Proudman Oceanographic Laboratory Coastal Ocean Modelling System Research Articles

The CO5 configuration of the 7 km Atlantic Margin Model: large-scale biases and sensitivity to forcing, physics options and vertical resolution

Abstract. We describe the physical model component of the standard Coastal Ocean version 5 configuration (CO5) of the European north-west shelf (NWS). CO5 was developed jointly between the Met Office and the National Oceanography Centre. CO5 is designed with the seamless approach in mind, which allows for modelling of multiple timescales for a variety of applications from short-range ocean forecasting to climate projections. The configuration constitutes the basis of the latest update to the ocean and data assimilation components of the Met Office's operational Forecast Ocean Assimilation Model (FOAM) for the NWS. A 30.5-year non-assimilating control hindcast of CO5 was integrated from January 1981 to June 2012. Sensitivity simulations were conducted with reference to the control run. The control run is compared against a previous non-assimilating Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) hindcast of the NWS. The CO5 control hindcast is shown to have much reduced biases compared to POLCOMS. Emphasis in the system description is weighted to updates in CO5 over previous versions. Updates include an increase in vertical resolution, a new vertical coordinate stretching function, the replacement of climatological riverine sources with the pan-European hydrological model E-HYPE, a new Baltic boundary condition and switching from directly imposed atmospheric model boundary fluxes to calculating the fluxes within the model using a bulk formula. Sensitivity tests of the updates are detailed with a view toward attributing observed changes in the new system from the previous system and suggesting future directions of research to further improve the system.

Modeling flocculation in a hypertidal estuary

When fine particles are involved, cohesive properties of sediment can result in flocculation and significantly complicate sediment process studies. We combine data from field observations and state-of-the-art modeling to investigate and predict flocculation processes within a hypertidal estuary. The study site is the Welsh Channel located at the entrance of the Dee Estuary in Liverpool Bay. Field data consist of measurements from a fixed site deployment during 12–22 February 2008. Grain size, suspended sediment volume concentration, and current velocity were obtained hourly from moored instruments at 1.5 m above bed. Near-bottom water samples taken every hour from a research vessel are used to convert volume concentrations to mass concentrations for the moored measurements. We use the hydrodynamic model Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) coupled with the turbulence model General Ocean Turbulence Model (GOTM) and a sediment module to obtain three-dimensional distributions of suspended particulate matter (SPM). Flocculation is identified by changes in grain size. Small flocs were found during flood and ebb periods—and correlate with strong currents—due to breakup, while coarse flocs were present during slack waters because of aggregation. A fractal number of 2.4 is found for the study site. Turbulent stresses and particle settling velocities are estimated and are found to be related via an exponential function. The result is a simple semiempirical formulation for the fall velocity of the particles solely depending on turbulent stresses. The formula is implemented in the full three-dimensional model to represent changes in particle size due to flocculation processes. Predictions from the model are in agreement with observations for both settling velocity and SPM. The SPM fortnight variability was reproduced by the model and the concentration peaks are almost in phase with those from field data.

Modelling temperature and salinity in Liverpool Bay and the Irish Sea: sensitivity to model type and surface forcing

Abstract. Three shelf sea models are compared against observed surface temperature and salinity in Liverpool Bay and the Irish Sea: a 7 km NEMO (Nucleus for European Modelling of the Ocean) model, and 12 km and 1.8 km POLCOMS (Proudman Oceanographic Laboratory Coastal Ocean Modelling System) models. Each model is run with two different surface forcing datasets of different resolutions. Comparisons with a variety of observations from the Liverpool Bay Coastal Observatory show that increasing the surface forcing resolution improves the modelled surface temperature in all the models, in particular reducing the summer warm bias and winter cool bias. The response of surface salinity is more varied with improvements in some areas and deterioration in others. The 7 km NEMO model performs as well as the 1.8 km POLCOMS model when measured by overall skill scores, although the sources of error in the models are different. NEMO is too weakly stratified in Liverpool Bay, whereas POLCOMS is too strongly stratified. The horizontal salinity gradient, which is too strong in POLCOMS, is better reproduced by NEMO which uses a more diffusive horizontal advection scheme. This leads to improved semi-diurnal variability in salinity in NEMO at a mooring site located in the Liverpool Bay ROFI (region of freshwater influence) area.

Remote sensing of seasonal stratification dynamics in the southern Irish Sea

In early summer, a well-defined front forms at the southern boundary of Irish Sea between thermally stratified and tidally mixed waters. The Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) was used to predict the location of this front, and data from the MODIS Aqua sensor was used to derive a five-year time series of red-waveband remote sensing reflectance (Rrs667) and sea surface temperature (SST) for two representative areas on either side of the frontal region. Pronounced annual cycles in Rrs667 and SST were observed at both locations, with summer minima in Rrs667 being roughly coincident with SST maxima. Previous studies have shown that Rrs667 in the Irish Sea is mainly determined by near-surface concentrations of suspended mineral particles. The annual cycles in reflectance therefore indicate that vertical mixing of the water column in winter supplies fine particles to the surface layer which settle out in calmer summer conditions. While the temporal resolution achievable by remote sensing in a single year was limited by the high incidence of cloud cover, monthly averaged data for the whole five-year period revealed differences of the order of one month in the timing of the reflectance cycle on the mixed and stratified sides of the front. This difference produced transient patterns of reflectance in early summer (and occasionally in autumn) in which sharp contrasts coincided with the location of the tidal front predicted by the POLCOMS model. We conclude that optical remote sensing in shelf seas can provide information not only on changing concentrations of optically significant materials, but also on the location and timing of dynamical processes such as seasonal front formation, and note that such information is of potential value for tuning and validating numerical models of shelf sea ecosystems.

Impact of sediment-induced stratification and turbulence closures on sediment transport and morphological modelling

Appropriate descriptions of turbulence are important in predicting sediment transport and seabed evolution. We use here a three-dimensional model to investigate the impacts of sediment-induced stratification, erosion parameterization, and turbulence closures on sediment transport and morphological simulations. The model is implemented in the Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS), which couples a hydrodynamic model to the General Ocean Turbulence Model (GOTM) and to a sediment transport model. This sediment transport model calculates both suspended sediment concentrations and bed evolution. A large number of turbulence closures can be tested and we focus on the methods used to calculate the turbulent length scale and the stability functions. Different modelling scenarios are assessed against experimental data of velocity profiles, suspended sediment concentrations, and trench migration in a laboratory flume. Numerical results show that the processes investigated have little impact on the flow velocity profiles, in spite of some differences on the bed shear stress. The only exception is the one-equation turbulence closure, which cannot reproduce velocity profiles appropriately. Sediment-induced stratification, erosion parameterization, and turbulence closures can all have significant impacts on the suspended sediment concentration and the bed evolution. Model-data comparisons are found to be particularly sensitive to the method used to calculate the turbulent length scale.

Impact assessment of advanced coupling features in a tide–surge–wave model, POLCOMS-WAM, in a shallow water application

Tide–surge–wave interaction is important in determining the nearshore 3D-current profile and the total water elevation. Liverpool Bay, northwest England, is used to assess the impact of coupling the 3rd generation spectral WAve Model (WAM), modified for shallow water, to the Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS), a tide–surge model, through 2-way interactions. Data are readily available from the Irish Sea Observatory to quantify the importance of each coupled term with the aim of producing the most accurate model setup for coastal forecasting. A storm event, 18th January 2007, has been hindcast to investigate extreme tide–surge–wave condition both offshore and inshore. The coupled terms investigated include standard processes already included (bottom friction, surface stress and the Doppler shift of the waves by the depth-averaged current), as well as advanced coupling procedures: use of the 3D current in the wave physics and calculation of radiation stress and Stokes' drift. During storm conditions it is found that the radiation stress is the most important term in this shallow water application as wave-setup increases coastal flood risk. However, WAM runs in near real time, making this model only practical for research purposes. Further work is therefore required to simplify this model setup.

The potential impacts of climate change on the hydrography of the northwest European continental shelf

Changes in global atmospheric conditions have the potential to substantially influence shelf sea environments with far reaching consequences for their ecosystems. Here we focus on the northwest European continental shelf, and review the mechanisms by which climate change might affect the temperature, salinity and stratification of this shelf sea. We explore results from a single pair of Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) simulations forced by the Hadley Centre regional climate model, for conditions typical of 1961–1990 and 2070–2098, under a ‘business as usual’ emissions scenario (SRES A1B). This provides a single, physically plausible, representation of the future and a consistent representation of the recent past. Comparing these simulations, the shelf sea regions of this model are shown to warm substantially more than the open-ocean, by between 1.5 and 4 °C depending on location. Across the whole domain the surface waters are projected to be ∼0.2 p.s.u. fresher by the end of the 21st century. The strength of seasonal stratification is shown to increase by ∼20% on the shelf, compared with 20–50% in the open-ocean. The former being controlled by temperature and the latter by salinity. In shelf seas away from the direct influence of river discharge, stratification is projected to start ∼5 days earlier and breakdown ∼5–10 days later each year, hence extending the stratified period. An ERA-40 re-analysis forced simulation provides a reference, along with validation from gridded monthly mean data from the ICES data base.

A case study of combined wave and water levels under storm conditions using WAM and SWAN in a shallow water application

Extreme storm events in Liverpool Bay, UK, are under investigation. A recent storm, 18th January 2007, has been used to investigate different modelling approaches to accurately simulate wave–surge conditions in shallow water. In Liverpool Bay wave and surge influence and their interaction are often considered in many studies. The tide–surge–wave interaction has been modelled in the Irish Sea using the Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) coupled to the state-of-the-art 3rd generation spectral WAve Model (WAM). Waves are locally generated in Liverpool Bay, while the surge is locally and externally generated. To include the external surge the 1.8 km Irish Sea model has been nested, using a 1-way approach, into the 1/9° by 1/6° operational Continental Shelf surge model. To investigate the performance of POLCOMS–WAM in shallow water a 180 m Liverpool Bay model has been nested within the Irish Sea model. For this high resolution model the state-of-the-art 3rd generation Simulating WAves Nearshore (SWAN) spectral model is used to further develop WAM, modified for shallow water. A POLCOMS–WAM and POLCOMS–SWAN (1-way) coupled model has been implemented to assess the effects of tide–surge–wave interaction at the coast. Investigation of the significant wave height and surge elevation reveals that when modified for shallow water WAM performs as well as SWAN. Two-way coupling between POLCOMS–WAM allows wave-setup to be included in the surge prediction greatly improving the hydrodynamic result. A drawback to using WAM is the computational expense, making SWAN more suitable for simulations exceeding a few days in duration.

An 11-year validation of wave-surge modelling in the Irish Sea, using a nested POLCOMS–WAM modelling system

In the future it is believed that extreme coastal flooding events will increase (in frequency and intensity) as a result of climate change. We are investigating the flood risks in the eastern Irish Sea posed by extreme storm events. Here, an 11-year simulation (01/01/1996–01/01/2007) including wave–current interaction has been validated. These data can then be used to investigate the potential for coastal flooding in the study area. To accurately model a storm event in the eastern Irish Sea both wave effects and the influence of the external surge need to be considered. To simulate the waves, we have set up a one-way nested approach from a 1° North Atlantic model, to a 1.85 km Irish Sea model, using the state-of-the-art 3rd-generation spectral WAve Model (WAM). This allows the influence of swell to be correctly represented. The Proudman Oceanographic Laboratory Coastal-Ocean Modelling System (POLCOMS) has been used to model the tide–surge interaction. To include the external surge we have set up a one-way nested approach from the 1/9° by 1/6° operational Continental Shelf surge model, to a 1.85 km Irish Sea model. For the high resolution Irish Sea model we use a POLCOMS–WAM coupled model, to allow for the effects of wave–current interaction on the prediction of surges at the coast. Using two classification schemes the coupled model is shown to be good and often very good at predicting the surge, total water elevation and wave conditions. We also find the number of low level surge events has increased in the study area over the past decade. However, this time period is too short to determine any long-term trends in the wave and surge levels.

Modelling SPM on the NW European shelf seas

Abstract The Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) has been developed to tackle multidisciplinary studies in coastal/shelf environments. The central core is a sophisticated three-dimensional hydrodynamic model that provides realistic flow fields to interact with, and transport environmental parameters. The model uses realistic forcing with ocean currents and hydrography at the boundary, atmospheric forcing and tides. Suspended particulate matter (SPM) is transported within the water, like any other scalar in the model; it is advected using a high-order scheme and mixed vertically using diffusion coefficients from a second moment turbulence closure. In addition, the SPM transport is influenced by a number of processes unique to this problem, such as: the transfer between the lowest level of the model and the bed layer in which the resuspension and deposition takes place is dependent on the erosion rate, the particle settling velocity and the critical stresses for erosion and deposition. The particular application described in this paper is an annual simulation for the NW European shelf for a single sediment class. The simulation shows the expected seasonal variation in spatial patterns and a clear tidal signal in the SPM resuspension. Also evident is a fortnightly/monthly variability due to the spring-neaps and the M2-N2 cycles. There are also quarter-diurnal and semi-diurnal cycles in SPM, due to tidal resuspension and advection respectively. The above tidal suspension behaviour, as well as, the net transport direction are in agreement with historical observations of SPM.

Modelling the hydrodynamics and ecosystem of the North-West European continental shelf for operational oceanography

This paper outlines an approach to complex spatio-temporal marine ecosystem modelling as applied to the North Western European Continental Shelf. The model presented here combines an eddy-permitting (approximately 6 km horizontal resolution) baroclinic model, the Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS), with the European Regional Seas Ecosystem Model (ERSEM). This has been run within an operational framework using operationally available high resolution atmospheric and lateral boundary forcing, allowing hindcast and near-real time nowcast simulations to be performed. The modelled surface temperature and chlorophyll distributions are presented, and interannual variations discussed. Validation of both the physical and ecosystem submodels show the system to be effective, whilst highlighting areas where improvements in the system can be made. Distinct regional differences in predictive skill are shown. The system presented is ready for operational implementation to provide products and services for use both scientifically and in coastal zone and shelf seas management activities. A programme of work to update the system is already in place.

An assessment of the fine-scale eddies in a high-resolution model of the shelf seas west of Great Britain

High-resolution (∼1.8 km) simulations of the shelf seas west of Great Britain with the Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) demonstrate the formation of an intense field of baroclinic eddies in open stratified waters such as the Celtic Sea. These eddies are most likely to be the result of an inverse energy cascade resulting from tidal flow over rough topography, as demonstrated by a series of idealised model experiments. They are shown to possess many of the properties of eddies identified in idealised laboratory and numerical simulations. Namely, they are predominantly anticyclonic and they merge to increase their size up to a limit set by the internal Rossby radius. An investigation of satellite sea surface temperature observations provides indirect evidence that these model eddies are too energetic, long-lived and frequent. The inclusion of a horizontal diffusion term with a shear dependent diffusivity is shown to significantly reduce the eddy kinetic energy and improve the simulation, without significantly affecting the results in other regions. An optimal parameter value is suggested for this resolution, but the need to put the treatment of horizontal turbulence in such models on a sounder theoretical or empirical footing is identified.