UV index and UV dose: surface albedo correction
data version 2.x

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UV radiation
monitoring:
UV index
UV dose
Main data product description page
 

Surface albedo correction

The amount of UV radiation received by someone at the Earth's surface is affected by the local surface albedo: the larger the surface albedo, the more UV radiation. For this reason the UV index and UV dose calculation applies the following correction function of surface albedo (A), which takes multiple reflections between the surface and the overlying atmosphere into account: 
  [1]    f(A) = ( 1 — 0.25·0.09 ) / ( 1 — 0.25·A )
Since the UV index parametrisation is empirally based on UV data collected at De Bilt and Paramaribo, the A at these (urban) sites -- with a 12-month average value of 0.09 -- is used as a normalisation factor for the calculation of f(A).

The data for A at each 0.25°×0.25° UV grid cell are taken at 340 nm from the monthly GOME-2A surface albedo climatology -- available through this page -- interpolated linearly in time to daily values, associating the monthly average values with the 15th of the month.

For UV index & UVdose data version 2.0 the surface albedo (LER) climatology versions 2.1 is used, while as of UV data version 2.1 LER climatology version 3.1 is used -- see below for some notes on the differences.

The surface albedo climatology comes with an error estimate, which is also time interpolated and subsequently propagated to the UV index and UV dose error estimates.

If you would like to adjust the albedo correction from the grid cell average evelation A_grid to a specific albedo value A_new, equation [1] shows how, since f(A) is a multiplication factor in the UV index or UV dose calculation: 

  [2]    UV_new = UV_grid * f(A_new) / f(A_grid)
with UV_grid the UV index or UV dose value from a gridded TEMIS UV data file.

 

Update of surface albedo

For UV index & UVdose data version 2.0 the surface albedo (LER) climatology versions 2.1 is used, while as of UV data version 2.1 LER climatology version 3.1 is used.

The v3.1 surface albedo database has a higher quality because it is based on observations over a much larger period. Data from both GOME-2A and GOME-2C are used, covering a period of more than 10 years (2007-2018). Also, the spatial resolution of the database has been increased to 0.25°×0.25° for coastlines and snow-covered mountain ranges using a method called dynamic gridding.

As a result of this change the UV index & UV dose values will changes, notably along coast lines and snow-covered areas. Based on the above equation [2] it is easy to calculate the relative change in UV index & UV dose for each of the monthly albedo climatologies: 

  [3]    UV_factor = UV_v2.1 / UV_v2.0 = ( 1 — 0.25·A_v2.1 ) / ( 1  — 0.25·A_v3.1 )
An example of the albedo and UV_factor changes for 15 January is given on a separate page.

There are areas where the UV index & UV dose change by more than 5%, but this occurs mainly in areas far North and South, i.e. where the UV index & UV dose values themselves are low. Apart from along some coastlines, snow-covered mountain ranges, the edge of the Antarctic (sea-)ice, and a few individual points the changes in the UV index & UV dose are not big. For these reasons the data from before the switch of the albedo climatology is not reprocessed.

 

Daily UV surface albedo data

 
Month:  
Day:  
 

   Select a day in the menu on the left to access the file with the surface albedo at 340 nm used for that date in the TEMIS UV processing through the following links:
UV data version 2.0, albedo version 2.1 :   uv_albv2.1_0101.hdf

UV data version 2.1, albedo version 3.1 :   uv_albv3.1_0101.hdf

 
The daily surface albedo data is calculated for days in a leap year for all years in the UV data processing, i.e. there is no difference in the time interpolation in for different years; in non-leap years 29 February is simply not used.

The UV_factor of Eq. [3] above for the the 15th of each month, computed from the surface albedo climatology, is given in the following file:

uvi-factor-albedo-change.nc
By time interpolation, this can be used to manually adjust version 2.0 UV data from before the change to version 2.1.
The file furthermore contains the UV_v2.0 UV index data of the 15th of each month of 2016 as well as the UV_v2.1 computed from that with Eq. (3) -- these two dataset were used to create the images on the example page. 		 

 

References:

See citation and documentation section at the surface albedo page

 

last modified: 20 July 2022
data product contact: Jos van Geffen & Michiel van Weele & Ronald van der A
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