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The reasons are not clear why IFS shows temperature and moisture differences and uncertainties in analysis and forecasts.  Possible causes and correlations:

• There is a dry bias during the dry season and first guess 2m temperatures can be too cold.  During the analysis process, soil moisture increments attempt to better represent 2m humidity.

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•  These increments can be substantial.  However, this results in excessive soil moisture near the surface despite no precipitation.  This moisture in IFS can then incorrectly percolate to lower layers or even runoff on the surface.  In contrast, the increase in soil moisture from actual rainfall is normally realistic and much less.   Increments can override the impact of any noteworthy rainfall.
• Surface ground moisture limits the amount of diurnal surface warming and nighttime cooling leading to lower 2m temperature and higher 2m relative humidity being observed and forecast.
• Vegetation could be lush and green after previous wet period and/or evapotranspiration could be mis-represented.
• Errors in forecast advection of boundary layer air.  Diurnal influxes of moist sea air may replenish moisture and/or not be well forecast either in depth or penetration.
• Observation representativeness.  Insufficient information on atmospheric structure due to the lack of radiosonde. 

Fig9.2.1.3-4: Mean soil moisture analysis increments for the top soil layers over the winter period 2024/25.  Systematic increments during the analysis process are:

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The increments shown are only a guide and do not imply that they occur in every case. 

Users should modify forecasts in the area of interest after assessment the IFS analysis of:

• temperature and dew points against observations.
• soil moisture and whether any increase is due to forecasts or observed rain, or perhaps due to model soil moisture increments.

ECMWF is currently investigating problems with soil moisture increments.

Summary of soil temperature errors:

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• comparing analyses of temperature, dew point and soil moisture with observed data.
• assessing soil moisture and whether any increase is due to forecast or observed rain, or perhaps due to model soil moisture increments.
• assessing future assessing future "background" conditions and the potential impacts thereof (e.g.  snowfall snowfall or cloud cover that might be different from atmospheric model predictions).

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Users should recognise the impact that low-level moisture has upon temperature forecasts; if humidity is too low then maximum temperatures can be forecast to be too high.  ECMWF is currently investigating problems with soil moisture increments.

See also Section 2.1.4.5 Modelling soil structure.

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