Modelling ocean surfaces

The majority of the surface of the earth is ocean and therefore the ocean/atmosphere interface is very important.  The wave model (ECWAM) provides information on sea-surface roughness and hence momentum loss in the boundary layer flow. The dynamic ocean model (NEMO) provides information on the sea-surface temperature.  Changes in these parameters as the forecast progresses impact strongly on monthly or seasonal forecasting.  This is particularly important with respect to El Niño/La Nina (ENSO) or other similar developments.

The state of the ocean surface can change on a daily time-scale.  Changes in the extent or movement of sea-ice, or modification of the structure of the upper ocean after the passage of hurricanes have an impact on the boundary energy and momentum interactions (e.g. a tropical cyclone can cool the sea surface through turbulent upwelling of colder water, particularly if the cyclone is slow-moving and/or the ocean's mixed layer is shallow).

Oceanic information is now derived in much the same way for all IFS model configurations.

• 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.  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)  
  

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:

• more than 50% sea water cover then fluxes are dealt with by NEMO.  A mixture of land and ocean 'tiles' is not allowed, i.e. a grid box is considered either 100% land or 100% ocean.
• more than 50% water cover, but the bodies of water are classed as lakes (e.g. large estuaries), then the HTESSEL 'tile' is "lakes and coastal waters" and fluxes are evaluated by FLake.
• less than 50% water cover, then the HTESSEL 'tile' is "lakes and coastal waters" and fluxes are evaluated by FLake (as if it were a lake).

Tides are not considered.