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, also Siberia).  In a relatively few areas elsewhere lakes have a notable impact on a regional scale (e.g. Switzerland, 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.

Areas of open water have an important impact upon the atmosphere - in particular:

  • 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.

  • 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 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)

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 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).

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 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). 

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


Fig2.1.4.2-1: 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.

aSAlbedo of water or iceTiTemperature of layer
KSDownward short wave radiationTskinTemperature at the surface of the water
LSDownward long wave radiation

Tξ

Thermocline
HSSensible heat fluxSySalinity
ESLatent heat flux

RSNet water flux at the surface (precipitation, evaporation, runoff) 

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)








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