Albedo

Albedo is the fraction of solar energy (shortwave radiation) reflected from the earth back into space.  Where reflectivity is:

• high (high albedo) less radiation is absorbed at the earth's surface.  In particular albedo is high where sea-ice is present or there is snow cover.
• low reflectivity (low albedo) more radiation is absorbed at the earth's surface.  The albedo depends upon the characteristics of the underlying surface.

Albedo is determined:

• 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 produced, 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.

  The albedo for:

• 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

Considerations

It is important to consider the effects of any day-to-day differences from model forecasts.  

Important points are:

• Albedo is not modified by the diurnal variation in solar zenith angle.
• Surfaces are assumed to be horizontal so no account is taken of land orientation or slope. 
• Differences in albedo that are not captured by the forecast (e.g. in mountainous areas or where observed conditions vary from forecasts).
• The albedo chart is purely climatological and anomalous conditions are not indicated.
• Widespread anomalous snowfall (increase of albedo) or snowmelt (decrease of albedo) will change the heat energy available to the varying surfaces.
• Transitory local but significant variations in surface characteristics (e.g. extensive burnt vegetation, seasonal variation in lake extent) are not represented.
• Tides can cover and uncover extensive areas of sand or mud (e.g. around the Frisian Islands) or the extent of a lake can vary substantially during the year (e.g. Lake Kati Thanda–Lake Eyre, Australia).

Temperature and consequent effects can be locally different where there are sub-grid scale variations in the forecast albedo (e.g. in 2 m temperatures or possible shower development, which in certain conditions in mountainous areas could both be more favoured over slopes facing the sun).

Additional sources of information

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

Read more on Albedo.

Fig2.1.18: Example surface albedo as shown in the surface parameters charts.  The albedo is taken from climatological data on a monthly basis.

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. 

 In the chart:

• 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). Anomalous snowfall has not had an effect.
• the sea is shown blue with high absorption because it reflects back very little solar radiation.

See the current albedo chart.