well as dramatic losses in ice cover (1%–8% decade−1) and ice volume (0–3 km3 decade−1) over the last 40 years. Estimated surface heating rates were strongest during the summer and fall, while subsurface warming was most rapid during the nearly isothermal winter and spring. Intensified (decreased) summer (winter) stratification led to shifts in lake turnover dynamics, with delayed fall turnover dates (2–6 days decade−1) and earlier spring overturn dates (2–9 days decade−1). Modeled surface temperatures (LST), bottom temperatures (LBT), and annual averaged ice cover (AAIC) were used to estimate low-frequency climate signals, which were highly correlated with the Atlantic multidecadal oscillation. Warming trends fit to residual climate signals (LST: 0.1°C decade−1; LBT: 0.03°C decade−1; AAIC: −1% decade−1), calculated by removing low-frequency variability from the raw climate signal, were lower than those fit to associated low-frequency components, suggesting that recent climate change in the Great Lakes may be strongly influenced by natural multidecadal climate variability.

Modeling investigation of wave-induced longshore current distribution patterns on barred beaches

A three-dimensional coupled wave-current model was employed to investigate the wave-induced longshore currents on barred beaches. The modeling system was developed by coupling the finite-volume community ocean model (FVCOM) and the simulating waves nearshore (SWAN) model to fully describe wave-current interactions in the nearshore region. The three-dimensional radiation stress, surface rollers and wave-enhanced turbulent mixing were incorporated as the major wave effects on currents. The model was first evaluated against data collected from multiple sets of laboratory experiments. Diagnostic simulations demonstrated that the contributions of the wave rollers and wave-induced mixing, in addition to the radiation stress, were important for ensuring accurate calculations. Then, the longshore current patterns on barred beaches were investigated through simulations of a series of test cases under controlled conditions. The cross-shore distribution of wave-induced longshore currents, which predominantly occurred within the barred beach surf zone, usually exhibited bimodal characteristics. The maximum peak longshore current velocity was positively correlated with the incident wave height and period. As the bar height and width increased, the first velocity peak of the longshore current strengthened, while the second peak weakened. In most cases, the maximum longshore current velocity on a barred beach coincided with the first peak, which could occur anywhere from the bar-trough region to the bar-crest region, depending on the wave and bar heights. This finding could be used to clarify the cross-shore location discrepancies in the maximum peak longshore current velocity reported in previously published studies.

A Circulation Model for Baynes Sound, British Columbia, Canada

ABSTRACT An application of the three-dimensional Finite Volume Community Ocean Model was developed for Baynes Sound, British Columbia, Canada and a simulation was carried for the period of May 2016 to April 2017. The objective was to provide highly resolved spatial estimates of the regional hydrodynamics that could be used in a coupled biogeochemical model to assess the cultured shellfish carrying capacity of the region. The issue of how representative the particular simulation period was in the context of typical seasonal features was addressed by comparing temperatures, salinities and river discharges with long-term statistics. Circulation model results were generally in good agreement with salinity, temperature, velocity, and sea surface height observations, providing confidence in subsequent seasonal estimates of volume fluxes through the two entrances and water-renewal. Approximately sixty percent of the variability in near-surface temperature observations at five locations was shown to be linearly dependent on a combination of the along-sound wind, air temperature, and the daily sea surface range, which was taken as a proxy for tidal mixing.

An improved method for water depth mapping in turbid waters based on a machine learning model

Water depth data products are fundamental for the study of near-shore marine science and coastal engineering; therefore, it is essential to identify the optimal technique for coastal water depth mapping. There are reliable mapping techniques for clear open waters. However, near-shore waters is typically turbid, which decreases the accuracy of previous methods for clear waters. To address these challenges, in this study, a machine learning-based topographic inversion method for turbid waters was developed. The log band ratio algorithm presented in previous studies was modified initially to enhance its application in turbid waters. Then, we established a model called AdaBoost-GBDT for bathymetric inversion in turbid waters using the Gradient Boosting Decision Tree (GBDT) algorithm fused with the Adaptive Boosting (AdaBoost) algorithm. Comparison of the results revealed that the model is highly reliable in all water depth intervals. To evaluate the quality of our model derived product in coastal research, a comparison of the data quality among four water depth datasets was conducted in this study. The comparison demonstrated that the inversion results of AdaBoost-GBDT have the best data coverage and accuracy. Furthermore, when applied to the Finite Volume Community Ocean Model (FVCOM), the AdaBoost-GBDT bathymetric product had the best performance. This study provides a new method for the bathymetric inversion of turbid waters, which can contribute to coastal management and engineering.

A Circulation Model for Inlets Along the Central West Coast of Vancouver Island

ABSTRACT An ocean circulation model has been developed for the central west coast of Vancouver Island, with particular attention to the numerous inlets where salmon farms are located. The ultimate goal is to provide a tool to assist in informing decisions on the siting and management of fish farms and to further understanding of the regional coastal oceanography. The complicated coastline and bathymetry are approximated with an unstructured triangular grid in the horizontal direction and terrain-following coordinates in the vertical. The period of 1 March to 31 July 2016, is simulated using the Finite Volume Community Ocean Model and results are evaluated through comparisons with a combination of acoustic Doppler current profiler, tide gauge, and temperature and salinity observations. The model was found to be accurate in representing (i) tidal elevations and currents everywhere throughout the model domain and (ii) subtidal currents and temperature and salinity along the shelf. However, it fared poorly in inlets where (i) the tidal currents were relatively weak and not the dominant source of mixing and/or (ii) steep bathymetry necessitated smoothing. Sources of these and other inaccuracies, and the changes needed to overcome them, are discussed. Overall, this work highlights the complex and diverse dynamics at play in a glacially-carved coastal ocean, providing an extensive discussion of modelling challenges and potential solutions.

Intermittent migration of mixing front driven by typhoon events on the inner shelf of the East China Sea: A FVCOM modeling study

Mixing fronts on the inner shelf of the East China Sea (ECS) are common under normal summer conditions, yet its migration during extreme weather events, such as typhoons, has been poorly understood due to limited availability of observational data. In this study, we used a well-validated Finite-Volume Community Ocean Model (FVCOM) to investigate the intermittent migration of mixing fronts on the inner shelf of the ECS during the passage of Typhoon Chan-hom in July 2015. The sedimentary and ecological effects of the front migration were also revealed. Results suggested that the potential energy anomaly (PEA) of 10 J/m3 could be a critical value to determine the location of the mixing front, which was consistent with that derived from the Simpson-Hunter (SH) parameter. Accordingly, the mixing front showed a rapid seaward migration from 15-m isobaths to 60-m isobaths under the drastic ocean turbulence during the typhoon and recovered shortly after its passage. This intermittent migration of the mixing front had the potential to facilitate cross-shelf material transport during the typhoon. On the one hand, the front migration could not only open the access for the seaward transport of nearshore resuspended sediments but also trap them within a certain area, causing an offshore deposition between 20 and 60 m isobaths south of the typhoon track. The mixing fronts had a boundary effect constraining the typhoon-induced sedimentation. On the other hand, unlike sediments that would settle down with weakened ocean dynamics, the offshore movement of coastal diluted water triggered by the typhoon remained after the front recovery. These nutrient-rich waters could be transported to the middle or even outer shelf of the ECS by the subsequent southwest summer monsoon, enhancing local primary production and potentially trigering significant algal blooms.

Evaluating the performance of turbulence closure models for tidal stream resource characterization

Harvesting tidal stream energy from the ocean for electricity generation has been considered as an energy resource alternative to fossil fuels for mitigating the negative impact of climate change and enhancing energy security and coastal resilience. Numerical models have been used extensively to characterize and assess tidal resources at potential tidal energy development sites. Turbulence plays an important role in site selection and tidal turbine farm deployment. However, most of the numerical modeling studies for tidal energy resource characterization do not include turbulence characteristics because of the limitation of Reynolds averaged Navier–Stokes coastal ocean model in resolving the inertial sub-range turbulence scales. However, studies also demonstrated that turbulence properties, such as turbulence intensity and turbulence kinetic energy, simulated by the coastal ocean models based on turbulence closures can be useful in assisting tidal resource characterization at tidal energy sites. In this study, we evaluated four General Ocean Turbulence Model (GOTM) turbulence closure models implemented in a tidal hydrodynamic model –Finite Volume Community Ocean Model (FVCOM) to characterize the tidal energy resource in the Western Passage, Maine, USA – a top ranked tidal energy site. The four turbulence closure models used in this analysis are k–kl (Mellor–Yamada Level 2.5), k-ε, k–ω, and a generic model. Model sensitivities showed that the MY2.5 model performed the best in comparison with the field measurements. In particular, the simulated time series of turbulence intensity and kinetic energy matched the observed data very well in the Western Passage. Detailed analysis was conducted to characterize turbulence properties on the horizontal plane and at selective cross-sections. This study demonstrated that turbulence properties simulated by a coastal ocean model, along with tidal hydrodynamic properties, can be very informative for tidal energy resource characterization and project site selection, as recommended by the International Electrotechnical Commission (IEC) technical specifications (IEC TS 62600-201).

Study on the Impact of the Coastline Changes on Hydrodynamics in Xiangshan Bay

Coastline changes have significant impacts on coastal hydrodynamics. Xiangshan Bay is a semi-closed and long-narrow bay located in Zhejiang Province, China. Its coastline has changed dramatically in recent decades; however, the variations in the small-scale hydrodynamics in the changed coastline areas have not been carefully studied. This study uses the Finite-Volume Community Ocean Model (FVCOM) to design a set of control experiments and five sets of compared experiments targeting the areas with significant coastline changes in Xiangshan Bay over the past 21 years. It was found that the coastline changes at the mouth of the bay, such as areas near Meishan Island and Dasong, have a significant impact on both residual currents and tidal currents, changing the amplitudes and phase distributions of the tides. Coastline changes in the inner bay have lesser impacts on hydrodynamics, mainly affecting the small-scale areas in the vicinity.

Simulating flood events at the Twin Ports of Duluth-Superior using a linked hydrologic-hydrodynamic framework

Generally, ports in the North American Great Lakes are not supported with navigational guidance (water level, water temperature, currents, ice) by NOAA’s Great Lakes Operational Forecast System (GLOFS). To examine the benefit of extending model coverage to this critical infrastructure, a linked hydrologic-hydrodynamic framework was developed for the Twin Ports of Duluth-Superior in western Lake Superior, and tested over three case studies of flooding due to storm surge and/or river flooding. Streamflow from 22 National Water Model (NWM) simulated and 3 gauged inflows within the domain was injected into a hydrodynamic model built on the Finite Volume Community Ocean Model (FVCOM), with a wet-dry grid that covered the harbor and surrounding floodplain. Model results from flood simulations compared well against time-series of water level and streamflow at gauges within the harbor, with root mean square error (RMSE) and bias values small relative to the typical fluctuations. Inclusion of NWM-simulated tributaries improved the accuracy of modeled water levels in the harbor, and increased simulated current speeds in navigational channels by up to 0.5 ms−1. Modeled inundation extent in the floodplain closely matched flood extent surveys conducted in response to a record flood event in 2012, demonstrating the capability of the modeling framework to provide flood guidance in the complex coastal setting.

Data assimilation system for the finite-volume community ocean model based on a localized weighted ensemble Kalman filter

The unstructured-grid finite-volume community ocean model (FVCOM) offers the potential to build a high-resolution operational prediction system because it can effectively adapt to complex boundaries and flexibly perform local mesh encryption. A matching assimilation system is an indispensable part of an operational forecast system, while the existing assimilation method of the FVCOM [e.g., the ensemble Kalman filter (EnKF)] has difficulty accommodating strong nonlinear model/observation operators and non-Gaussian errors. The localized weighted EnKF (LWEnKF) is a particle filter (PF) that combines the advantages of the EnKF and the PF, which can effectively address nonlinearity and non-Gaussianity and can provide a more accurate initial field for models. An observation preprocessing system and a new data assimilation system based on the LWEnKF for the FVCOM are developed. For the preprocessing system, a K-dimensional tree-based observation thinning method is proposed that can accommodate representative observation efficiently for unstructured grids. Through a 1-month global ocean data assimilation experiment, the effect of our assimilation system is compared with the EnKF and the Mercator analysis forecast product, which comes from a reduced-order Kalman filter system. The statistical verification includes five metrics for various sea surface fields, temperature, and salinity profiles. The results show that the system performs better than the EnKF, although there is still a gap with Mercator in some aspects.