3.3 Methodology
3.3.1 Data available and model setup
In the framework of the European project FIELD AC (contract no. FP7-SPACE- 2009-1-242284) two measurement campaigns have been carried out. The FIELD AC project is focused on the meteo-oceanographic forecasting for small regions such a beach, a harbour or the mouth of a river. To reach this goal, it is necessary to simulate with high resolution and in a coupled manner the meteorology, the circulation and wave fields, the land/river discharge and the resulting transport (e.g. for sediment) patterns.
Three of the instruments deployed off the Catalan coast were used to validate the wave model results: two AWAC directional systems located at the seabed and one directional wave buoy. The characteristics of these instruments, together with their location and the data provided can be found in table 3.1. The data was collected during two periods, the first one from the 12th of November 2010 to the 18th of
January 2011, and the second campaign going from the 11th of March 2011 to the
18th of April 2011. The campaign was scheduled during these months because, as
mentioned in chapter 2, from October to May the more energetic wind and wave events are expected in the area. However, the measurements show that during the first period only one storm from the north-west occurred, while during the second period several energetic episodes from the south and east were captured.
Table 3.1: List of the instruments location and information provided for the Catalan coast.
Institution Name Lon Lat Depth Parameters provided
FIELD AC campaign AWAC Bogatell 2.21o E 41.39o N 24 m H s, Dir, Tm02, Tp, Tm01 AWAC Besos 2.24o E 41.41o N 24 m Hs, Dir, Tm02, Tp, Tm01 Buoy Besos 2.26o E 41.40o N 50 m Hs, Dir, Tm02, Tp, Tm01 XIOM Blanes 2.28o E 41.65o N 74 m H s, Dir, Tm02, Tp, Tm01 Llobregat 2.14o E 41.28o N 45 m H s, Dir, Tm02, Tp, Tm01 Tortosa 0.98o E 40.72o N 60 m H s, Dir, Tm02, Tp, Tm01 Puertos de estado Barcelona 2.20o E 41.28o N 68 m H s, Dir, Tm02, Tp Tarragona 1.47o E 40.68o N 688 m H s, Dir, Tm02, Tp
Chapter 3. Wave energy balance for semi-enclosed domains
Additionally, five more instruments have been considered in the study. The local network XIOM (Xarxa d’Instruments Oceanogr`afics i Meteorol`ogics, www.xiom.cat) with four buoys along the coast and time series for more than 25 years has been used to characterize the wave properties in the study area. For this purpose, three of these buoys have been used to validate the obtained results (table 3.1). Comple- mentary to this local network, the general Spanish network of Puertos del Estado (www.puertos.es) presents three buoys in the study area. Two of them have also been used in this study; the first one is located offshore the Barcelona harbour en- trance, while the second one is located in the southern part of the domain at about 50km from the coast (Tarragona buoy, in deep water offshore wards the Tarragona harbour entrance).
All the buoy locations used are represented in figure 3.1. Table 3.1 shows that, except for the Tarragona buoy, all the instruments are moored near the coast. How- ever, due to the young wave age conditions prevailing in the area (consequence of the short fetch available and the highly variable wind conditions) all the instrument locations correspond to deep or intermediate water conditions.
The bathymetry used is obtained from GEBCO (General Bathymetric Chart of the Oceans, www.gebco.net) with a grid resolution of 30 arc-second (0.0083o).
The simulations have been structured in three nested domains covering the West- ern Mediterranean Sea (figure 3.1). The first one is a large domain all over the men- tioned area with a grid resolution of 9 kilometres. This domain is used to generate boundary conditions for the smaller domains. The second domain is located in the Balearic Sea, with a mesh size of 3 kilometres and the next one is a domain follow- ing the Catalan coast and shelf domains, represented by a curvilinear grid with a mean cell size of 1 kilometre. The grid dimensions and resolutions are presented in table 3.2.
Table 3.2: Description of the grids implemented in SWAN for the Catalan coast. Western
Mediterranean Sea
Balearic Sea Catalan Coast
Longitudes 4.90o W - 16.05o E 0.45o W - 5.26o E 0.17o E-3.591 E Latitudes 35.00o N - 44.52o N 39.00o N - 43.66o N 39.63o N - 41.83 oN Mesh size 196 x 119 160 x 174 208 x 106 Grid resolution 9km (0.107o x 0.081o) 3km (0.035ox0.027o) 1km (mean value)
3.3. Methodology
Figure 3.1: On top: The Western Mediterranean Sea. Represented the local distri- bution of the computational grids implemented in SWAN (in red) and the two atmo- spheric domains used as a forcing (in green). At the bottom: a detailed bathymetry for the finest grid (1km grid size in average) is presented. The dots represent the measurement locations.
The initial conditions for each domain have been obtained running the SWAN model in a stationary mode while the boundary conditions for the medium and small domains are generated by the enveloping larger domain. The largest grid covers almost the whole Western Mediterranean Sea which can be considered as a semi-enclosed basin. As the boundaries of this grid are far away from the area of interest, we do not expect that omitting the influx of energy along these boundaries will affect our model results near the Catalan coast.
Chapter 3. Wave energy balance for semi-enclosed domains
Two periods are considered in the present study, corresponding to the measure- ment campaigns mentioned before. These periods have been selected because of the different processes represented in each one such as offshore wind conditions; long fetch storm events and calm periods, all of them typical processes for the study area. During these dates two high resolution campaigns were carried out within the European project FIELD AC and there are available high resolution wind fields and a variety of observations. The selected wind field domains are represented in figure 3.1, and have been provided by the Barcelona Supercomputing Centre (BSC, www.bsc.es) using the Weather Research and Forecasting (WRF) Model (Skamarock et al., 2005). The first wind domain covers the full Western Mediterranean Sea, with a spatial resolution of 12 km and a temporal resolution of 1 hour and it is used for the Western Mediterranean Sea grid. The two smaller wave grids are forced by the second wind field provided by BSC, with a spatial resolution of 4 km and a time res- olution of 1 hour. As presented by Alomar et al. (2014) this resolution is considered enough to capture the wind space and time gradients in the studied coastal area. A more extensive validation of the wind patterns against remote-sensing measure- ments and a comparison to other wind options is presented by Bertotti et al. (2014), where the WRF model is considered to perform reasonably well, with a correlation coefficient for the study period around 0.83, a bias of 0.56 m/s and a RMSE = 1.98 m/s, quite high. The high-resolution wind field is also analysed with practically identical results, presenting some minor differences only very close to the coast.