Analysis of Infragravity Frequency Sediment Transport on Macrotidal Beaches

Many cross-shore sediment transport models use simple treatments of infragravity frequency (0.005- 0.05Hz) processes. For example, infragravity waves have been assumed to provide solely a 'drift velocity' for transport of sediment mobilised by incident frequency waves (0.05-0.5Hz) and be 100% reflected at the shoreline. Furthermore, numerous models calculate broken incident wave heights on the basis of water depth only. This work investigates both the processes underlying infragravity frequency variations in the crossshore velocity field, and the resulting effect of such variations on sediment suspension and transport. Data were selected from three beach experiments in order to compare observations from a range of energetic conditions and positions in the nearshore. Experiments conducted on a dissipative beach at Llangennith, and an intermediate beach at Spurn Head, form part of the pre-existing British Beach And Nearshore Dynamics dataset. The third deployment, at a dissipative site at Perranporth (Cornwall), provided new data for analysis. At Llangennith, high swell waves (significant wave height 3m) were observed, and the measurements come from an infragravity wave dominated saturated surf zone. At Perranponh, locally generated wind wave heights were 2m and measurements came from an incident wave dominated saturated surf zone. Conditions at Spurn Head saw swell wave heights of 1.5m, and observations were made in both an incident wave dominated non-saturated surf zone and the incident wave shoaling zone. Analysis of the data revealed that, in the surf zone, the nature of the infragravity wave field was dependent upon the distribution of energy between higher (>0.02Hz) and lower (<0.02Hz) infragraviiy frequencies. Lower frequency infragravity waves were found to shoal as free waves, while higher frequency infragravity waves were dissipated near to shore on low gradient beaches. Inftagravity wave reflection coefficients showed a dependence on frequency and beach slope (parameterised by the Iribarren number), varied between 50-90% for lower infragravity frequencies, and could be less than 50% for higher infragraviiy frequencies. Incident wave heights were modulated in the shoaling zone with a 'groupy' form. Modulation was also observed in the surf zone, but in the form of individual large waves occurring at low frequency. In the shoaling zone and very close to shore, non-linear interactions occurred between the incident and infragravity components, and calculated phase values between modulated incident waves and infragravity waves indicated a phase shift from a value of less than 180° in the shoaling zone toward 0° close to shore. However, the two signals were not significantly correlated for much of the surf zone. High velocities resulting from a combination of the mean, infragravity and incident wave components drove sediment suspension. Large suspension events occurring at infragravity frequencies were correlated with incident wave groupiness in the shoaling zone, and in high energy conditions with infragravity waves near to the swash zone. Such variations in suspension were related not only to velocity magnitude, but the duration for which a threshold for suspension was exceeded. The bed response to forcing also varied during a tide, possibly as a result of changing bed conditions (e.g. due to bedforms). The infragravity contribution to suspension was independent of the magnitude of suspended sediment concentration, and increased from approximately 30% at the breaker line to 90% in an infragravity wave dominated inner surf zone. The contribution of the infragravity component to transport did not show a similar behaviour, due to phase effects, which produced a reversal in the transport direction between higher and lower infragravity frequencies. Comparison of the observations of sediment transport with energetics predictors identified several cases where the observed transport was qualitatively different from the model prediction as a result of sediment transport thresholds being exceeded at, or for, infragravity timescales

Quantification of the uncertainty in coastal storm hazard predictions due to wave‐current interaction and wind forcing

Coastal flood warning and design of coastal protection schemes rely on accurate estimations of water level and waves during hurricanes and violent storms. These estimations frequently use numerical models, which, for computational reasons, neglect the interaction between the hydrodynamic and wave fields. Here, we show that neglecting such interactions, or local effects of atmospheric forcing, causes large uncertainties, which could have financial and operational consequences because flood warnings are potentially missed or protection schemes underdesigned. Using the Severn Estuary, SW England, we show that exclusion of locally generated winds underestimates high water significant wave height by up to 90.1%, high water level by 1.5%, and hazard proxy (water level + 1/2 significant wave height) by 9.1%. The uncertainty in water level and waves is quantified using a system to model tide‐surge‐wave conditions, Delft3D‐FLOW‐WAVE in a series of eight model simulations for four historic storm events

Wave-tide interaction modulates nearshore wave height

The combined hazard of large waves occurring at an extreme high water could increase the risk of coastal flooding. Wave-tide interaction processes are known to modulate the wave climate in regions of strong tidal dynamics, yet this process is typically omitted in flood risk assessments. Here, we investigate the role of tidal dynamics in the nearshore wave climate (i.e. water depths > 10 m), with the hypothesis that larger waves occur during high water, when the risk of flooding is greater, because tidal dynamics alter the wave climate propagating into the coast. A dynamically coupled wave-tide model “COAWST” was applied to the Irish Sea for a 2-month period (January–February 2014). High water wave heights were simulated to be 20% larger in some regions, compared with an uncoupled approach, with clear implications for coastal hazards. Three model spatial resolutions were applied (1/60°, 1/120°, 1/240°), and, although all models displayed similar validation statistics, differences in the simulated tidal modulation of wave height were found (up to a 10% difference in high water wave height); therefore, sub-kilometre-scale model resolution is necessary to capture tidal flow variability and wave-tide interactions around the coast. Additionally, the effects of predicted mean sea-level rise were investigated (0.44–2.00 m to reflect likely and extreme sea-level rise by the end of the twenty-first century), showing a 5% increase in high water wave height in some areas. Therefore, some regions may experience a future increase in the combined hazard of large waves occurring at an extreme high water

Radiational tides: their double-counting in storm surge forecasts and contribution to the Highest Astronomical Tide

Tide predictions based on tide-gauge observations are not just the astronomical tides; they also contain radiational tides – periodic sea-level changes due to atmospheric conditions and solar forcing. This poses a problem of double-counting for operational forecasts of total water level during storm surges. In some surge forecasting, a regional model is run in two modes: tide only, with astronomic forcing alone; and tide and surge, forced additionally by surface winds and pressure. The surge residual is defined to be the difference between these configurations and is added to the local harmonic predictions from gauges. Here we use the Global Tide and Surge Model (GTSM) based on Delft-FM to investigate this in the UK and elsewhere, quantifying the weather-related tides that may be double-counted in operational forecasts. We show that the global S2 atmospheric tide is captured by the tide-and-surge model and observe changes in other major constituents, including M2. The Lowest and Highest Astronomical Tide levels, used in navigation datums and design heights, are derived from tide predictions based on observations. We use our findings on radiational tides to quantify the extent to which these levels may contain weather-related components

The UKC3 regional coupled environmental prediction system

This paper describes an updated configuration of the regional coupled research system, termed UKC3, developed and evaluated under the UK Environmental Prediction collaboration. This represents a further step towards a vision of simulating the numerous interactions and feedbacks between different physical and biogeochemical components of the environment across sky, sea and land using more integrated regional coupled prediction systems at kilometre-scale resolution. The UKC3 coupled system incorporates models of the atmosphere (Met Office Unified Model), land surface with river routing (JULES), shelf-sea ocean (NEMO) and ocean surface waves (WAVEWATCH III®), coupled together using OASIS3-MCT libraries. The major update introduced since the UKC2 configuration is an explicit representation of wave–ocean feedbacks through introduction of wave-to-ocean coupling. Ocean model results demonstrate that wave coupling, in particular representing the wave-modified surface drag, has a small but positive improvement on the agreement between simulated sea surface temperatures and in situ observations, relative to simulations without wave feedbacks. Other incremental developments to the coupled modelling capability introduced since the UKC2 configuration are also detailed. Coupled regional prediction systems are of interest for applications across a range of timescales, from hours to decades ahead. The first results from four simulation experiments, each of the order of 1 month in duration, are analysed and discussed in the context of characterizing the potential benefits of coupled prediction on forecast skill. Results across atmosphere, ocean and wave components are shown to be stable over time periods of weeks. The coupled approach shows notable improvements in surface temperature, wave state (in near-coastal regions) and wind speed over the sea, whereas the prediction quality of other quantities shows no significant improvement or degradation relative to the equivalent uncoupled control simulations

Can wave coupling improve operational regional ocean forecasts for the north-west European Shelf?

Operational ocean forecasts are typically produced by modelling systems run using a forced mode approach. The evolution of the ocean state is not directly influenced by surface waves, and the ocean dynamics are driven by an external source of meteorological data which are independent of the ocean state. Model coupling provides one approach to increase the extent to which ocean forecast systems can represent the interactions and feedbacks between ocean, waves, and the atmosphere seen in nature. This paper demonstrates the impact of improving how the effect of waves on the momentum exchange across the ocean–atmosphere interface is represented through ocean–wave coupling on the performance of an operational regional ocean prediction system. This study focuses on the eddy-resolving (1.5 km resolution) Atlantic Margin Model (AMM15) ocean model configuration for the north-west European Shelf (NWS) region. A series of 2-year duration forecast trials of the Copernicus Marine Environment Monitoring Service (CMEMS) north-west European Shelf regional ocean prediction system are analysed. The impact of including ocean–wave feedbacks via dynamic coupling on the simulated ocean is discussed. The main interactions included are the modification of surface stress by wave growth and dissipation, Stokes–Coriolis forcing, and wave-height-dependent ocean surface roughness. Given the relevance to operational forecasting, trials with and without ocean data assimilation are considered. Summary forecast metrics demonstrate that the ocean–wave coupled system is a viable evolution for future operational implementation. When results are considered in more depth, wave coupling was found to result in an annual cycle of relatively warmer winter and cooler summer sea surface temperatures for seasonally stratified regions of the NWS. This is driven by enhanced mixing due to waves, and a deepening of the ocean mixed layer during summer. The impact of wave coupling is shown to be reduced within the mixed layer with assimilation of ocean observations. Evaluation of salinity and ocean currents against profile measurements in the German Bight demonstrates improved simulation with wave coupling relative to control simulations. Further, evidence is provided of improvement to simulation of extremes of sea surface height anomalies relative to coastal tide gauges

The UKC2 regional coupled environmental prediction system

It is hypothesized that more accurate prediction and warning of natural hazards, such as of the impacts of severe weather mediated through various components of the environment, require a more integrated Earth System approach to forecasting. This hypothesis can be explored using regional coupled prediction systems, in which the known interactions and feedbacks between different physical and biogeochemical components of the environment across sky, sea and land can be simulated. Such systems are becoming increasingly common research tools. This paper describes the development of the UKC2 regional coupled research system, which has been delivered under the UK Environmental Prediction Prototype project. This provides the first implementation of an atmosphere–land–ocean–wave modelling system focussed on the United Kingdom and surrounding seas at km-scale resolution. The UKC2 coupled system incorporates models of the atmosphere (Met Office Unified Model), land surface with river routing (JULES), shelf-sea ocean (NEMO) and ocean waves (WAVEWATCH III). These components are coupled, via OASIS3-MCT libraries, at unprecedentedly high resolution across the UK within a north-western European regional domain. A research framework has been established to explore the representation of feedback processes in coupled and uncoupled modes, providing a new research tool for UK environmental science. This paper documents the technical design and implementation of UKC2, along with the associated evaluation framework. An analysis of new results comparing the output of the coupled UKC2 system with relevant forced control simulations for six contrasting case studies of 5-day duration is presented. Results demonstrate that performance can be achieved with the UKC2 system that is at least comparable to its component control simulations. For some cases, improvements in air temperature, sea surface temperature, wind speed, significant wave height and mean wave period highlight the potential benefits of coupling between environmental model components. Results also illustrate that the coupling itself is not sufficient to address all known model issues. Priorities for future development of the UK Environmental Prediction framework and component systems are discussed

The Regional Coupled Suite (RCS-IND1): application of a flexible regional coupled modelling framework to the Indian region at kilometre scale

A new regional coupled modelling framework is introduced – the Regional Coupled Suite (RCS). This provides a flexible research capability with which to study the interactions between atmosphere, land, ocean, and wave processes resolved at kilometre scale, and the effect of environmental feedbacks on the evolution and impacts of multi-hazard weather events. A configuration of the RCS focussed on the Indian region, termed RCS-IND1, is introduced. RCS-IND1 includes a regional configuration of the Unified Model (UM) atmosphere, directly coupled to the JULES land surface model, on a grid with horizontal spacing of 4.4 km, enabling convection to be explicitly simulated. These are coupled through OASIS3-MCT libraries to 2.2 km grid NEMO ocean and WAVEWATCH III wave model configurations. To examine a potential approach to reduce computation cost and simplify ocean initialization, the RCS includes an alternative approach to couple the atmosphere to a lower resolution Multi-Column K-Profile Parameterization (KPP) for the ocean. Through development of a flexible modelling framework, a variety of fully and partially coupled experiments can be defined, along with traceable uncoupled simulations and options to use external input forcing in place of missing coupled components. This offers a wide scope to researchers designing sensitivity and case study assessments. Case study results are presented and assessed to demonstrate the application of RCS-IND1 to simulate two tropical cyclone cases which developed in the Bay of Bengal, namely Titli in October 2018 and Fani in April 2019. Results show realistic cyclone simulations, and that coupling can improve the cyclone track and produces more realistic intensification than uncoupled simulations for Titli but prevents sufficient intensification for Fani. Atmosphere-only UM regional simulations omit the influence of frictional heating on the boundary layer to prevent cyclone over-intensification. However, it is shown that this term can improve coupled simulations, enabling a more rigorous treatment of the near-surface energy budget to be represented. For these cases, a 1D mixed layer scheme shows similar first-order SST cooling and feedback on the cyclones to a 3D ocean. Nevertheless, the 3D ocean generally shows stronger localized cooling than the 1D ocean. Coupling with the waves has limited feedback on the atmosphere for these cases. Priorities for future model development are discussed

SEASTAR: a mission to study ocean submesoscale dynamics and small-scale atmosphere-ocean processes in coastal, shelf and polar seas

High-resolution satellite images of ocean color and sea surface temperature reveal an abundance of ocean fronts, vortices and filaments at scales below 10 km but measurements of ocean surface dynamics at these scales are rare. There is increasing recognition of the role played by small scale ocean processes in ocean-atmosphere coupling, upper-ocean mixing and ocean vertical transports, with advanced numerical models and in situ observations highlighting fundamental changes in dynamics when scales reach 1 km. Numerous scientific publications highlight the global impact of small oceanic scales on marine ecosystems, operational forecasts and long-term climate projections through strong ageostrophic circulations, large vertical ocean velocities and mixed layer re-stratification. Small-scale processes particularly dominate in coastal, shelf and polar seas where they mediate important exchanges between land, ocean, atmosphere and the cryosphere, e.g., freshwater, pollutants. As numerical models continue to evolve toward finer spatial resolution and increasingly complex coupled atmosphere-wave-ice-ocean systems, modern observing capability lags behind, unable to deliver the high-resolution synoptic measurements of total currents, wind vectors and waves needed to advance understanding, develop better parameterizations and improve model validations, forecasts and projections. SEASTAR is a satellite mission concept that proposes to directly address this critical observational gap with synoptic two-dimensional imaging of total ocean surface current vectors and wind vectors at 1 km resolution and coincident directional wave spectra. Based on major recent advances in squinted along-track Synthetic Aperture Radar interferometry, SEASTAR is an innovative, mature concept with unique demonstrated capabilities, seeking to proceed toward spaceborne implementation within Europe and beyond

Use of probabilistic medium- to long-range weather-pattern forecasts for identifying periods with an increased likelihood of coastal flooding around the UK

A medium- to long-range forecast highlighting periods with an increased likelihood of coastal flooding is useful to United Kingdom governments and response agencies when considering proactive, large-scale and multiregional responses. For this purpose, a new operational forecasting tool called Coastal Decider was developed for use by the Flood Forecasting Centre (FFC). Coastal Decider is used for internal preparedness within the FFC and provides the likelihood of coastal flooding around the UK, which is associated with the occurrence of specific predefined weather patterns in combination with high astronomical tides. Forecasts are available for medium-range and monthly time-scales using output from several ensemble prediction systems, whereby ensemble members are objectively assigned to the closest matching weather pattern definition. Ensemble members already clustered by weather pattern are then grouped to provide objective probabilities of a broader cluster of coastal-risk weather patterns occurring. The coastal-risk weather patterns were objectively derived for 21 coastal sites by relating daily historical weather pattern classifications to a wave hindcast, using a Met Office configuration of WAVEWATCH III, as well as observed skew surges, derived using tide-gauge data from the British Oceanographic Data Centre (BODC). Finally, metrics based on the pressure-anomaly difference between ensemble members and their assigned weather patterns, alongside forecasts of daily maximum 10 m wind speed are used to estimate the magnitude of coastal flooding