4.2 Background: model coupling
4.3.2 Two-way coupling experiments description
For the evaluation of the two-ways coupling, the entire 2012 year is simulated and analysed. During the full year several north-west wind jet events occurred in the Ebro River shelf, so one of them was selected as the energetic event. The episode lasted from the 19thof May 2012 to the 23rd of May 2012. The sequence of wind field modelled in the Catalan coast mesh during the wind jet period is characterized by a rise of wind intensity during the 20th and 21st of May, leading to a wind jet in the northern margin of the Ebro delta (see daily-averaged wind intensity in figure 4.3). Then, the offshore winds remains strong during the 22nd of May, decreasing during
the 23rd of May 2012.
As noted in section 4.2, the air-sea momentum transfer presents high complexity due to the relation of wave characteristics and the sea surface roughness, which in turns affect the wind field. In order to investigate the air-sea momentum transfer in the wind jet, a set of simulations have been designed applying different sea surface roughness formulations included in the COAWST modelling system. The sensitivity
Chapter 4. The effect of coupling on the wave modelling in the Catalan Coast
tests pursue an evaluation of the coupling effects on two principal variables involved in the air-sea momentum transfer: wind intensity and significant wave height. In this sense three different formulations have been tested (equations 4.3 to 4.5 described in section 4.2) and compared with the configuration set up by default in WRF.
Figure 4.3: Sequence of the wind jet intensity on four days for a wind jet event in the domain of the Catalan coast (in m/s).
In consequence, the coupled results are directly compared with an uncoupled simulation where the sea surface roughness length is only a function of the wind stress. The sensitivity tests are as follows: ‘CHK’ for the simulation considering the sea surface roughness as a function of the wind stress (uncoupled with the wave sea state) using the Charnock coefficient equal to 0.016 (typical value for young seas), ‘T-Y’ simulation considering the Taylor and Yelland formulation (Taylor and Yelland, 2001), ‘DRE’ using the Drennan formulation proposed by Drenann et al. (2003) and ‘OOST’ simulation considering the formulation introduced by Oost et al. (2002).
As a part of the European project NEPTUNE a buoy was moored in the north- ern margin of the Ebro shelf where the wind jet develops (figure 4.4) that was used to validate the results for the second section episodes. The buoy was moored 3.1 km from the coast at 43.5 m bottom depth, measuring wind, waves and water currents
4.3. Methodology
for one year (November 2011 to December 2012). A TRIAXYS directional wave sensor mounted on the moored buoy was used to record statistical wave spectral parameters. Wind speed and direction were measured at 4 m height every 10 min using an ultrasonic wind sensor (Gill Instruments), and water currents were mea- sured with a SonTek acoustic Doppler currentmeter profiler (ADCP) at 500 kHz every hour using 20 vertical layers (layer depth was 2m).
Figure 4.4: Orography and bathymetry of the study area (top). The red boxes represent the SWAN and ROMS meshes and the green boxes the WRF domains for the two-way episodes (resolutions detailed in table 4.3). In the bottom the Ebro delta domain is represented and the Neptune buoy and control point locations marked with pink dots.
During the wind jet event two evaluation locations are considered to compare the results for the sensitivity test simulations. One point corresponds to the Neptune buoy position (where the numerical results are also compared with the measure-
Chapter 4. The effect of coupling on the wave modelling in the Catalan Coast
ments) and the second point is located 30 km offshore of the measurement point (see control point in figure 4.4). This point has been chosen in order to capture the wave growth due to off-shore winds and evaluate properly the coupling - uncoupling differences
Additionally satellite-measured winds are used for the numerical model vali- dation. Sea wind intensity and direction were obtained from the National Cli- matic Data Center (NCDC- NOAA, http://www.ncdc.noaa.gov/oa/rsad/air-sea/ seawinds.html). This product is the result of a spatial and temporal interpolation of the data received from the different satellites passing through the study area during a time interval, with a 6 h time resolution and 15 km spatial resolution.
The bathymetry used in all the episodes tested is obtained from GEBCO (Gen- eral Bathymetric Chart of the Oceans, www.gebco.net) with a grid resolution of 30 arc-second (0.0083o).
The system strategy in the two-way coupling is very similar to the one presented previously with an extra curvilinear grid located in the Ebro Delta with a spatial resolution of 250 m and nested to the Catalan Coast grid (table 4.2). The main purpose of this local grid is to implement the ocean-atmospheric-wave two-ways coupling because is in the coastal areas where the scale of the coupling process may be more evident in the results.
Table 4.2: Description of the grids implemented in SWAN for the Catalan coast. The Ebro Delta grid is only valid for the two-ways coupling episodes.
Western Mediterranean Sea
Balearic Sea Catalan Coast Ebro Delta Longitudes 4.90o W 16.05o E 0.45o W 5.26o E 0.17o E 3.59o E 0.25o E 1.26o E Latitudes 35.00o N 44.52o N 39.00o N 43.66o N 39.63o N 41.83o N 40.27o N 41.03o N Mesh size 196 x 119 160 x 174 208 x 106 253 x 226 Grid Resolution 9 km (0.107o x 0.081o) 3 km (0.035ox0.027o) 1km (mean value) 250m (mean value)
In the two-way coupling exercises the WRF atmospheric model and the ROMS circulation models run simultaneously to the SWAN model, providing the forcing for the wave model. In table 4.3 a description of the downscaling system of meshes used in the two-ways coupling episodes is presented.
4.3. Methodology
Table 4.3: Resolution of the different domains used in the two-ways coupled system as a function of each model and regional scale.
Model Western
Mediterranean Sea
Balearic Sea Catalan Coast
Ebro Delta
WRF 27 km 9 km 3 km 1 km
SWAN 9 km 3 km 1 km 250 m
ROMS - - 1 km 250 m
The water circulation system for the study area consist of two domains, matching the Catalan Coast and Ebro Delta wave domains (table 4.2). The largest oceanic domain is nested into the daily MyOcean-MEDSEA product (Tonani et al., 2014), with a horizontal resolution of 1/16ox 1/16oand 72 unevenly spaced vertical levels,
in order to provide suitable boundary conditions for the oceanographic variables in terms of water velocity, sea level, temperature and salinity.
The Western Mediterranean Sea atmospheric model is nested into the ECMWF ERA-Interim reanalysis product (www.ecmwf.int) considering four downscaling mesh- es, with resolutions of 27km, 9km, 3km and 1km respectively, to obtain suitable grid resolution for the complex orography of the region (figure 4.4).
In the COAWST system designed for the Ebro delta domain the information be- tween models is exchanged every 10 min of simulated time. The computer used to run the coupled simulations belongs to the Centro de Investigaciones Energ´eticas, Medioambientales y Tecnol´ogicas from the Spanish Government (CIEMAT, www .ciemat.es) and has 48 processors available that were distributed as follow: 32 pro- cessors to run the WRF meteorological model, 8 processors to run the ROMS circu- lation model and 8 processors to run the SWAN wave model. The distribution was designed to minimize the waiting time between the different models on the instant when the information is exchanged.
Chapter 4. The effect of coupling on the wave modelling in the Catalan Coast
4.4
Analysis of the results
As previously mentioned, the chapter is divided in two sections. In the first one the effects of the currents on the wave modelling are considered while in the second one a two-way coupling between the wave model and the circulation and atmospheric model is performed.
The results are presented following the same structure, including two episodes, a calm period and an intensification event, for each section.