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• for snow-covered areas:
  • at analysis time by a combination of the background climate (climatological 10-day average), satellite measurements, and observed surface fields (notably snow depth and extent).
  • throughout the forecast by snow cover or ice sheets that are developed, maintained or reduced by the model.  The albedo varies throughout the forecast period.  
• for snow-free land areas:
  • by a climatology from the MODIS satellite.  This is slow to change through the forecast period.

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The albedo varies throughout the forecast period and is determined at analysis and forecast times time via a combination of the background monthly climate, satellite measurements, and observed and forecast surface fields (notably snow depth and extent).  Snow cover or ice sheets that are produced by the model also modify the values throughout the forecast.  However, it should be noted that the snow-free land albedo is taken from a climatology from the MODIS satellite and thus is slow to change through the forecast period.  Also albedo is not modified by the diurnal variation in solar zenith angle and surfaces are assumed to be horizontal so no account is taken of land orientation or slope.  It is important to consider these aspects and their effects when appraising model forecasts (e.g. 2m temperatures or possible shower development, which in certain conditions in mountainous areas could both be more favoured over slopes facing the sun).

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(Note: In older material there may be references to issues that have subsequently been addressed)

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