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

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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).

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Table1: List of symbols for parameters shown in Fig3.

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.
• 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 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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