The density and temperature of snow is not usually uniform throughout the snowpack. Density at all snow levels is related to how much air is trapped, the ice or water content, and also linked to the temperature of the snow itself. The upper snow layer, especially if of fresh snow, is largely uncompressed and has relatively low density. Lower layers in the snowpack generally have greater density due to compaction by the snow above.
Heat flux differs through each layer of snow according to its density and temperature. Snow, especially new dry snow, is a good thermal insulator. Percolation, freezing and melting of water also has an effect upon the transfer, release and absorption of heat in each layer and on the surface. Nevertheless, the flux of heat through the snowpack, though relatively small, is important. In particular, upward heat flux from the ground through the layers of snow influences surface snowmelt and sublimation, and of course, surface temperature.
In snow-free areas, there is a ready exchange of heat, moisture and momentum between the atmosphere and underlying surface. Snow-covered regions have reduced heat conductivity, higher surface albedo, and reduced roughness compared to areas without snow. Thus for snowy areas there is an effective thermal, hydrological, and mechanical decoupling between the overlying atmosphere and underlying soil.
The skin temperature of the upper snow layer is governed by the balance between:
Forecasts of air temperature at 2m are derived from the forecast temperature at the lowest level of the atmospheric model and the forecast temperature of the model surface (the skin temperature). The skin temperature is itself derived using HTESSEL which employs one or more “tiles” to describe the characteristics of the land. These “tiles” evaluate heat fluxes into and from the underlying surfaces.
Calculation of the skin temperature of snow is rather complex and depends upon the characteristics of the snowpack throughout its depth.
To address this, a multi-layer snow model is used in two “tiles” within HTESSEL:
The IFS multi-layer snow model uses up to five layers to represent the snowpack and the complex heat fluxes and interactions between them. It represents the vertical structure and evolution of snow temperature, snow mass, density, and liquid water content in each layer. The energy flux at the top of the snowpack is the balance of the upward and downward energy fluxes at the snow surface including the effect of any snow evaporation. Albedo and surface fluxes vary according to the snow extent, depth and ground coverage (with account taken of trees in areas of forest), and age of the snow, . Heat flux from the underlying ground is also incorporated. The fluxes are illustrated and explained in Figs1.
The multi-layer snow model has a fairly realistic representation of the vertical density and temperature profiles of the snowpack which allows a good representation of its thermal properties.
The model represents:
When fresh snow falls or melts away, it is added to or subtracted from the top of the snowpack. Then the layers are reanalysed such that relatively shallow layers of snow are maintained at the top (5cm thick) and at the base (15cm thick) so that the atmosphere/snow and soil/snow heat fluxes can be best modelled.
The skin temperature (Tskin) over snow cannot rise above 0°C and any net positive heat flux at this temperature is used to warm or melt the snow layer. The flux of heat might be:
Over flat terrains:
Fresh snowfall is added to the top layer, with a new snow density depending on air temperature and wind speed. Melted snow is removed from the top layer. The snow mass is redistributed across the different layers but relatively shallow layers of snow are maintained at the top and at the base so that the atmosphere/snow and soil/snow heat fluxes can be best modelled. At snow depths between 12.5cm and 27.5cm additional snow is added proportionately to the layers as they are introduced. Where the snow depth is >27.5cm, only the layer 4 is used as the snow accumulation layer.
Fig1A: Schematic representation of the multi-layer snow scheme. Shallow snow layer. Snow depth <12.5cm. (Note: Snow depth < 10cm implies only a partial cover of snow)
Fig1B: Schematic representation of the multi-layer snow scheme. Deep snow. Snow depth >27.5cm. Any additional snow accumulation is added into the fourth snow layer in order to preserve the characteristics and thermal flux qualities of thinner layers at base and top of the snowpack. For snow depths between 12.5cm and 27.5cm additional snow is added proportionately to the layers as they are introduced.
rsnow | Conductive resistance between exposed snow and atmosphere | |||
rforest | Conductive resistance between forest snow and atmosphere | |||
KS | Downward short wave radiation | Ti | Temperature of snow layer i | |
LS | Downward long wave radiation | ρi | Density of snow layer i | |
HS | Sensible heat flux | Si | Mass of frozen water in snow layer i | |
ES | Latent heat flux | Wi | Mass of liquid water in snow layer i | |
RS | Net (precipitation and evaporation) water flux at the surface | di | depth of I-layer in the snowpack | |
aS | Albedo of exposed snow | Ki | Short wave radiation between snow layers I and I+1 | |
aF | Albedo of forest snow | Gi | Conductive heat flux between snow layers I and I+1 | |
Ri | Liquid water flux between snow layers I and I+1 | |||
TSO | Temperature of uppermost soil layer | GB | Conductive heat flux at snow-soil surface | |
WSO | Liquid water of uppermost soil layer | KB | Short wave radiation at snow-soil surface | |
dSO | Depth of uppermost soil layer | RB | Liquid water flux at snow-soil surface | |
rsoil | Conductive resistance between snow and soil |
Table1: List of symbols for parameters shown in Figs1.
A different algorithm is applied to define the snow layers in regions of complex or mountainous terrain where snow depth >25 cm. These layers are thicker than used for a snowpack with same depth over a flat region (e.g. in complex terrain an 85cm deep snowpack is discretised with layer depths: 16.00cm, 17.25cm, 17.25cm, 17.25cm, 17.25cm).
Complex terrain is defined as regions where the standard deviation of the sub-grid-scale orography is greater than 50 m. Ground height data from internationally available datasets at 1km resolution are interpolated to model resolution but smoothing misses important detail. Statistical parameters (e.g. standard deviations of the mean height, slopes, and direction of unresolved orography) are fed into the model via the sub-grid-scale parametrisation of orography.
In permanent snow areas (e.g. Greenland, Antarctica and glaciers) a fixed snow layering it is used. The top four layers (counting from the one in contact with the atmosphere) have a constant depths of 50 cm, whereas any additional snow accumulation is added into the bottom layer.
There is no representation of snow on top of sea ice or ice on lakes. Snow cover on ice acts to increase its persistence by increasing the albedo and reducing the heat flux into the modelled ice. Thin sea ice or lake ice covered by thin snow grows or melts much faster than does thick ice with deep snow.
Snow cover, snow depth and snow compaction affect all IFS atmospheric forecast models. It is important the IFS monitors actual values and updates the background fields accordingly. Any discrepancy will cause errors in the forecast as several physical properties of snow influence:
Model variables of snow need to be reanalysed at each analysis cycle. These are:
Snowfields are initialized every day at 00UTC from continuous offline data.
Snow data assimilation at ECMWF relies on:
Incorrect analyses and forecasts of snow are possible:
At high levels (altitudes >1500m) IMS data is not used and observations of snow depth are sparse or non-existent. In these cases snow depth prediction depends only upon the short range IFS evolution. Thus there can be little or no decrease in snow water equivalent (if it remains cold enough), though an increase after further forecast or actual snowfalls. Snow depths may also reduce because the density of the snow tends to increase with time through compaction in the model (and also in reality). Snow depths in such regions rise in response to forecast snowfall but may not decrease sufficiently at other times (See example in Fig2.1.12D).
Lake ice and sea ice do not have snow cover in the model.
The snow depth in the model changes when fresh snow falls or when snow on the ground melts, evaporates or is compressed. The response in dry periods at different altitudes is shown in Fig2.1.12D.
Snow depth is computed using:
At some high-latitude or ‘glacial’ grid points in the model it is common for snow depth to be extremely high.
Snow cover is diagnosed from the water equivalent of the modelled snow:
See the section Prognostic variables that affect energy fluxes for more information on snow data and its assimilation into the model.
Users should be aware of possible impacts on model forecasts, especially where snow cover and associated colder surface temperatures may persist for longer than they should and influence other parameters too.
Cloud and freezing fog strongly influence the energy fluxes into and from the snowpack. The IFS may not correctly capture or forecast the extent or thickness of cloud. It is very important to consider the possible formation, persistence or clearance of cloud and to assess the possible changes in energy transfer between cloud and snowpack. Thick cloud at any level will reduce solar radiation, but low cloud could be warmer than the underlying snow surface resulting in a net increase in downwards long wave radiation.
The characteristics of each grid box and areal extent of each tile type are updated through the forecast period and can vary in a rapid and interactive way.
model forecast snowfall might increase the area or depth of snow cover incorrectly. Partial cover of snow may become full cover as the accumulated model snow depth becomes >10cm. This means "tiles" in HTESSEL describing land surfaces may incorrectly cease to be used.
snow may accumulate then melt (e.g. with rain, or as as a warm front advances over a cold area).
Differing snowfall among the ENS members can cause increasing differences in evolution during the remainder of the model forecast period. Nevertheless each member remains equally probable.
The statistical information on the slope and aspect of orography within each grid box (e.g. south-facing, steepness) is not detailed enough for forecasts at an individual location. This can be important in mountainous areas and HTESSEL may under- or over-estimate solar heating and runoff. Incorrect analyses and forecasts of snow are quite possible at altitudes above 1500m in data sparse areas, or after a prolonged period without observations. Forecasts of snow depths can be too great at altitudes above >1500m due to insufficient melting of snow more especially at very high locations (e.g. Tibet).
Forest snow night time temperatures fall too low. Even if the forest is dominant, the vertical interpolation to evaluate T2m is done as for an exposed snow tile (because verifying SYNOP stations are always in a clearing). In reality, forest generated turbulence maintains turbulent exchange over the clearing and prevents extreme cooling.
(Note: In older material there may be references to issues that have subsequently been addressed)
Fig2.1.12A: Snow depth (cm) and sea-ice cover (%) in the high resolution forecast (HRES). DT 12UTC 07 Feb 2023 T+00. Note frozen lakes (e.g. NW Russia, north Caspian Sea, Uzbekistan) are also plotted as "sea ice". FLake represents or generates ice on coastal or inland water.
Fig2.1.12B: Conversion of background and forecast snow water equivalents to snow cover. Forecast snow water equivalents of 10cm or greater are considered as associated with full cover of snow on the ground; snow water equivalent of 5cm is considered to be associated with half cover.
Fig2.1.12C: Conversion of IMS information into an estimate of snow water equivalent for data assimilation. IMS delivers binary information on the presence of snow for each grid cell but does not give information on snow depth.
IMS strongly impacts upon any updates to the background snow depth field. Only if both IMS and background fields indicate snow is the IMS information not used.
Fig2.1.12D: Forecast snow water equivalent at high level stations (blue) and low level stations (red) during the winter of 2019/20.
At low levels background fields are updated using IMS data and numerous observations of snow depth. Forecasts show a gradual decrease in snow water equivalent during a dry period.
At high levels observations are more sparse and IMS data is not used (>1500m). Background fields rely on earlier snow depth forecasts. Forecasts show constant snow water equivalent during a dry period.