Daily erythemal UV dose from SCIAMACHY -- product information

near-real time UV dose:

last two days | archive

Brief product information

The daily erythemal UV dose, usually given in kJ/m2, is the "amount" of UV radiation that reached the earth, taking cloud cover into account. Note that the UV dose data is still preliminary and that a validation has to take place.

The UV dose 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. 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 UV dose.

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 1-hourly Meteosat cloud cover fraction (CCF) information needed in the integration is at the moment available only for the region of Europe used in the HIRLAM model, which is in use at KNMI. Areas where no cloud information is available are marked as "no data" and show up as grey in the plots. Note that on some days cloud information is missing altogether, so that the UV dose cannot be computed.

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.

===> Structure and reading of the HDF data files
[no link back to this page]

Current processing version: 1.1

Version history:

version	date	remarks	cloud-cover correction
1.1	10 February 2004	updated to also process SCIAMACHY and some other minor changes	1.0 for: CCF < 0.02 0.5 for: CCF > 0.98 0.965081-0.255512CCF for:* other CCF
1.0	14 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.
TM3-DAM: Assimilated ozone fields based on GOME data,
on-line documentation of the data assimilation software