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The Forecasting System 1/45°
The Model
The ocean model used is POM (Princeton Ocean Model). It is a three-dimensional finite difference, free surface, primitive equation numerical model, utilizing the Boussinesq and the hydrostatic approximation and a split mode time step. The model contains a second-order turbulence closure submodel providing the vertical mixing coefficients (Mellor and Yamada, 1982). Horizontal diffusion is parameterized following the scheme of Smagorinsky (1993), as coded into POM by Mellor and Blumberg (1985). Density is calculated by an adaptation of the UNESCO equation of state devised by Mellor.
Compared to the standard version of POM an important change in the model structure has been implemented into this version of POM. The default POM centred difference scheme for the advection of tracers has been substituted with the MUSCL scheme (Estubier and Lévy, 2000).
Implementation for the Adriatic Sea
The POM model has been implemented in the Adriatic Sea (this implementation is called AREG: Adriatic REGional model), and its domain encompasses the whole Adriatic basin and extends south of the Otranto channel into the northern Ionian Sea , where the only open boundary is located. Grid points located over the Thyrrenian Sea have been masked out. The AREG grid has a horizontal resolution of about 1/45° (about 2.2 km), on 31 s -layers. The bottom topography was obtained from the U.S. Navy 1/60° bathymetric database DBDB1, by bilinear interpolation of the depth data into the model grid. All the depths shallower than 10 metres have been flattened to 10. The model domain and the bathymetry are shown in below.
The surface fluxes are interactively computed using model predicted sea surface temperature and realistic atmospheric data provided by the European Centre for Medium Range Weather Forecast (ECMWF) with a frequency of six hours and a resolution of 0.5°.
Realistic fresh water has been implemented in the surface boundary condition for the vertical velocity.
The river input into the basin has been implemented through river climatology for all the rivers except for the Po, which is a very important forcing for the Adriatic Sea . The Po runoff mplemented is on a daily basis. For what concerns the simulations the runoff values are calculated from the observed level of the river at the closure cross section of Pontelagoscuro, while for what concerns the forecast, the last observed value is persisted for the whole period. The runoff is partitioned over several grid points approximately representing the proportion of the fresh water discharged through the mouth of the delta (Provini et al. 1992).
The initial and lateral boundary conditions for temperature, salinity and velocity come from the model OPA (Madec et al, 1998), operationally implemented in the entire Mediterranean
Forecasting System Details (1/45°)
A nine day ocean forecast for all the Adriatic Basin is done and released via ftp and on a web bulletin every day.
In order to have the best estimate of the state of the sea for the production of a new forecast, the daily cycle of the system is combined once a week with a weekly cycle, necessary in order not to have a degeneration of the system. In fact, since the Adriatic system is nested into the Mediterranean System ( http://www.bo.ingv.it/mfs), and this latter is based on a weekly cycle of assimilation, also the Adriatic system needs to be rewinded once a week to have the best estimate of the sea state for the forecast runs. This rewind of the simulation cycle takes place every Wednesday morning, and it provides the restart conditions for the 9-day forecast. During the other days of the week the forecast is based on a daily simulation which provides the restart condition for the next 9-day forecast.
A scheme of the system is shown in the panel below.
Figura 1 : Daily assimilation cycle
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