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• mountainous regions, particularly where model and actual surface altitudes are dissimilar.  
• snow covered areas, particularly in extremely stable conditions.

Diurnal

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Range of temperatures

The amplitude of the diurnal cycle is generally underestimated over land (a deficiency shared by most forecasting models).  This is especially the case in Europe during summer when the underestimation of temperature range reaches ~2°C across large areas.  Near-surface temperatures are generally too warm during night-time and slightly too cold during the day, although the degree to which the amplitude of the diurnal cycle is underestimated depends on region and season.  Night-time 2m temperatures are about 1–2°C too warm and surface temperatures about 2°C too warm.

Diurnal temperature changes are strongly influenced by incoming and outgoing heat flux. This is principally governed by the extent and thickness of cloud cover.  Model analysis and forecast of cloud

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Diurnal temperature changes are strongly influenced by incoming and outgoing heat flux. This is principally governed by the extent and thickness of cloud cover.  Model analysis and forecast of cloud and fog can have a strong impact on forecast errors.  

Most large errors seem to occur when the surface temperature is very cold and the lowest levels of the atmosphere may become extremely stable.  Tiny amounts of energy can correspond to large temperature changes at the surface because there is no convection to mix energy through the lower atmosphere.

Temperature errors often don’t depend strongly on the forecast range.

Effects contributing to temperature errors

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• Forecast maximum 2m temperatures can be too low particularly during anomalously hot weather.
• If the predicted humidity is too low then maximum temperatures can be forecast to be too high.
• Post-processing (e.g. using a calibrated statistical technique) usually improves 2m temperature forecasts, sometimes substantially.
• Model 2m temperature output corresponds to short grass cover, because by meteorological convention observations are ordinarily made over such a surface.  This strategy may not work so well in:
  • complex terrain - e.g. forests with clearings.
  • over snow areas.  The algorithm which derives 2m temperature uses the model surface temperature while the snow surface is above the earth's surface.  This can be an important consideration where there is deep snow.  See section on snow effects.  

Diurnal Range of temperatures

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Suggested considerations to offset temperature errors

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