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The fluxes are illustrated and explained in Figs2Fig2145.A.  

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.

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• soil texture (defines water retention and hydraulic conductivity in the soil):
  • Coarse.
  • Medium.
  • Medium-Fine.
  • Fine.
  • Very Fine.
  • Extra-tropical Organic.
  • Tropical Organic.
• hydraulic properties (define amount of water in the soil and availability for vegetation):
  • saturation.
  • field capacity.
  • permanent wilting point.
  • residual moisture.
  • plant available soil moisture.
• infiltration capacity:
  • ability of water to percolate downwards from one layer to another (rain or dew at the surface).
  • surface evaporative fluxes consider separately the contributions from snow cover, wet and dry vegetation, and bare soil.
  • base of lowest layer is considered as free draining.
• surface run-off:

  • when the water flux at the surface exceeds the maximum infiltration rate the excess water is considered surface runoff.

• extraction of water by plant root:
  • root depth varies according to plant species as defined by the "tile".

Fig2Fig2145.1.16A: Schematic of four-level soil model with land surface tiles.  Surface heat and moisture are illustrated in the schematic using the maximum selection of six different surfaces ('tiles').  The four layers of soil have differing moisture contents which vary:

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Table1Table2145.A: List of symbols for parameters shown in Figs2Fig2145.A.

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.  

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• 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 2.1.14Fig2145.C).

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. 

Fig2Fig2145.1.13B: Examples of Soil Moisture at T+00 and T+192 DT 00UTC 06 March 2023.  

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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.

Fig2Fig2145.1.14C: 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.

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