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• open snow (includes snow on low vegetation) changes with age but is broadly ~0.85 for fresh snow to ~0.50 for old snow.  
• forest snow (snow beneath high vegetation) depends on vegetation type.  It is broadly ~0.3 just after a heavy snowfall  decreasing to ~0.2 after a few days due to snowmelt (for sufficiently warm temperatures) or wind drift (for cold temperatures).
• ice depends on snow cover but varies between ~0.7 with snow and ~0.4 without snow.
• open water, lakes and sea is <0.1

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).

The albedo used by IFS is a climatological 10-day average.  However the user should be alert to potential local temporary variations which cannot be modelled such as:

• due to tides that can cover and uncover extensive areas of sand or mud (e.g. around the Frisian Islands) or where the extent of a lake varies substantially during the year (e.g. Lake Kati Thanda–Lake Eyre, Australia).
• due to large areas of burnt vegetation after extensive forest fires.

 Image Added

Fig2.1.15:  Illustration of the albedo field used by the model during the forecast:

• darker colours indicate the greater radiation absorption qualities of vegetation
• lighter colours indicate areas of high reflectivity (snow over mountains and Russia, and from the sands of the Sahara),
• the sea is shown blue with high absorption because it reflects back very little solar radiation. Solar noon is assumed when assigning model albedo values and so is probably an under-estimate of the true albedo for most of the time in daylight. 

Considerations

It is important to consider the effects of any day-to-day differences in albedo values between those deriving from actual conditions and those used in model forecasts.  

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