Numerical model implementationThe model code is NEMO-OPA (Nucleus for European Modelling of the Ocean-Ocean PArallelise) version 3.2 off-line coupled with WAM (Wave Analysis Model). NEMO-OPA has been developed by Institut Pierre Simon Laplace, Laboratoire d’Oceanographie DYnamic et de Climatologie, Paris. A detailed description of the code can be found in Madec et al. (2008). The model is primitive equation in spherical coordinates.
NEMO-OPA and WAM has been implemented in the Mediterranean at 1/16° x 1/16° horizontal resolution and 71 unevenly spaced vertical levels (Oddo et al., 2009). The model domain and the bathymetry are shown in below.
The Digital Bathymetric Data
Base Variable Resolution (DBDB-V)
has been used to make the model
cost line and bathymetry. The bathymetry
has been manually interpolated along
the Croatian coast by a comparison
with detailed nautical chart.
The model covers the entire Mediterranean Sea and also extends into the Atlantic. The model uses vertical partial cells to fit the bottom depth shape. The model is forced by momentum, water and heat fluxes interactively computed by bulk formulae using the 6-h, 0.5° horizontal-resolution operational analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the model predicted surface temperatures (details of the air-sea physics are in Tonani et al., 2008).
For what concerns the water balance (Evaporation-Precipitation-Runoff)
- Evaporation is derived from the latent heat flux;
- Precipitation is taken from monthly mean Climate Prediction Center Merged Analysis of Precipitation (CMAP) Data (Xie and Arkin, 1997);
- Runoff is composed of monthly mean climatological data. Only seven major rivers have been implemented: the Ebro, Nile and Rhone monthly values are from the Global Runoff Data Centre (Fekete et al., 1999) and the Adriatic rivers (Po, Vjosë, Seman and Bojana) are from Raicich (Raicich, 1996). In this model configuration the Dardanelles inflow has been parameterized as a river and its monthly climatological net inflow rates were taken from Kourafalou and Barbopoulos (2003).
The advection scheme for active tracers (temperature and salinity) is a mixed up-stream/MUSCL (Monotonic Upwind Scheme for Conservation Laws, Estubier and Lévy, 2000) scheme. The up-stream scheme is used in proximity of the river mouths, in the Gibraltar Strait and close to the Atlantic lateral boundaries. At Gibraltar, the up-stream scheme, together with an artificially increased vertical diffusivity, parameterizes the large mixing acting in this area due to the internal wave and tide breaking, which is not explicitly resolved by the model.
In the Atlantic the model is nested within the monthly mean climatological fields computed from the daily output of the ¼° x ¼° degrees global model (Drevillon et al., 2008). Details on the nesting techinique and major impacts on the model results can be found in Oddo et al., 2009.
The model levels are exactly at the following depths (meters):
1.472, 4.587, 7.944, 11.559, 15.449, 19.633, 24.133, 28.968,
34.164, 39.743, 45.733, 52.161, 59.058, 66.456, 74.390, 82.895,
92.011, 101.780, 112.247, 123.459, 135.467, 148.325, 162.092, 176.829,
192.603, 209.485, 227.548, 246.875, 267.551, 289.666, 313.320, 338.615,
365.663, 394.582, 425.499, 458.547, 493.870, 531.621, 571.962, 615.066,
661.117, 710.311, 762.857, 818.977, 878.906, 942.896, 1011.211, 1084.136,
1161.970, 1245.031, 1333.657, 1428.206, 1529.057, 1636.611, 1751.292, 1873.549,
2003.855, 2142.711, 2290.645, 2448.210, 2615.993, 2794.607, 2984.700, 3186.948,
3402.060, 3630.780, 3873.883, 4132.178, 4406.510, 4697.753, 5006.818, 5334.648
WAM model has been implemented on Mediterranean basin in the exact same region and with the exact same space resolution of the NEMO-OPA. The configuration used is with current interaction and without bottom interaction. The directions of the wave propagation have been divided in 24 bins. The range of the frequencies taken in to account for the external barotropic waves is within 0.05 Hz and 0.79316 Hz. This range has been discretized in 30 bins.
Data assimilation systemThe data assimilation system is the 3DVAR scheme developed by Dobricic and Pinardi (2008 Ocean Modelling).
The background error correlation matrix is estimated from the temporal variability of parameters in a historical model simulation. Background error correlation matrices vary seasonally and in 13 regions of the Mediterranean which have different physical characteristics (Dobricic et al 2006). The mean dynamic topography used for the assimilation of SLA has been computed by Dobricic et al. (2005).
The assimilated data include: sea level anomaly, sea surface temperature, in situ temperature profiles by VOS XBTs, in situ temperature and salinity profiles by ARGO floats, and in situ temperature and salinity profiles from CTD.
Satellite OA-SST data are used for the correction of surface heat fluxes with the relaxation constant of 40 W m-2 K-1.
Forecast and analysis cycle
The analysis is done weekly using a daily assimilation cycle. This means that in order to produce an analysis, the model is run for 24 hours and the analysis is produced at the end of the day assimilating all and only the data available in that time window (filter mode). The daily analysis cycle is done once a week, each Tuesday, producing 13 past analyses and the present day analysis. Each day a 10 days forecast is produced starting on Tuesday from an analysis and each of the successive six days from a model simulation. The figure below shows the detailed structure of the assimilation and simulation cycle for the daily forecast production.
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