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- sensible heat fluxes and latent heat fluxes
- downward long wave and shortwave radiation
- upward fluxes of heat through the layer(s) in the snowpack from the underlying ground.
Forecast air temperature at 2m is derived by interpolation between:
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Generally, skin temperature is derived using HTESSEL. This uses one or more “tiles” describing the land characteristics and evaluate heat fluxes into and from the underlying surfaces.
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To address this, a multi-layer snow model is used in two “tiles” within HTESSEL:
- exposed snow - for areas without vegetation above the snowpack, and therefore without interference to the weak incoming and outgoing heat and momentum fluxes.
- forest snow - for areas where there is tree cover sufficient to interfere with incoming and outgoing heat and momentum fluxes. The heat flux at the snow/atmosphere interface is rather larger than over exposed snow.
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Forecasts of 2m air temperature use the skin temperature of the snow as if it were at the model ground surface rather than at the elevation of the snow surface. This may lead to errors in derivation of 2m temperatures forecast in cases of deep snow.
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Representation of snow on top of sea ice or ice on lakes is introduced in Cy50r1 in May 2026. Coupling between the sea ice model to the atmosphere allows sea-ice and variable ice thickness to be represented in the atmospheric forecast model. Snow cover on ice acts to increase its persistence by increasing the albedo and reducing the heat flux into the modelled ice. Reduces the warm bias seen in winter over the ice surface, especially in cloud free situations. Thin sea-ice or lake-ice covered by thin snow grows or melts much faster than does thick ice with deep snow.
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Representation of glaciers is introduced in Cy50r1 in May 2026 by use of a new land-ice tile within HTESSEL. This allows representation of the proportion of land-ice or glaciers that partly (or wholly) cover the grid box. The parametrisation scheme uses four layers of equal thickness on top of the soil column. There is an exchange of energy and water fluxes between the ice and the atmosphere and also at the bottom where the soil (ground) temperature is used as boundary condition and to compute the ice basal heat flux.
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- Total snow cover is assumed where snow depth is diagnosed as >10cm. Only exposed snow or forest snow "tiles" are used by HTESSEL.
- Partial snow cover is assumed where snow depth is diagnosed as <10cm. A snow water equivalent of 6cm is considered to be associated with 60% snow cover (Fig2A.1.4.4-7). In addition to the exposed snow or forest snow "tiles"the remainder of the grid-box is described by other HTESSEL "tiles".
Snow is not intercepted by a tree canopy and will accumulate on the ground. Snow accumulation on glaciers, land-ice, sea-ice and lake-ice was introduced in Cy50r1 in summer 2026.
The albedo of snow in forested areas is given by a look-up table depending on (high) vegetation type (Table 2.1.4.4-2). The albedo of exposed snow decays with time between 0.85 for fresh white snow to 0.5 for older snow. It is reset to 0.85 after large snowfall events.
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Some considerations should be made before issuing forecasts when interpreting model output of snow.
Fig2A.1.4.4-6:. Weather station at Røldalsfjellet (Norway). The temperature sensor is mounted at 5m above the ground (left picture). This allows sufficient clearance beneath the sensor with high snow accumulation (right picture). Photos:MET Norway.
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- IFS DOCUMENTATION – Cy48r3 Operational implementation PART IV: PHYSICAL PROCESSES.
- Hydrological Impact of the New ECMWF Multi-Layer Snow Scheme
- Impact of a Multi-Layer Snow Scheme on Near-Surface Weather Forecasts
- Read more on the model snow depth and sea ice with example chart and model sea-surface temperature with example chart.
- Read more on snow data assimilation or land surface monitoring by satellite (LDAS).
- Reads more on modelling of glaciers.
(FUG Associated associated with Cy50r1).
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