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The increments shown are only a guide and do not imply that they occur in every case.
Temperature and relative humidity and assimilation response in soil moisture.
The data assimilation process modifies soil moisture in an attempt to compensate for errors in forecast temperature and humidity.
If first guess temperatures are:
- too high - assimilation decreases soil moisture.
- too low - assimilation increases soil moisture.
If forecast humidities are:
- too high - assimilation increases soil moisture.
- too low - assimilation decreases soil moisture.
Overall, the soil moisture assimilation of SYNOP data has a positive impact on the scores for forecast 2m temperature.
However:
- During cold/wet conditions that persist for a few days, each assimilation further removes soil moisture. The model soil then can become too dry compared with reality. Model rainfall can offset this effect.
- During warm/dry conditions that persist for a few days, each assimilation further increases soil moisture. The model soil then can become too wet compared with reality. Actual evaporation in hot weather can offset this effect.
To avoid excessive modifications to soil moisture:
- Soil moisture is not allowed to go below 0.01.
- Deeper soil moisture analysis is not carried out below the wilting point nor above the field capacity.
Generally, Level1 is a more freely evolving of the model soil layers. This is because it has the strongest coupling to the atmosphere so responds more rapidly and can recover quickly.
Fig9.2.1.3-5: 12UTC 09/07/2025. First guess differences from observations of 2m temperature and relative humidity and the response of assimilation on the level1 soil moisture. Over France the first guess 2m temperature is higher and 2m relative humidity is lower than observed values. Assimilation assumes the first guess values have been caused by excessive soil moisture and dries the level1 soil accordingly.
Fig9.2.1.3-6:00UTC 09/07/2025. First guess differences from observations of 2m temperature and relative humidity and the response of assimilation on the level1 soil moisture. Over France the first guess 2m temperature is lower than observed values. First guess 2m relative humidity is generally similar to observed values except for a few pockets. Assimilation assumes the first guess 2m temperature values (particularly across central parts) have been caused by insufficient soil moisture and moistens the level1 soil accordingly.
Fig9.2.1.3-7: An example of Layer1 soil moisture changes. Layer1 soil moisture analysis at DT00UTC 08/07/2025. Layer1 soil moisture IFS control forecast (ex HRES) T+48 VT 00UTC 10/07/2025. Layer1 soil moisture analysis at DT00UTC 10/07/2025. The Layer1 soil moisture over France appears to be drying out, but a lot more in the analysis than in the forecast. The forecast is insufficiently drying the soil within the 48 hours.
Summary of soil temperature errors:
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