Page History

Modelling the soil

The soil is important since it represents the main land storage of heat and water that is available for subsequent release into the atmosphere.  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.  

...

There is an exchange of heat, moisture and momentum between the atmosphere and underlying surface according to the vegetation type.   

The multi-layer soil model

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.  The heat and moisture energy flux is represented by the model:

...

Table1: List of symbols for parameters shown in Figs2.

Soil temperature

Soil temperature is a forecast variable in IFS.  It needs to be initialised at each analysis cycle but there are relatively few directly measured observations.  Soil surface (skin) temperature is derived from the expected air temperature structure in the lowest 2 m together with energy fluxes (from HTESSEL) and an analysis of observed screen level (2 m) temperatures.  

Soil moisture

Soil moisture is a measure of the water content within the ground.  It is commonly expressed as a percentage of the soil water content compared with the water that the ground could hold when fully saturated.  The evaluation and prediction of soil moisture is important as this governs the efficiency of evapotranspiration from vegetation.  Thus:   

...

Fig2.1.14: Measurements from the Soil Moisture and Ocean Salinity satellite mission (SMOS)  polar orbiter satellite data.  At L-band frequency (1.4 GHz) the surface emission is strongly related to soil moisture over continental surfaces. Surface radiation at this frequency is influenced by the vegetation layer (and hence soil moisture if the vegetation type is known), but proximity of lakes etc cause difficulties with interpretation.

Considerations

• Actual soil characteristics can vary widely within a grid box.  Users and forecasters should take into account the peculiarities of a location when interpreting model output.
• The assigned average soil type for a grid box is not necessarily representative of an individual location.
• Runoff can be up to 30% of rainfall in complex orography or mountainous regions. 

Additional sources of information

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

...