High-resolution nested model for the Lebanese basin, East Levantine Basin
eastern Mediterranean Sea: implementation and climatological runs.

• N. Kabbara¹, R. Sorgente², and S. Natale²
• ¹ National Center for Marine Sciences, Beirut, Lebanon
• ² International Marine Center, Oristano, Italy
• The Regional Workshop on Marine Sciences & Natural Resources 25-26 May 2004.

Abstract

A high-resolution nested flow model for the coastal, shelf and open sea area of the Lebanese Basin, East Levantine Basin, eastern Mediterranean, based on the Princeton Ocean Model (POM), is used to study the climatological variability of circulation of the Lebanese Basin. The model is run perpetual year run simulation for five years, with nesting to the course regional model ALERMO (Aegean and Levantine Eddy Resolution Model) covering the Eastern Mediterranean and developed by MFSPP project (Korres and Lascaratos, 2003). The MED6 climatology was used for initialisation and the ECMWF (European Center for Medium Weather Forecast) Re-Analyses databases for perpetual year surface forcing. The numerical solution is used to highlight the characteristics of important dynamic features in the study area.

POM has been used extensively to simulate the coastal circulation in various regions around the world, including the Mediterranean Sea (e.g. Zavatarelli and Mellor, 1995). It is a three-dimensional, time-dependent model based on the primitive equations (Blumberg and Mellor, 1983, 1987; Oey et al., 1985a, b; Galperin and Mellor, 1990a, b; Mellor and Ezer, 1991, Lascaratos and Nittis, 1998). It has a bottom-following vertical sigma coordinate system, a free surface and a split mode time step. It consists of prognostic equations for the two components of the horizontal momentum, potential temperature, salinity, and the free surface elevation. Three diagnostic equations consisting of the hydrostatic equation, equation of state, and the vertical velocity, which is derived from the mass continuity equation, supplement these. In addition, POM contains an imbedded higher order turbulence closure scheme to simulate the vertical mixing (Mellor and Yamada, 1982). The modelled area extends from 32.9 N to 34.7 N and from 34.4 E to 35.9 E. The grid resolution is chosen to be 1°/40 for better representation of the mesoscale eddy activity in the study area. The number of grid points used is 62 in the x-direction, 72 in the y-direction, and 16 sigma levels in the vertical. The external time step is 4 sec. The model bathymetry is based on the (1/60)°×(1/60)° U.S. Navy bathymetric database DBDB1 with bilinear interpolation of depth data into the model grid. Additional light smoothing is applied to reduce the sigma coordinate pressure gradient error (Mellor et al., 1994). The resulting topography is shown in Figure 1. The maximum depth is about 2000m.

The model was initialized with temperature and salinity fields from MED-6 monthly Mediterranean Sea gridded 1 deg. temperature and salinity fields from MEDATLAS. The model has three open boundaries. At the open boundaries, the normal and tangential total velocity components are fully specified by a bilinear interpolation of the 10-day averaged coarse resolution model fields (ALERMO) onto the high resolution model grid (POM). The free surface elevation is not nested (zero gradient boundary condition). These boundary condition has been developed during the MFSPP project.

At the free surface, the climatological atmospheric forcing data set consists of heat and water flux fields, and wind stress components, on a monthly basis, derived from the ECMWF range Forecast 6-hourly Re-Analysis (ERA) data set covering the period 1979-1993 for the whole Mediterranean Sea (Korres and Lascaratos, 2003). Starting from the 1st of January the model is integrated for five successive years with surface and lateral boundary conditions as described above. Simulated horizontal fields at 5m taken from the fifth year of model integration are shown in the Figures 1-3. They depicted the general characteristics of the circulation in the region, with the prevalent occurrence of sub-basin scale gyres and intense coastal boundary currents, exhibiting a high temporal and spatial variability.

Key words: sea currents, physical oceanography, hydrodynamic modelling, nested model.