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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 , also Siberia).  In a relatively few areas elsewhere lakes have a notable impact on a regional scale (e.g. Wales, Switzerland etcSwitzerland, Hungary (Lake Balaton), East Africa (Lake Victoria), Cambodia (Tonle Sap), Peru (Lake Titicaca) or similar).  The extent and location of lakes is taken from a lake cover mask consistent with the land-sea mask.

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

  • Areas of open water They 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.

  • Cause They cause stabilisation or destabilisation of the temperature structure of the real and model atmosphere.  The effects of a lake can provide:

    • a cooling effect during spring and summer.
    • a source of heat and moisture during autumn - this aids cloud formation and/or intensification of precipitation.
    • weaker effects overall during effects overall during the ice covered period -  though can be very significant locally.
  • 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)

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  • 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 is classed as a lake and the HTESSEL 'tile' is "lakes and coastal waters" and fluxes are evaluated by FLake (as if it were a salty water lake).

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  • the freshness or salinity of the water.
  • the extent and mean depth.  This very important as it gives information on the reservoir of potential capacity for heat storage.
  • 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). 

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