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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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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 FLakeby 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 by FLake (as if it were a salty water lake).

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

Fig3Fig2.1.4.2-1: 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.

Table1Table2.1.4.2-1: List of symbols for parameters shown in Fig3Fig2.1.4.2-1.

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 nighttime 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.

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