Comparison between nested grids and unstructured grids for a high-resolution wave forecasting system in the western Mediterranean sea

Abstract

Traditionally wave modelling uses a downscaling process by means of successive nested grids to obtain high-resolution wave fields near the coast. This supposes an uncertain error due to internal boundary conditions and a long computational time. Unstructured grids avoid multiple meshes and thus the problem of internal boundary conditions. In the present study highresolution wave simulations are analysed for a full year where high-resolution meteorological models were available in the Catalan coast. This coastal case presents sharp gradients in bathymetry and orography and therefore correspondingly sharp variations in the wind and wave fields. Simulations with SWAN v.4091A using a traditional nested sequence and a regional unstructured grid have been compared. Also a local unstructured grid nested in an operational forecast system is included in the analysis. The obtained simulations are compared to wave observations from buoys near the coast; almost no differences are found between the unstructured grids and the regular grids. Simultaneously, tests have been carried out in order to analyse the computational time required for each of the alternatives, showing a decrease to less than half the time when working with regional unstructured grids and maintaining the forecast accuracy and coastal resolution with respect to the downscaling system. ARTICLE HISTORY

Design and Evaluation of an Efficient High-Precision Ocean Surface Wave Model with a Multiscale Grid System (MSG_Wav1.0)

“…There is a slight but acceptable impact on MWD. A similar phenomenon can also be seen in Pallares et al (2017), where the MWD is the most sensitive among these three variables when the used grid is changed. With the same computational resources (128 computing cores) simulating the same time length (31 days), 𝑊𝑆 𝑚𝑢𝑙𝑡𝑖4 𝑢𝑡𝑚𝑠 takes 0.63 hours, saving about 70% of the calculational time compared to the reference 𝑊𝑆 0.125…”

The Mw 7.5 Tadine (Maré, Loyalty Islands) earthquake and related tsunami of 5 December 2018: seismotectonic context and numerical modeling

“…One uses a sequence of nested structured-grid models; the other relies on a single unstructured-grid model. Both techniques aim at obtaining high-resolution wave fields in shallow areas and provide similar results (Harig et al, 2008;Pallares et al, 2017), even though several studies have highlighted that the use of only one unstructured mesh grid for tsunami modeling provides better reproduction of tsunami observations and records in comparison to nested-grid-scheme use (e.g., Harig et al, 2008;Shigihara and Fujima, 2012). When considering the presence of many archipelagos forming the Melanesian volcanic arc (Solomon Islands and Vanuatu, Fig.…”

The land–sea coastal border: a quantitative definition by considering the wind and wave conditions in a wave-dominated, micro-tidal environment

“…The calculated anisotropies should be as robust as the starting wave or wind fields that are employed in the analysis. Because of that, the SWAN code has been calibrated with local atmospheric and hydrodynamic conditions (Pallarés et al, 2017). Special emphasis has been put on using high-quality wind fields, both for the direct assessment linked to meteorological fields and for the indirect effect they exert on the behaviour of the forced hydrodynamics.…”

“…SWAN employs an unstructured grid with spatial resolutions of 600 m-40 km, denser near the land-sea boundary. Mesh sizes are proportional to bathymetry gradients and distance to the coastline, following the same criteria than in Pallarés et al (2017). Such a non-structured-grid approach avoids nesting and internal boundary conditions, while maintaining a good spatial resolution to capture bottom and coastline irregularity (submarine canyons and capes or pro-deltas that are found in the Catalan continental shelf).…”