Salinity (PM3D model) Salinity is determined using the PM3D hydrodynamic model. The calculations take into consideration the inflows from the 150 or so largest rivers carrying fresh water, and also the water exchange with the North Sea, from where saline waters enter the Baltic. Salinity maps with a resolution of 1 km are obtained from the spatial interpolation of the results of the PM3D model, the spatial resolution of which varies from ca 0.9 km in the southern Baltic to ca 1.9 km in other regions of this sea. In the vertical, regardless of depth, the model has 18 layers of varying thickness, thus enabling the salinity to be calculated at different depths. The SatBałtyk System service provides salinity distributions in the Baltic in practical salinity units (PSU) four times a day.	An example of the surface salinity distribution of the Baltic Sea calculated by the model PM3D.
Methodology of determining salinity using the PM3D model The three-dimensional PM3D hydrodynamic model (Parallel Model 3D) is a version of the numerical [1] M3D model, modernized within the framework of the SatBałtyk project, which is based on the POM (Princeton Ocean Model). As a result of the parallelization of the calculations performed on computers equipped with multi-core processors and the implementation of procedures for assimilating satellite data, the PM3D enables a more accurate calculation of salinity and other water parameters at high resolution. Information about the weather is supplied from the UM numerical weather model (Unified Model) operating at ICM with a resolution of 4 km. Over the entire Baltic Sea the PM3D model has a resolution of 1 nautical mile (ca 1.9 km). This model enables the resolution to be increased for certain areas with two-way downscaling, which means that the salinity can be calculated at a resolution of 0.5 nautical miles (ca 0.9 km) in the southern Baltic (see figure). The PM3D works operationally, and the precision of the salinity is updated daily on the basis of current remote sensing data during the assimilation of such data. As the boundary in the western Skagerrak is open, the model simulates the inflow of saline water from the North Sea. It solves the problem of the transport of salt under the influence of drift currents, which are associated with the wind action and gradients due to the spatial variability of temperature and salinity.	An example map of water salinity of the Baltic Sea with spatial resolution of the numerical model
Validation (accuracy assessment) The PM3D model has been validated many times with respect to various parameters and regions [2] [3]. The accuracy of salinity determination was evaluated on the basis of the salinity difference calculated using the model with measurements made during ICES monitoring work in 2010-2014. The statistical error, expressed as the standard deviation of these differences, was estimated at 0.41 PSU. The systematic error (the mean difference) was 0.57 PSU. The dependence between the values calculated using the model and those measured in the sea during monitoring cruises is shown in the figure.	Comparision of modelled (Smodel) and empirical values (S obs.) of surface temperature of the Baltic Sea, regression equation, R²-coefficient of determination
Interesting phenomena visible on salinity maps The spread of river waters Plumes of low salinity due to the spread of fresh waters from the mouths of large rivers into much more saline sea waters are clearly visible on salinity maps. The shapes and extents of such freshened water plumes depend on the volume of river water entering the sea and on the speed and direction of surface sea currents, which depend largely on the speed and direction of the wind. The figure exemplifies the spread of fresh waters from the River Vistula in the Gulf of Gdańsk. Link to the parameter in SatBaltic System: Sea surface salinity [1] Kowalewski M.,1997, A three-dimensional, hydrodynamic model of the Gulf of Gdańsk; Oceanol. Stud., 26 (4); 77–98 [2] Jędrasik J., 2005, Validation of hydrodynamic part of the ecohydrodynamic model for the southern Baltic, Oceanologia, 47(4),. 517-541 [3] Kowalewski M., Kowalewska-Kalkowska H., 2011, Performance of operationally calculated hydrodynamic forecasts during storm surges in the Pomeranian Bay and Szczecin Lagoon, Boreal Environment Research, Res. 16 (suppl. A): 27–41

Aktualności

Salinity (PM3D model)

Interesting phenomena visible on salinity maps

Link to the parameter in SatBaltic System: