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Modelling lake and coastal water surfaces

Lake or shallow coastal waters are treated as a further HTESSEL tile within a grid box with its influence proportional to the coverage of water.

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• The presence or absence of lake ice:
  • can drastically change the amount of snowfall downstream of lake areas.
  • modifies heat fluxes from the lake - but heat flux through ice cover is nevertheless greater than for surrounding frozen land.

The fresh water lake model (FLake)

The Fresh-water Lake model (FLake) is a lake and shallow coastal waters parametrisation scheme that evaluates the fluxes of heat, moisture and momentum over areas of water.  It includes all sub-grid and resolved lakes, reservoirs, and rivers independent of their size and shape where the water covers ≥1% surface area of each grid box (i.e. around 2km² for Ensemble Control Forecast (ex-HRES) grid resolution).

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Table2.1.4.2-1: List of symbols for parameters shown in Fig2.1.4.2-1.

Considerations

The presence of lakes has a range of effects on weather and local climate:

• In mid-latitude regions, lakes help to foster mild micro-climate conditions by acting as thermal inertial bodies, and they trigger locally higher precipitation rates.  This happens especially when lakes are shielded by mountainous regions, which is often the case given the geomorphological origin of many lakes (e.g. Lago Maggiore area straddling Switzerland and Italy).
• in high latitude regions, lakes tend to freeze almost every winter.  It is important to predict when that happens as freezing changes the surface albedo and thermal capacity.  This affects the surface fluxes exchanged with the atmosphere.  In winter this can make the difference between light or heavy snowfall downwind from a lake (e.g. as often seen in the vicinity of the Great Lakes).
• Currently there is no representation of snow on top of ice, the effect is:
  • to allow rather too much heat to transfer downwards or upwards.
  • to reduce the albedo from more realistic high values.
  • to reduce the albedo locally where melt ponds exist or form on otherwise extensive sea ice.
• In temperate and tropical areas, lakes are often linked with high-impact weather by contributing to the formation of convective cells.  This happens mostly at night due to moisture convergence and breeze effects (e.g. regularly occurring in Lake Victoria, one of the African Great Lakes).
• Currently there is no representation of base sediment so flux of heat to or from the underlying soil is not included.
• Extensive areas of sand or mud exposed by low tides and re-covered by incoming tides are not considered.

Additional sources of information

(Note: In older material there may be references to issues that have subsequently been addressed)

• Read more information on the background of FLake.
• Read more on the impact of lakes on the ECMWF surface scheme or on lakes in weather prediction or a discussion of the impact of interactive lakes in the IFS (pages 30-34).
• Read more in depth information on the contribution of lakes in predicting near-surface temperature in NWP.
  
  
• Other sources of information on FLake (external to ECMWF) are at:
• https://wgne.net/bluebook/uploads/2017/docs/09_Balsamo_Gianpaolo_CouplingOceansLandECMWF.pdf
• https://www.mdpi.com/2073-4433/12/6/723

(FUG Associated with Cy49r1)