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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.

Relationship between 2m temperature and humidity with soil moisture 

It has been observed with Cy49r1 that even quite significant amounts of forecast rainfall had only minor impact on the modelled soil moisture.  The impact of soil moisture increments induced by observed 2m temperature and 2m humidity was greater than the effect of rainfall.  Extremely active soil moisture increments in drier periods generally overrode the impact of any noteworthy rainfall.  This has caused some unrealistic results.   

The effect of positive/negative increments to model soil moisture has been to increase/reduce model 2m temperatures or reduce/increase model 2m RH.  A reverse effect was also seen; a wetter observation of 2m humidity was acting to dry the soil, or warmer observation of 2m temperature was acting to wet the soil.

To combat this effect, the soil moisture adjustment scheme in Cy49r1 has been adjusted with effect from 14 May 2025.  From that date, no soil moisture increments are made:

• to use the effect to raise/lower model 2m temperatures where model soil moisture values are too low/high.
• to use the effect to raise/lower model soil moisture where observed 2m temperatures are too high/low.
• to use the effect to lower/raise model 2m humidity values where model soil moisture values are too high/low.
• to use the effect to lower/raise model soil moisture where observed 2m relative humidity values are too high/low.
• where soil moisture is above field capacity.                          

By day, these modifications result in a better representation of 2m temperature and 2m humidity.

By night, experience suggests lower/higher soil moisture usually results in lower/higher 2m temperatures.  But the adjustment scheme now in use means there will be no soil moisture changes even though this might be desirable.

The relationship between night time 2m dew points and soil moisture is uncertain.

Probably daytime-based adjustments are the more substantive.

This is a very complex area.  It is very difficult to get things right.  Cycle 49r1 included substantial changes to vegetation, flux representation, etc.  It is also very challenging to balance the needs of, for example, hydrologists and fire prediction algorithms, which can be contradictory, particularly when the model has intrinsic biases for which soil moisture is only one small part of the equation.

Image Added

Fig2.1.4.5-6: Forecast 24h ppn to 00Z 03Apr25, DT00Z 02Apr25.  Rain over SW France is 10-30mm.  The impact on soil moisture metric in Levels 1,2,3 between 00UTC 02Apr and 00UTC03 Apr is very minor.  There is even some drying out in the wettest areas shown at T+0 .  To change soil moisture metric appears to need a lot of rain. 

Surface water budget in a dry desert

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Soil moisture charts consistently give an indication of water below the surface in mid-Sahara (near 23N 7E).  This should not be relied on.  However, it may well be correct as the area is around the oasis town of Tamanrasset in Algeria.  There is indication of water in layers 2 and 3 in Arabia, locally as high as 60%, but there is little data to confirm this.

 Fig2.1.4.5-67:  Example of soil moisture in desert areas.  DT and VT 12UTC 07 Sep 2023.  In the area around Tamanrasset (Algeria) and much of Saudi Arabia, soil moisture charts show dry surface layers (level 1, orange) and about 20% moisture in lower layers (levels 2 and 3, green) and locally as high as 60% in Saudi Arabia.

Fig2.1.4.5-78: 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  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.

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• 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.
• Soil moisture increments have less impact on 2m temperature and 2m humidity (and vice versa) since 14 May 2025.
• Recycling of moisture by evaporation from surface often has an impact on maintaining cyclones over the dessert.
• Availability of water in the soil for plant uptake allows plant roots to better extract water, especially in relatively dry conditions.  Evapotranspiration can affect the surface specific humidity.
• Impacts of errors associated with soil moisture. 

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