Erythemal UV index from SCIAMACHY -- brief product information

Near-real time UV index:

Brief product information

The erythemal 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 total ozone column data used in the data assimilation comes from the near-real time service of global ozone fields based on observations by the SCIAMACHY instrument.

The algorithm that is used applies a functional relation between the erythemal 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 the CIE spectral action function. The CIE action spectrum 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).

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.

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 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 averages are the average over the errors, i.e. not an error on the average.

The UV index is 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. The UV index data is then used for making plots.

Note that the data files themselves are not made available here in order to prevent confusion: data and images are namely updated 3-4 times a day, when new ozone field information is available.

Current processing version: 1.1

Version history:

version	date	remarks
1.1	29 January 2004	updated to also process SCIAMACHY and some other minor changes
1.0	14 April 2003	first operational version

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.
TM3-DAM: Assimilated ozone fields based on GOME data,
on-line documentation of the data assimilation software