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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.

Lakes are plentiful in some areas (e.g. parts of Finland, Sweden, Canada, northwest part of Russia including Siberia).  In a relatively few areas elsewhere lakes have a notable impact on a regional scale (e.g. Wales, Switzerland etc).  The extent and location of lakes is taken from a lake cover mask consistent with the land-sea mask.

Open water surfaces have an important impact on the atmosphere - in particular:

• Areas of open water are strong local sources of moisture and are important for determining local and regional climates.

• The fluxes of heat, moisture and momentum over a lake differ significantly from the surrounding land, mainly due to large differences in albedo, heat capacity, roughness, and energy exchange.

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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 HRES and ENS).

Shallow coastal waters are difficult for the oceanic models (NEMO) to analyse or forecast.  Heat, moisture and momentum fluxes are evaluated according to the proportion of the area of the grid box that is covered by open water.  Where there is:

• more than 50% sea water cover then fluxes are dealt with by NEMO.
• more than 50% water cover, but the bodies of water are classed as lakes, then the HTESSEL 'tile' is "lakes and coastal waters" and fluxes are evaluated by FLake.
• less than 50% water cover, any sea that happens to be encapsulated the HTESSEL 'tile' is "lakes and coastal waters" and fluxes are evaluated by FLake (as if it were a salty water lake).

Each lake "tile" has its own description of the lake or coastal water and takes into account:

• the freshness or salinity of the water.
• the extent and mean depth.  This very important as it gives information on the reservoir of heat.
• the bathymetry.  FLake  uses an assumed temperature profile with: 
  • a near-surface mixed layer (implies a uniform temperature).
  • a thermocline (with upper boundary at the base of mixed layer, lower boundary at the lake bottom).
• the ice cover. FLake contains an ice model.
• (other parameters, such as base sediment, are scheduled to be incorporated in the future). 

Prognostic variables from the previous forecast are used for FLake initialisation.  

Image Modified

Fig3: Schematic of Lake tiles with heat and moisture fluxes from ice and water surfaces.  The volume of the water (given by area and depth) gives an indication of the capacity for heat storage.  Sufficient levels are modelled to define the thermocline (ξ). The skin temperature of the lake governs the heat and moisture flux to the lowest layer of the atmosphere.  Radiation to or from any ice cover is also modelled.  The coupling coefficient controls how tightly the skin layer temperature follows the temperature of the water or ice.

Table1: List of symbols for parameters shown in Fig3.

Considerations

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

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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.
  
  
• Another source of information on FLake (external to ECMWF) at:
  
• http://www.lake15.cge.uevora.pt/wp-content/uploads/presentations/FLakeInDWD-Dmitrii_Mironov_LAKE2015.pdf
• https://wgne.net/bluebook/uploads/2017/docs/09_Balsamo_Gianpaolo_CouplingOceansLandECMWF.pdf
• https://www.mdpi.com/2073-4433/12/6/723