Comment on gmd-2022-189

Abstract

The sensitivity of a shelf sea model of the Gulf area to changes in the bathymetry, lateral and vertical resolution, vertical coordinates and external forcing is explored. Two new Gulf models with a resolution of 1.8 km named GULF18-3.6 and GULF18-4.0 differing only in the vertical coordinate system and the NEMO codebase employed (NEMO-3.6 and NEMO-4.0.4, respectively) are introduced. We compare them against the existing 4 km PGM4 model, which is based on NEMO-3.4 and is developed and used by the Met Office. PGM4 and GULF18-3.6 use a similar type of quasi-terrain-following vertical levels while GULF18-4.0 employs the multi-envelope method to discretise the model domain in the vertical direction. Our assessment compares non-assimilative hindcast simulations of the three Gulf models for the period 2014–2017 against available observations of tides, hydrography and surface currents. Numerical results indicate that both high resolution models have higher skill than PGM4 in representing the sea surface temperature and the water column stratification on the shelf. In addition, in the proximity of the shelf-break and the deep part of the domain GULF18-4.0 generally presents the highest accuracy, demonstrating the benefit of optimising the vertical grid for the leading physical processes. For the tides and the surface currents the three models give comparable results. However, our tidal harmonic analysis suggests that future work may be needed in order to get real benefit from using a more realistic bottom topography as in the case of the GULF18 models.