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 (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 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.

  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.
• melt ponds on otherwise extensive sea ice can reduce albedo locally.
• 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.

Fig2.1.4.6-1: 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.

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.  

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.
• Albedo can be affected by melt ponds over polar sea ice sheets.
• 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.