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Modelling ocean surfaces
The majority of the surface of the earth is ocean and so 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 and hence heat exchanges in the boundary layer flow. 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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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 full coupling. The formation, evolution and decay of ice over open waters is controlled by SI3 (part of NEMO). In effect, NEMO and SI3 together move ice around (according to ocean drift etc.) and melt or form ice (according to sea-surface temperatures etc.).
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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- In the extra-tropics from:
- In the tropics from:
- Sea-ice forecasts are derived from the ocean model (NEMOVAR and OCEAN5).
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 Fig2A.1.4.3-1.
- Sea surface temperature and sea-ice analysis are closer aligned 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.
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:
- the low-level atmosphere affects the ocean through its:
- heat (causing changes in sea surface temperature and stability in the ocean).
- moisture and density (by precipitation and/or evaporation).
- low-level wind (affecting sea surface currents and causing wave development).
- the ocean affects the atmosphere through its:
- sea surface temperature (causing changes in stability in the lower atmosphere).
- ocean surface current (causing changes in roughness and momentum exchanges).
- ice concentration (causing changes in roughness, momentum exchanges and boundary layer cooling).
Full atmospheric/ocean coupling (from Cy50)
Full atmospheric/ocean coupling is used by Cy50r1.
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 (pre Cy50)
Partial coupling was used to day 4 then gradually switched off (if sea surface temperatures from atmospheric and ocean models are in line). Partial atmospheric/ocean coupling was used by IFS Cy45r1 to Cy49r1.
Fig2A.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.
Fig2A.1.4.3-2: The cold trace due to the passage of tropical cyclone Erin is clearly visible in the Sea Surface Temperature anomaly from the ocean monitoring valid 24 August 2025. The colder sea temperatures have an impact on energy flux from the ocean and hence affect the development of subsequent depressions passing across the colder waters.
Modelling coastal waters
For a variety of reasons coastal regions are important for many customers. Sea that is 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
Additional Sources of Information
(Note: In older material there may be references to issues that have subsequently been addressed)
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