Page History

Modelling lake and coastal water surfaces

Within each grid box lake or shallow coastal waters are treated as a further HTESSEL tile.  Its influence is proportional to the coverage of 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.  This is ainly 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 resolved lakes, reservoirs, and rivers, which cover ≥1% surface area of each grid box (i.e. around 2km² for Ensemble control forecast grid resolution).

...

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

Image RemovedImage Added

Fig2A.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.

...

Table2.1.4.2-1: List of symbols for parameters shown in Fig2A.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).
• 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.
• The fluxes of heat, moisture and momentum differ over open water and ice.

Additional sources of information

(Note: In older material there may be references to issues that have subsequently been addressed)

...