SHORT DESCRIPTION

CMEMS BS-MFC logo

The Black Sea analysis and Forecasting System (BSFS, EAS4.1 version) is the operational system that provides regular and systematic information about the physical state of the Black Sea region. It is developed by CMCC, which is responsible for the Black Sea Physics Production Unit as part of the Black Sea Monitoring and Forecasting Centre (BS-MFC) in the framework of the Copernicus Marine Environment and Monitoring Service (CMEMS, https://marine.copernicus.eu/). The core model is based on NEMO v4.0 online coupled with OceanVar, a 3D variational scheme for assimilation of in-situ and satellite data. The BSFS catalogue offers near real time products – analysis and forecast – for 3D temperature, salinity, currents and 2D sea surface height, mixed layer depth and bottom temperature. The Black Sea Physics Production Unit provides also reanalysis product for the past reconstruction of the ocean state in the basin from 1993.

More information on the Black Sea Physics analysis and forecast product is available here: https://resources.marine.copernicus.eu/product-detail/BLKSEA_ANALYSISFORECAST_PHY_007_001/INFORMATION

SYSTEM

THE BSFS HYDRODYNAMICAL CORE

The BSFS numerical core is based on the NEMO v4.0 ocean general circulation model (Nucleus for European Modelling of the Ocean, Madec et al. 2019), online coupled with OceanVar, a 3DVAR scheme for the assimilation of insitu and satellite observations provided by CMEMS (Ciliberti et al., 2021).

The model covers the whole basin except the Azov Sea: it includes a portion of the Marmara Sea as in Gunduz et al., 2020, to optimally interface the Black Sea with the Mediterranean Sea through the Marmara Sea and, consequently, provide a solution for the Bosporus Strait dynamics. The primitive equations are discretized over a horizontal grid at at 1/40° x 1/40° resolution and over 121 z* levels. The bottom topography has been reconstructed from the GEBCO 30” resolution dataset (https://www.gebco.net) combined with a high-resolution dataset for the Marmara Sea box – Bosporus Strait – Bosporus Exit (Gürses, 2016) and interpolated at the BS-PHY spatial grid. Coastline has been revised to account for the main coastal peculiarities and structures by using the NOAA shoreline dataset (https://www.ngs.noaa.gov/CUSP/).

The model is forced by momentum, water and heat fluxes interactively computed by bulk formulation as used in the Med-PHY system (Pettenuzzo et al., 2010; Clementi et al., 2021) and applied to BSFS EAS3 system (Ciliberti et al., 2022), using ECMWF IFS analysis and forecast atmospheric forcing (including precipitation) at the highest resolution at today available – 0.01° in horizontal and 1-3-6 hours frequency in time. Atmospheric fields used by BSFS are zonal and meridional components of 10 m wind (ms-1), total cloud cover (%), 2 m air temperature (K), 2 m dew point temperature (K) and mean sea level pressure (Pa) and precipitation (kg m-2s-1).

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Figure 1: The Black Sea bathymetry

A total number of 72 rivers are accounted as land forcing: for each river inflow, monthly climatological discharge values from SESAME project (Ludwig et al., 2009) with imposed zero salinity have been used. The Danube River has deserved a more dedicated study in order to improve its representation: it uses a distributed freshwater source to proper represent the main branches – the Chilia, the Sulina and the St. George arms – accounting for river discharge interannual historical dataset as provided by the National Institute of Hydrology and Water Management (NIHWM, partner of the BS-MFC consortium). For the major rivers, imposed non zero salinity is accounted, considering monthly climatological values as provided by SeaDataNet for the Black Sea at the closest river mouth location to represent the riverine salt contribution to the ocean. The BSFS implements lateral open boundary conditions at the Marmara Sea box: 3D temperature, salinity, currents and 2D sea surface height are provided by the new Unstructured Turkish Straits System (U-TSS), a Shyfem-based model implemented for the Dardanelles-Marmara Sea-Bosporus (Ilicak et al. 2021) whose spatial domain is represented in Figure 2.

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Figure 2: The U-TSS spatial grid (Ilicak et al. 2021)

The BSFS data assimilation system

BSFS data assimilation scheme is based on OceanVar, a 3D variational scheme developed by CMCC (Dobricic and Pinardi, 2008; Storto et al., 2011). The background covariance matrix is modelled using a set of empirical orthogonal functions (EOF) that provides a variable transformation to pre-condition the cost function minimization. The system uses a spatially varying set of 45 EOF to describe the covariance of sea surface height and temperature and salinity in the water column. The EOF are derived from a 10-year integration of the hydrodynamical core without DA. To account for seasonal variability the EOF have a monthly time dependence. Horizontal correlations are modelled through a third-order recursive filter (Farina et al., 2015), specified as a function of the distance from coast, ranging approximately from 9 to 27 km.

The observational error covariance matrices are spatially varying and include a depth and (monthly) time dependence where appropriate. The matrices have been calculated by a series of experiments in which the error is iteratively updated using the method of Desroziers et al., 2015. BSFS assimilates the list of observations as reported in Table 1, operationally provided by CMEMS INS, SST and SLA TACs. The assimilation of SLA imposes local hydrostatic adjustments as multi-variate balance between the sea level innovation and vertical profiles of temperature and salinity (Storto et al., 2011). The DA system runs with a daily frequency and uses a 24-hour assimilation time window.

The observations assimilated in the BSFS include: i) in-situ temperature and salinity profiles (mostly ARGO floats); ii) along-track L3 sea level anomalies, currently from AltiKa, Cryosat-2 and Jason-2/3, Sentinel3A and Sentinel3B satellites; iii) sea surface temperature L4 satellite observations. Observations are provided by CMEMS Thematic Assembly Centers (TACs).

The BSFS system evaluation

The quality assessment of the system is monitored weekly by the calculation of the root mean square statistics of difference between observations and model background fields (so-called misfits): http://oceanlab.cmcc.it/bsfs-evaluation/

The BSFS performs data quality control and rejection during the assimilation phase through an online procedure. In-situ data – temperature and salinity profiles – and satellite data – sea level anomaly and sea surface temperature – are checked with respect to 1) time and position quality check, 2) valid range and quality flags.

In the following, the rejection criterion applied to in-situ and satellite data as performed by the BSFS data assimilation scheme:

QC = 21 ARGO in-situ observation with data assimilation rejection in the upper water column
QC = 05 Sea level anomaly: level of no motion inapplicable

The Processing chain

The BSFS processing system consists on two different cycles (Figure 3). One cycle is daily, during which the system produces 3-days analysis, 1-day hindcast and 10-days forecast every day. Once a week, the BSFS performs a 14-days analysis, 1-day hindcast and 10-days forecast to incorporate a large number of in-situ and satellite observations into the data assimilation. The system produces 3D temperature, salinity, currents and 2D sea surface height, mixed layer depth and bottom temperature fields, as daily and hourly means. The available time series starts on Jan 2017 and is daily updated.

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Figure 3: The BSFS processing system

The BSFS catalogue DOI is here available:

https://doi.org/10.25423/cmcc/blksea_analysisforecast_phy_007_001_eas4

Essential references