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• Initial sea surface temperatures and sea ice concentration are given by NEMOVAR.  In practice the following procedures are adopted to deliver the T+0 fields used as the starting point for the coupled model forecasts:
  
  • For Sea-Ice, the ocean analysis assimilates OSTIA sea-ice fields and in effect is blended with the background (as happens in atmospheric assimilation).
  • For Sea Surface Temperature (SST), the latest OSTIA sea surface temperature analysis (re-gridded) is used but the approach depends on latitude.  Between 20S and 20N the NEMO ocean sea surface temperature analysis is relaxed towards the latest OSTIA sea surface temperature analysis.  North of 25N and South of 25S, the latest OSTIA sea surface temperature analysis is used unchanged.  Between 20 and 25 degrees a hybrid of these approaches is used.
    
• Throughout the forecast period, NEMO provides the oceanic temperature structure near and at the surface.  ECWAM provides wave data, and therefore an indication of surface roughness.  From these, fluxes of heat, moisture and momentum are evaluated for passing to the lowest layers of the atmosphere by partial coupling (and later in the forecast by full coupling).  The formation, evolution and decay of ice over open waters is controlled by LIM2 (part of NEMO).  In effect, NEMO and LIM2 together move ice around (according to ocean drift etc.) and melt or form ice (according to sea-surface temperatures etc.).  The albedo over the sea-ice surface uses a climatology prescribed in the IFS rather than the model albedo of LIM2 (this is because LIM2 does not model some of the key processes important for albedo such as melt ponds).  Note: ECMWF uses LIM2 which is an earlier version of the Louvain-la-Neuve sea ice model currently available (Version 3.6)  
  

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Difficulties in the assimilation of sea surface temperature:

• OSTIA and EUMETSAT OSI-SAF data are sometimes quite delayed – possibly by some 60hr.
• Uncertainty whether it’s better to adjust IFS model sea temps to sea ice extent or vice-versa.
• Rapid changes in sea surface temperature (e.g. upwelling) take a long time to be absorbed into OSTIA.  Rapid changes are captured rather better with NEMOVAR and OCEAN5 than OSTIA.  Errors can lead to:
  • over prediction of intensity due to unrealistic heat availability from the ocean.
  • day-to-day consistency is not always good.
  • spurious or missing areas of ice.

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For a variety of reasons coastal regions are important for many customers.  Seas immediately adjacent to coastlines are difficult for the oceanic models (NEMO) to analyse or forecast, so coastal areas are dealt with by FLake as if they were salty water lakes.  Heat, moisture and momentum fluxes are evaluated according to the proportion of the area of the grid box that is covered by open water defined by the Land-Sea Mask.  Where there is:

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