TEMIS -- UV index & UV dose based on GOME -- archive

UV index & UV dose based on GOME	UV radiation monitoring
Overview of the data product archive

Brief product information of the UV index and UV dose
The clear-sky UV index is the effective UV irradiance (1 unit equals 25 mW/m2) reaching the Earth's surface under clear-sky conditions given for local solar noon, when the Sun is highest in the sky.
The UV index is computed from the assimilated global ozone field at local solar noon. With the use of forecast meteorological fields of the ECMWF and data assimilation, KNMI provides a forecast of the ozone fields. Hence, UV index forecasts for today and four days ahead can be made.
The daily UV dose, usually given in kJ/m2, is the "amount" of UV radiation that reached the earth, taking cloud cover into account, which means that it can only be computed afterwards.
The total ozone column data used in the data assimilation comes from: the GDP version 3.0 from DLR for the archive of monthly averaged UV index data; ozone data is available since July 1995, but as the first month of these data was used to initialise the data assimilation scheme, the UV index data is available since August 1995.
The algorithm that is used applies a functional relation between the clear-sky UV index, the local solar noon ozone fields and the solar zenith angle at local solar noon (Allaart et al., 2003), which has proved to work well. This parametrisation has been based on the UV irradiance at the ground weighted by an action spectrum. Currently two action spectra have been used:

The erythemal action spectrum
This is a model for the susceptibility of the caucasian skin to sunburn (erythema). It is proposed by McKinlay & Diffey (1987) and adopted as a standard by the Commission Internationale de l'Éclairage (International Commission on Illumination, CIE).
The DNA-damage action spectrum
This action spectrum is suggested by R.B. Setlow (1974) in a paper entitled "The wavelengths in sunlight effective in producing skin cancer: a theoretical analysis". A parameterization of this action spectrum was suggested by G. Bernhard and G. Seckmeyer (1997).

Three additional corrections are applied to further improve the UV index estimates:

correction for the varying Earth-Sun distance
correction for the elevation (the UV index increases with height)
correction for ground albedo
There is no correction (yet) for aerosols in the atmosphere, though since the algorithm was derived from real UV observation, there is some ("zero-th order") aerosol correction already built in the algorithm.

Integrating the UV index value from sunrise to sunset, with a time dependent solar zenith angle and 1-hourly Meteosat cloud cover information, then results in the daily UV dose. Since the Meteosat cloud cover is available only for Europe, the daily UV dose can be computed only for Europe, In order to have data for the whole globe, ISCCP cloud data base is used, but these are monthly averages and so the UV dose for the whole world is only given as monthly averages.
The local solar noon ozone fields are supplied with an error estimate of the analysed ozone fields. This error is converted into an error in the UV index and UV dose, and is reported in the data files. Possible errors in the other terms are not accounted for; these are expected to be smaller than the error due to an error in the ozone field. Errors reported in the data files with UV index and UV dose averages are the average over the errors, i.e. not an error on the average.
The UV index and UV dose are computed at latitude/longitude grid with cells measuring 0.5 by 0.5 degrees, which amounts to about 50 x 50 km at the equator. The resulting UV index values are written to a data file in HDF 4 format. Grid cell that lack sufficient data are marked as "no data": the have '-1' in the data file and are grey in the plot.

===> Structure and reading of the HDF data files

Current processing version: 1.2
Version history:

version date remarks cloud-cover correction

1.2 April 2008 reprocessed all the data in the UV index & UV dose archive with (a) ozone fields based on GDP-4 ozone data (was GDP-3), and (b) ISCCP cloud data extending the full data range (used to end mid 2001). as version 1.1

1.1 Feb. 2004 used to produce all the data in the UV index & UV dose archive 1.0 for: CCF < 0.02
0.5 for: CCF > 0.98
0.965081-0.255512*CCF
for: other CCF

1.0 April 2003 first operational version 1-0.5*CCF

version	date	remarks	cloud-cover correction
1.2	April 2008	reprocessed all the data in the UV index & UV dose archive with (a) ozone fields based on GDP-4 ozone data (was GDP-3), and (b) ISCCP cloud data extending the full data range (used to end mid 2001).	as version 1.1
1.1	Feb. 2004	used to produce all the data in the UV index & UV dose archive	1.0 for: CCF < 0.02 0.5 for: CCF > 0.98 0.965081-0.255512CCF for:* other CCF
1.0	April 2003	first operational version	1-0.5*CCF

References

M. Allaart, M. van Weele, P. Fortuin and H. Kelder: 2003,
"UV-index as function of solar zenith angle and total ozone,"
Meteorological Applications, in press.
A.F. McKinlay and B.L. Diffey: 1987,
"A reference action spectrum for ultra-violet induces erythema in human skin,"
International Congress Series 6, 17-22.
R. B. Setlow: 1974,
"The wavelengths in sunlight effective in producing skin cancer: a theoretical analysis",
Proc. Nat. Acad. Sci., USA, 71, No. 9, pp. 3363-3366.
G. Bernhard and G. Seckmeyer: 1997,
"Measurements of spectral solar UV irradiance in tropical Australia",
Journal of Geophysical Research, 102, No. D7, 8719-8730.
TM3-DAM: Assimilated ozone fields based on GOME data,
on-line documentation of the data assimilation software
The International HIRLAM Project
GOME fast delivery service at KNMI
GOME home page at DLR