Albedo

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

Albedo is determined:

  The albedo for:

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:


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:

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:

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.