The soil is important since it represents the main land storage of heat and water available for release into the atmosphere later. The multi-layer soil model has a fairly realistic representation of the vertical density and temperature profiles of the soil which allows a good representation of its thermal properties.
Structure of the soil
The structure of the soil is not usually uniform throughout the top layers of the earth but for a given location is normally similar within the top ~1.3m of soil. Variations of density of the soil and fluxes of heat and moisture are more related to the texture and the water or ice content, and vegetation roots have an impact on the retention of water.
There is an exchange of heat, moisture and momentum between the atmosphere and underlying surface according to the vegetation type.
The IFS multi-layer soil model uses four layers to represent the top ~1.3m of soil and the complex heat fluxes and interactions between them. These are sufficient to represent correctly all timescales from one day to one year. The soil model represents the vertical structure of the soil and the evolution of soil temperature and liquid water content in each layer. Heat and moisture energy flux:
an interception layer collects water from precipitation and dew fall. Infiltration and run-off are represented depending on soil texture and standard deviation of subgrid orography. A fraction of the water flux (rain or snow melt) is considered runoff according to the soil texture, soil water content and the standard deviation of orography (runoff can be up to 30% of rainfall in complex orography or mountainous regions).
The fluxes are illustrated and explained in Figs2.
The characteristics of each grid box are updated through the forecast period (e.g. model snowfall might increase the area or depth of snow cover; model rainfall might increase soil moisture rather than be removed by run-off). The areal extent of each land surface tile type (listed above) can vary in a rapid, interactive way during the model run, as rain falls then evaporates or snow accumulates then melts, etc. The slope and aspect of orography within each grid box (e.g. south-facing, steepness) is not taken into account and HTESSEL may consequently under- or over-estimate solar heating and runoff.
The soil type for each land grid box is defined by an offline dataset and this soil type is used for all the layers. Each soil type has its own physical characteristics:
when the water flux at the surface exceeds the maximum infiltration rate the excess water is considered surface runoff.
(Note: In older material there may be references to issues that have subsequently been addressed)
Fig2.1.16: Schematic of four-level soil model with land surface tiles. Surface heat and moisture fluxes using maximum selection of six different surfaces ('tiles'). The four layers of soil have differing moisture contents which vary:
aS | Albedo of weighted average of tiled surfaces | Ti | Temperature of soil layer i |
KS | Downward short wave radiation | Fi | Mass of frozen water in soil layer i |
LS | Downward long wave radiation | Wi | Mass of liquid water in soil layer i |
HS | Sensible heat flux | Gi | Conductive heat flux between soil layers I and I+1 |
ES | Latent heat flux | Ri | Liquid water flux between soil layers I and I+1 |
RS | Net water flux at the surface (precipitation, evaporation, runoff) | RB | Water flux at base of model soil layer (Free draining, Downward only |
GB | Conductive heat flux at base of model soil layer = 0 |
Table1: List of symbols for parameters shown in Figs2.
Fig2.1.xx: Example soil moisture chart VT 00UTC 14 Feb 2023 showing moisture in soil level 1, the surface layer. The legend shows:
See the current soil moisture chart. Select "Layer 1 2 3" from the drop down menu for the average moisture in the top metre of the earth.