Scottish Shelf Model. Part 4: East Coast of Lewis and Harris Sub-Domain

4 Summary and Conclusions

4.1 Introduction

This report documents the work carried out in developing the East Coast of Lewis and Harris ( ECLH) model. This work includes: data collated for the numerical modelling, setup and calibration of the flow model, and the longer term six month climatological simulation required for this study.

The FVCOM model was chosen because of its capabilities as well as it being freely available, which then fulfils the aim for this and other models developed under the same project to become community models.

4.2 Hydrodynamic model calibration

The ECLH hydrodynamic model was setup using bathymetry taken from a number of sources, from the freely available but coarser EMODnet/ NOOS data, to the UKHO and Marine Scotland higher resolution datasets. Where data from these sources was not readily available, Admiralty Charts were digitised (with permission from the Hydrographic office) to fill in any gaps. All bathymetry was reduced to mean sea level as the common datum.

The model mesh was created with the SMS mesh generator using a spherical coordinate system (latitude and longitude). The model was run with 10 vertical sigma layers with a vertical datum of Mean Sea Level ( MSL).

An analysis of the data available for forcing the hydrodynamic ( HD) model showed that periods in 2009 were the most appropriate providing all of the necessary forcing data required by the model. Datasets for calibration and validation of the model in the form of timeseries of water levels and current speeds were available close to shore and at various locations throughout the model domain. Additionally temperature and salinity profiles were available for comparisons with the model.

Boundary conditions for water levels, depth-averaged currents, temperature and salinity were taken from the Atlantic Margin Model ( AMM) developed by NOC-L. These were applied using a nested boundary approach. Water levels and currents were provided at hourly intervals, whereas the temperature and salinity were provided at daily intervals for each of the 40 layers in the AMM. Meteorological forcing was provided by NOC-L and derived from the Met Office model. The heating input was calculated internally by FVCOM rather than provided externally. This was found to provide the best results for sea surface temperature. River flow data was provided by CEH from their Grid to Grid model. Salinity was set at 0 psu, and temperature at 7 degrees Celsius which was felt appropriate when considering the observed sea water temperatures.

Comparisons between the model results and measurements of water level and current speeds showed generally good agreement. Comparisons of the 10 layer baroclinic model showed that salinity comparisons with data were generally within the 1 psu in line with our target. Temperature was within 1 degree Celsius, although most of the time it was in the order of 0.5 degrees.

4.3 Climatological simulations

One requirement of this study was to produce a six month climatic run based upon climatological forcing to represent a typical annual cycle. The model was therefore run for the period May to October. Mean boundary forcing for water levels (mean yearly tides), currents, temperature and salinity were taken from the Scottish Waters Shelf Model climatology results. An efficient method was developed to interpolate the forcing data onto the nested boundary nodes and elements. River climatology was also provided by CEH and used for this study following analysis by NOC-L. Meteorological forcing was derived by NOC-L from ECMWF (ERA-Interim) averaged data to provide monthly mean wind-stress, pressures, heating and evaporation minus precipitation from the period 1981-2010.

Average monthly temperature and salinity simulated by the model were compared against sea surface temperature and salinity climatological datasets and residual currents for the months of May, August and October; the results compared well with this data.

Mean spring and neap tidal ranges and currents were also calculated using M2 and S2 water level and current constituents and then compared against an ABPmer model of the area. Comparisons are generally good, with the main difference found around Skye; here both the Shelf Model and the ABPmer model have similar tidal ranges, but both models do not resolve the channel between Skye and the mainland. The ECLH model does however resolve this channel and therefore this is likely to be the reason for the differences observed and the benefit of the finer resolution ECLH model.

Consideration of the seasonal variations in temperature, salinity and residual currents in the context of existing studies has shown that the ECLH model can reproduce the spatial variations in temperature and salinity fields. Its results support the findings of existing research into the hydrodynamics of the area and provide further insights into the residual flows around this complex region.