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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.

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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)  
  

Coupling of sea surface temperature between IFS atmospheric and oceanic models.

Importance of atmospheric/ocean coupling

IFS needs to model the exchange of heat, moisture and momentum at the interface between the model atmosphere and the underlying model surface.  The ocean-atmosphere coupling is achieved by a two-way interaction:

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Fig2.1.4.3-1: An example of the beneficial effect of using coupled atmospheric/ocean to realistically simulate the cool wake after a tropical cyclone (TC Neoguri in 2014).  The forecast changes in sea surface temperature agree closely with those measured by DRIBUs close to the wake of the cyclone.  Particularly well modelled is the sharp fall in sea surface temperature after passage of the tropical cyclone followed by successive pulses of warmer and colder water.

Importance of analysis of sea surface temperatures

IFS needs a representative and timely analysis of sea surface temperatures and sea-ice to derive radiances over the ocean surfaces.  These factors are important because:

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• 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.

Full atmospheric/ocean coupling

Full atmospheric/ocean coupling was used by IFS Cy44R and earlier.  

The ocean model (NEMO) takes sea surface temperature from the ocean data assimilation system (NEMOVAR and OCEAN5).  Sea surface temperature observations may be deficient both spatially and in timeliness.

Partial atmospheric/ocean coupling 

Partial atmospheric/ocean coupling is used by IFS Cy45R1 and later.

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Fig2.1.4.3-2: Schematic diagram showing partial coupling is used for the first four days of the forecast then gradual reduction between day4 and day8 followed by full coupling after day8.

Advantages

• There is assimilation rather than interpolation of OSI-SAF sea-ice.
• Small-scale structures in the sea surface temperature field of OSTIA are preserved.
• The coupled model is able to simulate realistically the cool wake after a TC (and possibly also after deep depressions).  Verification is often difficult as there are only few available relevant observations. See Fig.
• It moves sea surface temperature and sea-ice analysis closer to the 4D-VAR analysis, and with better timeliness.
• Tropical sea surface temperature analyses from dynamic ocean assimilation systems seem to be better than no-model assimilation systems.
• Atmosphere/ocean interface are more dynamically consistent.
• There is an increase in atmosphere/ocean coupling in the analysis.
• Ensemble scores improve (e.g. root mean square errors in Geopotential heights) decrease.

Problems

• The increased variability in sea surface temperatures can lead to difficulties in verification.

Modelling coastal waters

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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See also Section on Lakes and Coastal Waters

(FUG Associated with Cy49r1)