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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.

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Table1: List of symbols for parameters shown in Figs2.Image Removed

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Soil temperatures

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.  Its evaluation and prediction is important as soil moisture governs the efficiency of evapotranspiration from vegetation:

• If soil moisture is too little (below the Permanent Wilting Point, PWP) then plants die.
• Higher soil moisture implies greater evapotranspiration efficiency, reaching a maximum (Field Capacity, CAP) when plants can flourish best.  The soil does not have to be saturated for this to happen.  As soil moisture increases beyond this the efficiency of plant evapotranspiration stays the same.
• If soil moisture is above Field Capacity then there is more water in the soil than it can hold with possible flooding.  

For each soil type and location there is a pre-defined value of the ability to hold moisture and this is used to assess the impact of model rainfall.  The HTESSEL system includes allowance for water capture by interception of precipitation and dew fall, and at the same time, there are infiltration and run-off schemes that take account of soil texture and the standard deviation of sub-grid scale orography.

Soil moisture is a forecast variable in IFS.  It needs to be initialised at each analysis cycle but there are very few directly measured observations.  Soil surface (skin) moisture is derived from:

• the expected air temperature and moisture structure in the lowest 2 m together with energy fluxes (from HTESSEL) and an analysis of observed screen level (2 m) humidities.
• satellite soil moisture data from the ASCAT sensor on the MetOp satellites
• data from the Soil Moisture and Ocean Salinity satellite mission (SMOS) is (used for operational monitoring (see Fig 2.1.14).

The 2m temperature and humidity are diagnostic parameters of the model, so their analysis only has an indirect effect on atmosphere through the soil and snow variables. 

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Fig2.1.13: Examples of Soil Moisture at T+00 and T+192 DT 00UTC 06 March 2023.  

xx:Example soil moisture chart VT 00UTC 14 Feb 06 March 2023 showing moisture in soil level 1, the surface layer.  The legend shows:

• Sandy shades: Soil moisture SM < Permanent wilting point PWP.  Living vegetation cannot be sustained. Values show soil moisture as a percentage of the permanent wilting point  value.
• Yellow/Green shades: Permanent wilting point PWP < soil moisture SM < field capacity CAP.  Evapotranspiration efficiency in percent increases as soil moisture increases.
• Blue shades: Capacity CAP < Saturation SAT.  Soil moisture super-saturation. Dark blue suggests flooding (in the model).

Note the change in soil moisture over France from ~60% field capacity (greens) to above 60% saturation (blues). This is largely due to rain exceeding evaporation in these areas during the forecast period.  Conversely, parts of northern Morocco, northern Algeria and northern Tunisia have become a little drier.

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.

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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.

Read more on Soil Moisture and Evapotranspiration Efficiency with example chart.