The Scottish Shelf Model. Part 1: Shelf-Wide Domain

2 Available data for model development

2.1 Introduction

In order to carry out the numerical modelling works for the Scottish Shelf Waters ( SSW), the following data were collated and/or identified:

• Bathymetry data, required for creating the bathymetry for the numerical model.
• Forcing data, required for specifying the forcing conditions in the numerical flow models.
• Calibration and validation data, required for calibrating and validating the numerical models.

This section of the report describes the data collated/identified for the Scottish Shelf Waters ( SSW) model area. Where appropriate reference is made to the overall project data review report (Halcrow, 2012). Note that the proposed model domains shown in this section are not the final model domains but an approximation.

2.2 Bathymetric Data

2.2.1 Coastline Data

Two coastline data sets were obtained for use in this study: the Global Self-consistent, Hierarchical, High-resolution Shoreline ( GSHHS) distributed by National Geophysical Data Centre ( NGDC) in the US, and Ordnance Survey Mapping. Some reference has also been made to coastline data from the US National Geophysical Data Centre, Marine Geology & Geophysics Division ( MGGD), https://www.ngdc.noaa.gov/mgg/ for the whole NW European Shelf model.

The GSHHS coastline comes in different resolutions. For the UK, the best resolution available is the World Vector Shoreline ( WVS), designed to be used at a resolution of 1:250,000. The GSHHS coastlines have been data processed to ensure they are free of internal inconsistencies such as erratic points and crossing segments.

The Ordnance Survey ( OS) Vector Map District contains tidal boundary polylines, which are at Mean High Water Spring level ( MHWS) in Scotland and MHW in England and Wales. These are at higher spatial resolution than the GSHHS shoreline dataset. The GSHHS data are considered appropriate for use in areas where the model resolution is coarse, the OS vector map district MHWS line should be used in areas of higher resolution, such as for the Pentland Firth and Orkney Waters.

2.2.2 Global/Regional Gridded Data Sets

Three existing coarse resolution bathymetry data sets were identified which cover the study area: the GEBCO_08, the ETOPO-1 grid and the EMODnet grid. These are described briefly below. Details regarding these datasets are provided in Halcrow (2012).

2.2.2.1 General Bathymetric Chart of the Oceans ( GEBCO)

The GEBCO_08 data set is a global DTM at 0.5 minute resolution generated from a database of bathymetric soundings with interpolation between soundings guided by satellite-derived gravity data. The dataset is produced by GEBCO ( http://www.gebco.net, note that a new version has come out in 2014, but too late for use in setting up the models).

Known errors or discontinuities in the data set occur between regions where data is derived from satellite data and detailed bathymetric survey - this is evident in a grid pattern in the Southern North Sea Region, and a discontinuity at 0°E. Marine Scotland has highlighted errors where false banks occur on the shelf around the Shetland Island (Hughes, 2014).

Figure 2-1 shows the NOOS bathymetry ( see section 2.2.4 below). Figure 2-2 shows the GEBCO_08 bathymetry for the NW European Shelf and the source of the data. The discontinuity at 0°E and the grid pattern in the North Sea are clearly visible.

2.2.2.2 ETOPO-1

ETOPO-1 is a global DTM at 1 minute resolution produced by NOAA National Geophysical Data Center. The documentation states that this uses the GEBCO_08 data set for the British Shelf. Due to the lower resolution this dataset was not considered further.

2.2.2.3 European Marine Observation and Data Network ( EMODnet)

The European Marine Observation and Data Network ( EMODnet) have produced DTMs for the Greater North Sea and Celtic Seas at 0.25 minute resolution (about 250m east-west direction and 450m north-south directions). The grids are based on bathymetric surveys and terrain models developed by external data providers including the UK Hydrographic Office ( UKHO), and the GEBCO_08 Grid 0.5 minute resolution dataset where no other data is available. Data sets are made available through the EMODnet website http://www.emodnet-hydrography.eu/

Further details of EMODNet are provided in Halcrow (2012).

Figure 2-3 a and b show where UK Hydrographic Office data has been incorporated into the EMODnet dataset and the differences between the EMODnet and GEBCO_08 bathymetry. Comparison of the EMODnet and GEBCO_08 data sets shows significant differences where the data from the UKHO and other hydrographic offices have been included. Differences are generally greater in areas where the GEBCO_08 has been interpolated, and the UKHO data has been used in the EMODnet bathymetry, for example around 1.5°W 56.3°N, due east of the Firth of Tay. The large differences west of Norway are due to incorporation of Norwegian hydrographic office data. There are also differences north west of the British Shelf around Iceland, where the EMODnet data is sourced from the GEBCO_08 grid. However these were not investigated as they were not considered important for the study area.

Due to the inclusion of the majority of the UKHO data, the EMODnet bathymetry is considered appropriate for use as the base bathymetry for model construction in areas where the resolution will be of the order of one kilometre. Higher resolution bathymetry data are however required in areas where the model mesh is finer to represent bed or flow features. Therefore other datasets were required as described below.

2.2.3 Hydrographic Data

Three sources of hydrographic survey data were identified: the United Kingdom Hydrographic Office ( UKHO), the International Council for Exploration of the Sea ( ICES) and Marine Scotland's data sets.

The UKHO has a memorandum of understanding with Marine Scotland making their high resolution bathymetric survey data available. Most of these data have already been incorporated into the EMODnet bathymetry, however further data have since become available. The location of the UKHO data is shown in Figure 2-3a. Marine Scotland has carried out recent bathymetric surveys for the Pentland Firth which are not listed in the UK Hydrographic office data sets.

The ICES surface dataset holds over 100 years of ship based observations, including soundings. There are over 2 million data points in the ICES data set within the study area, providing a good coverage over most areas. The ICES website ( http://ocean.ices.dk/) states that data are quality controlled by the contributing organisations and visually inspected by experienced staff to further improve the quality of these data. However it is expected that due to the age of some of the sounding data, and the differences in measurement methods, data logging and processing, that there may be significant differences or scatter between the soundings. Marine Scotland used the ICES dataset to identify and correct anomalies in the GEBCO_08 data set off the coast of Shetland. See Halcrow (2012) for more detail regarding hydrographic data and the differences observed between datasets.

2.2.4 NOOS 1.0

NOOS 1.0: A gridded dataset for the UK continental shelf at 1 arc-minute resolution was produced under the aegis of NOOS (an operational oceanography organisation for the NW European Shelf (see Halcrow, 2012 for more information). The NOOS bathymetry incorporates local datasets made available by oceanographic institutions in countries around the North Sea, however no detailed source attribution information is available for the bathymetry, and it was last revised in 2004. Bathymetric survey collected by the UKHO post-2004 is therefore not incorporated into the bathymetry, and it is uncertain to what extent earlier UKHO and other national hydrographic office datasets were incorporated.

The NOOS bathymetry, as gridded in the NOC-L high resolution continental shelf model (1.5 minute by 1 minute resolution, Figure 2-1), was compared with ICES ship track soundings and the EMODnet bathymetry. The NOOS bathymetry does not have a discontinuity in the North Sea at 0°E and is more consistent with the ICES ship track soundings than the EMODnet bathymetry east of 0°E. The false islands in the EMODnet and GEBCO bathymetry east and north east of Shetland are not present in the NOOS bathymetry. It is therefore considered more appropriate to use the NOOS bathymetry than the EMODnet bathymetry for the PFOW and the shelf model in the North Sea east of 0°E, except in areas where it is known that UKHO data has been incorporated into the EMODnet bathymetry. Where UKHO data has been incorporated into the EMODnet bathymetry the difference between the EMODnet bathymetry and the ICES shiptrack soundings is less than for the NOOS bathymetry.

As the EMODnet bathymetry is also at higher resolution it is not considered appropriate to use the NOOS bathymetry east of 0°E where it is known than hydrographic office data has been incorporated into the EMODnet bathymetry.

However, south east of Shetland (0.1°W,59.6°N to 0.2°E 60°N ) the NOOS bathymetry is shallower than the EMODnet bathymetry and less consistent with the ICES ship track soundings, and comparison with chart data is needed in this region. Differences between the NOOS bathymetry and the ICES ship track soundings are also larger than for the EMODnet bathymetry for the west of Scotland, including the Inner and Outer Hebrides. It was therefore not considered appropriate to use the NOOS bathymetry west of 0°E.

2.2.5 Other data sources

Other identified data sources include digital Admiralty charts ( C-MAP) and SeaZone. However, these datasets were not used for this study due to licensing restrictions as discussed fully in Halcrow (2012). A licence enabling Halcrow to digitise the required Admiralty Charts was obtained from the Hydrographic Office and the digitising undertaken. This allows the data to be used into the future for this project without paying a licence fee every year. The digitised Admiralty Charts were used to fill the gaps in the digital bathymetry data available for the PFOW model.

2.2.6 Summary of bathymetry data availability for Scottish Shelf Waters

This section summarises the availability of bathymetry data for the SSW area.

High resolution bathymetric data is available, for most of the core study area of the Pentland Firth and Orkney Islands. EMODnet will form the base bathymetry for SSW with the NOOS data providing depths to the east of longitude 0°E.

To summarise, there appears generally to be sufficient bathymetry data in the open water areas, however there is limited data in the channels within the islands of Orkney and Shetland as well as in the shallow areas of these islands. These gaps were filled with data obtained by digitising the appropriate Admiralty Charts (after first obtaining a licence to do so from the Hydrographic Office). The final bathymetry for the SSW area was derived as a sub-sampled composite from various data sources, to harmonise with the high resolution case study areas.

2.3 Forcing Data

2.3.1 Introduction

Forcing data were required for a one year climatological model run of the SSW flow model and for calibration using observed data for approximate 1 month periods. The following forcing data were required;

• meteorological - including wind speed/stress, atmospheric pressure, surface heat flux, precipitation and evaporation
• hydrological - river flux
• oceanic open boundaries - including temperature, salinity and velocity
• tides

2.3.2 Meteorological forcing

2.3.2.1 UK Met Office Unified Model ( UM) Data

Two data streams from the Met Office forecast models have been archived at NOC (Liverpool) for operational modelling:

• for operational tide-surge modelling on the continental shelf, using the 2D tide-surge model ( CS3 and CS3X).
  • These data comprise of surface wind and atmospheric pressure only, at 1-h intervals, from May 1991 to present. From 1991 to 1995 the data is at 50 km resolution, post 1995 the data is at 12 km resolution.
• for the Irish Sea Observatory operational modelling system, running the 3D baroclinic hydrodynamic model, POLCOMS, on (i) the Atlantic Margin Model ( AMM, ~12km) and (ii) the nested Irish Sea model ( IRS, ~2km). The data comprise the following, from 2004 to 2007 with some gaps, and continuously from 2007 to 2011, all at 12 km resolution:
  • Global model output for the Atlantic at 6-hour intervals - 10m wind (E and N components); sea level pressure; low, medium and high level cloud coverage; specific humidity at 1.5m, air temperature at 1.5m; total accumulated precipitation; sensible heat flux
  • Mesoscale ( NAE) model output at 3-hour intervals - same variables
• Access to the UM model outputs was obtained via the BADC website

2.3.2.2 Climatological Forcing

Climatological forcing was derived from the ERA40 and ERA-Interim datasets, which were used to force the POLCOMS AMM (~12km) model for the 45 year hindcast (1960-2004), see Wakelin et al. (2012) and Holt et al., (2012). A licence to use these data was provided by the European Centre for Medium range Weather Forecasting ( ECMWF) for this study.

A one-year climatological forcing dataset for the temperature and salinity ( i.e. heat flux and precipitation) has been derived. Further work would be necessary to define a typical year in terms of the high-frequency wind forcing i.e. storm climatology, see e.g. Wolf and Woolf (2006). A detailed description of the methodology used to derive the climatology forcing is provided in Section 5 of this report.

2.3.3 Meteorological observations

The Marine Scotland Science survey vessel MV Scotia undertook two surveys for this project, one in St Magnus Bay, Shetland (October 2012) and the other in the Hoy Sound, Orkney (Dec, 2012). During these surveys wind measurements were made from the vessel.

2.3.4 Hydrological Data (Fresh Water Inflows)

In order to simulate the effect that river flow has upon salinity in coastal waters, river flux data are required. The Centre for Ecology and Hydrology ( CEH) Grid-to-Grid ( G2G) model was used to supply freshwater inflows to the various coastal models for this study. The G2G model was extended to provide conditions for the Shetland Isles which were not available previously.

The outputs provided by CEH from the G2G model were:

1. Provision of river discharge data (time series data) at all coastal locations in Scottish waters from the G2G model. The data were supplied for a period covering 1 March 2007 to 30 September 2010 at 15 minute intervals.

2. Provision of river discharge data (time series data) at all coastal locations around Shetland and Northern Ireland from the G2G model. The data were supplied for a period covering 1 March 2007 to 30 September 2010.

3. Provision of river discharge climatological data (long term daily/seasonal discharge data) at all coastal locations for Scotland (including Shetland) and Northern Ireland from the G2G model. Daily averaged data were supplied for a period covering 1962-2011.

2.3.5 Tide

For the Shelf Model, the boundary data were derived from the NOC-L Atlantic Margin Model ( AMM) with a 12km resolution, which was also used to force the NOC-L HRCS 1.8km model. Water levels along with temperature and salinity time-series were applied at the model boundaries for specific periods coincident with times for which calibration data were available.

Climatological runs were forced using results from the POLCOMS model hindcast from 1960-2004, which was run on the AMM 12km grid. These are available for monthly means but also held in-house at NOC-L as daily mean 3D temperature and salinity and current residual fields, together with hourly barotropic currents and elevations.

2.4 Validation and Calibration Data

2.4.1 Introduction

Model validation and calibration were undertaken against observation datasets for periods of up to 1 month. Calibration is required for water level, currents, temperature and salinity.

In addition calibration was required for the 1 year climatological runs against accepted general flow characteristics including current speed and direction (seasonal variability) and seasonal temperature and salinity cycles.

2.4.2 Water Level

Figure 2-4 shows all the locations of water level observations that are available in the PFOW region. These come from three main sources: tide gauge data from the BODC National Oceanographic Database ( NODB); bottom pressure data from the NODB, analysed tidal data from NOC and tide gauge data from SEPA. All of the SEPA gauges (except Rothesay, which ends on 17 ^th April 2007) have data between 2009 and 2012; most go back to 2002. Their locations are shown in Figure 2-5.

In addition, we have access to tidal data from TotalTide - a digital version of the UK Admiralty tide tables, from the UK Hydrographic Office. The locations of these datasets are shown in Figure 2-6. These data are based on harmonic analyses, and thus water level estimates for any past or future date are obtainable from these, or via the use of constituents from the Admiralty tide tables.

2.4.3 Currents

Datasets of currents have been found from a number of sources; all locations are shown in Figure 2-7. These come from the BODC National Oceanographic Database ( NODB) and the TotalTide software, from the UK Hydrographic Office. As Figure 2-8 shows, there are only a few datasets from the BODC National Oceanographic Database since the year 2000. In addition, some of these datasets (shown in red) may not be freely available. In some cases, vertical current profiles are available; these are shown in Figure 2-9.

In the Pentland Firth, interest in tidal energy has led to the existence of other datasets. Baston and Harris (2011) presented results from Acoustic Doppler Current Profiler ( ADCP) data collected in 2001. Also the Environmental Research Institute collected current data via ADCP in the Pentland Firth in 2009. These datasets have been obtained for calibration purposes.

The methodology used by TotalTide for calculating currents is not known. In addition, these data have been estimated for the use of shipping; therefore, a greater weighting may be placed on surface currents than currents near the sea bed.

The Atlas of UK Marine Renewable Energy Resources ( www.renewables-atlas.info) contains information on peak tidal current speeds over a mean spring and a mean neap tide. The dataset was derived from the POL HRCS Model, with peak spring and neap current speeds calculated from the major 2 or 4 tidal harmonics. Although this dataset is limited, it is freely available on a 0.0167° x 0.025° (latitude x longitude) grid throughout the region shown in Figure 2-10.

2.4.4 Temperature and Salinity

Temperature and salinity validation and calibration has been carried out using selected hydrographic stations which were identified from the British Oceanographic Data Centre data holdings for the UK. There are a very large number of datasets from CTD and bottle casts, both from the BODC National Oceanographic Database and the ICES database. Additionally, some of the CEFAS WaveNet buoys record sea surface temperature.

Figure 2-11 shows the locations of the temperature observations and Figure 2-12 shows the locations of the salinity observations. As Figure 2-13 shows, the temperature and salinity observations have occurred throughout the last two decades, with many observations throughout all model domains having occurred over the last two years. Figure 2-14 shows which of these observations include profiles over the entire water depth. Most temperature and salinity observations occurred at the same location and time.

In addition, the Ocean Data analYsis System for SEA ( ODYSSEA) dataset is a re-analysis of satellite observations of sea surface temperature. Daily mean average sea surface temperatures since 01/10/2007 have been obtained, on a 0.1° x 0.1° grid.

A 3-month calibration period for the baroclinic model was selected based on data availability and quality.

The results from the climatology run have been compared with climatological atlas information for temperature and salinity from the World Ocean Atlas ( WOA) and International Council for Exploration of the Seas ( ICES) climatological datasets.

2.4.5 Summary of data availability for the SSW model

This section summarises the availability of calibration and validation data for the SSW model area and identifies any gaps in the available data. Furthermore, recommendations are made on how to fill the gaps.

Table 2‑1 summarises the available water level, current, temperature/salinity and meteorological/river flow data available for calibration of the SSW model.

Table 2‑1 SSW model and available data

Model	Year	Water level	Currents	Temperature/salinity	Meteorological	Wind	River
Scottish Shelf Waters	2001	√	√	√	X	√	X
	2009	√	√	√	√	√	√
	2012	√	√	√	X	√	X

It can be seen that the year 2009 appears to be the period where a complete set of the required data is available for calibration. The 3-month period of April-June 2009 has been used for calibration with other periods 2008-2009 being used for validation.

2.5 Conclusions and Recommendations

A review has been undertaken to identify and in many cases request / obtain data that are relevant to the setting up, forcing and calibration of the SSW model. It was found that there are many datasets available providing coverage over a wide spatial and temporal extent.

2.5.1 Bathymetry

The EMODnet data are considered appropriate for use as the base bathymetry for model construction. These data formed the basic coarser resolution data for the SSW model.

Further UKHO data and other higher resolution datasets from ICES and Marine Scotland were used to replace the coarser resolution data in areas that they overlap, with appropriate checks for consistency. However even with these data there are areas which were identified in the data review report (Halcrow, 2012) as not having sufficient bathymetry data at a fine enough resolution. In this case data from digitised Admiralty Charts have been used, for the high resolution models.

2.5.2 Forcing data

Temperature and salinity data have been obtained from the NOC-L AMM mode to provide boundary conditions to the SSW model. The AMM model was initially used for the tidal forcing at the open boundary, however it became necessary to move the boundary into the Atlantic and the alternative method of using the TPXO7.2 global tidal inversion from TOPEX/Poseidon altimeter data was then used (Oregon State University, Egbert and Erofeeva, 2002).

Meteorological forcing for the SSW model has been derived from the Met Office model data that NOC-L holds. The Met Office data provides wind data from 1991 to present day, however other parameters such as sea level pressure, low, medium and high level cloud coverage, specific humidity at 1.5m, air temperature at 1.5m, total accumulated precipitation and sensible heat flux are only available from 2007 to 2011. This therefore limited the periods where calibration data are available coincident with full meteorological forcing. Due to the lack of full meteorological forcing during many of the potential calibration periods all calibration runs will be during 2009 , although no current measurements are available for this period harmonic analysis of the results can be carried out for comparison with observed data.

Extra UM fields were downloaded from the BADC website.

Fluvial inputs have been derived from G2G river flow data obtained from CEH for the SSW area.

2.5.3 Validation and Calibration Data

Section 2.4.5 presents information about which data are available for the SSW model. In general there are sufficient data with which to undertake calibration for water levels, currents, temperature and salinity. A summary of the dates where suitable calibration data is available is provided in Table 2‑1.