Aegean Sea POSEIDON (HCMR)

The Aegean Sea hydrodynamic model is based on the Princeton Ocean model (POM) and was initially developed as part of the Poseidon-I system (Soukisian et al., 2002). POM is a primitive equations free surface ocean model which operates under the hydrostatic and Boussinesq approximations. The model equations are written in sigma-coordinates and discretized using the centered second-order finite differences approximation in a staggered “Arakawa C-grid” with a numerical scheme that conserves mass and energy.

The model domain in the latest version compatible with MOON V1 covers the geographical area 19.5°E – 30°E and 30.4°N – 41°N with a horizontal resolution of 1/30° and 24 sigma layers along the vertical with a logarithmic distribution near the surface and the bottom. The model includes parameterization of the main Greek rivers (Axios, Aliakmonas, Nestos, Evros) while the inflow/outflow at the Dardanelles is treated with open boundary techniques.

The Aegean Sea is located to the northeast of the Ionian and to the northwest of the Levantine Sea. It is the third major sea of the Eastern Mediterranean basin. The topographical structure of the Aegean is very complicated. It is bounded to the east by the Turkish coasts (Asia Minor), to the north and west by the Greek mainland and to the south by the island of Crete. Its coastline is very irregular and hundreds of islands are scattered all over the Aegean. The Aegean and Ionian Seas constitute one of the main links of Europe to the Eastern Mediterranean and Russia. Therefore, these waters serve as the main routes of oil transportation from source to Europe. This results to a continuous pressure to the marine environment, as indicated by the following remarks: i) During 2000, the Greek authorities reported 379 oil-pollution incidents, the major part of them caused by ships ii) According to the map published by JRC for the illicit vessel discharges in the Mediterranean during 1999, over 400 oil-spills were detected by SAR imagery around Greece. A recent review of the circulation characteristics of the Eastern Mediterranean and the Aegean Sea is given by Karageorgis et al. (2008) who synthesized the results of Robinson and Golnaraghi, 1994, Malanotte-Rizzoli et al., 1997 and Theocharis et al., 1999.

http://www.poseidon.hcmr.gr/

Aegean Sea : 30.4°N to 41°N & 19.5°E to 30°E

Princeton Ocean Model - POM (Primitive Equations)

Every 6 hours

Orthogonal grid 1/30°

* 25 sigma levels with logarithmic distribution near the surface and the bottom boundary layer

* Free surface

ETOPO 1

* Bulk formula for sensible, latent heat flux and upward longwave radiation (Korres et al., 2002) * Net short-wave and downward longwave radiation is provided by the atmospheric model (Korres et al., 2002) * HCMR non-hydrostatic (1/20°) ETA model analysis & forecasts (hourly frequency) * Monthly run off

* Zero gradient condition for free surface elevation. Modified Flather type boundary condition for barotropic velocities. Internal velocities are prescribed from the parent model. T,S are advected upstream in cases of outflow and prescribed from parent model in cases of inflow into the model domain. (Korres et al., 2003) * Lateral forcing data from the Mediterranean SYS2b model

Nonslip lateral boundary condition

Non linear bottom friction

Laplacian diffusion with Smagorinsky scheme

Laplacian diffusion with Smagorinsky scheme

Mellor & Yamada 2.5 Turbulence closure

No tide

Localized SEEK filter (Korres et al., 2009): The assimilation scheme is based on the Singular Evolutive Extended Kalman (SEEK) filter which is an error subspace extended Kalman filter that operates with low-rank error covariance matrices as a way to reduce the computational burden. The filter is additionally using covariance localization and partial evolution of the correction directions. The approximation of “partial evolution” of the correction directions was first proposed by Hoteit et al. (2002) and subsequently by Korres et al. (2009) who found that this approach has a limited impact on the filter performance. Its use in the high resolution Aegean Sea model can be supported by the fact that the evolution of the last (i.e. least significant) EOFs in the SEEK filter can be problematic and might introduce noise in the filter correction basis, and this was observed in preliminary experiments. This happens because the evolution equation of the SEEK filter might fail to track fast fine-scale variations similar to the ones resolved by the last EOFs of the high resolution coastal model, bearing in mind that the filter is designed to follow slow dynamical changes only.

Hoteit I., D.T. Pham and J. Blum, 2002. A simplified reduced Kalman filtering and application to altimetric data assimilation in the Tropical Pacific. J. Mar. Sys. 36, 101-127.

Korres, G., K. Nittis, I. Hoteit and G.Triantafyllou, 2009. A high resolution data assimilation system for the Aegean Sea hydrodynamics. J. Mar. Sys. 77, 325-340.

3D EOFs

Altimetric absolute sea level (Jason-1, Envisat, GFO), GOS optimally interpolated SST and in situ T/S vertical profiles in a fully multivariate way