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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.  

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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:

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Table2.1.4.5-1: List of symbols for parameters shown in Fig2.1.4.5-1.

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:   

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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.

Measurement of soil moisture

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:

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Fig2.1.4.5-2: 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.

Soil moisture charts

Fig2.1.4.5-3: Examples of Soil Moisture at T+00 and T+192 DT 00UTC 06 March 2023.  

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Grid point data is plotted for Europe.  Elsewhere, for (most) other parts of the world, soil moisture is interpolated from surrounding grids points.  Field capacity, saturation, wilting point etc. depend on the soil type so can consequently be affected.  Users should check nearby soil moisture before accepting misleading soil moisture actual and forecast data.

Contrasting examples of surface and soil water budgets

Surface water budget in a typical mid-latitude agricultural landscape reacting to high rainfall in the model.

Recent periods of persistent rain over Britain over the winter of 2023/24 increased the soil moisture content in the river valleys and countryside around Reading.  Soil water storage in all model soil layers had been consistently between 120% and 150% of field capacity but generally below saturation.  Nevertheless there were areas of standing water in low-lying areas.

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Fig2.1.4.5-5: Example of surface and soil water budget.  DT12UTC 12 Feb 2024, VT12-14 Feb 20-24.  Temperate mid-latitudes.

Surface water budget in desert soil reacting to extreme rainfall in the model.

A tropical system moved over the Northern Territories, Australia depositing a period of significant rainfall.  

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Fig2.1.4.5-4: Example of surface and soil water budget.  DT00UTC 21 Jan 2024, VT21-23 Jan 20-24.  Desert areas.

Surface water budget in a dry desert

The model soil moisture charts sometimes show moisture layers below the surface in dry desert areas.  There is very little ground truth so there must be some uncertainty.

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Fig2.1.4.5-7: Afilal Oasis lies within the area where the model indicates soil with moderate moisture content in level 2 (~25% of Field Capacity) and level 3 (~40% of Field Capacity).  Level 1 has water content below the wilting point) and remains so as there is no vegetation and roots to bring water upwards from lower layers.  Nevertheless, subterranean water is locally sufficient to reach the surface at Afilal Oasis as springs. Soil moisture is a mean over a grid square.  Local details and individual oases are unlikely to be captured.  Soil moisture and soil type is not necessarily representative of an individual location.

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.
• Recycling of moisture by evaporation from surface often has an impact on maintaining cyclones over the dessert.
• Impacts of errors associated with soil moisture. 

Additional sources of information

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

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

(FUG Associated with Cy49r1)