Short description of UV-index and the NUV product

UV-index

The UV-index is a unit of measure of UV levels relevant to the effect on human skin. It is defined as the effective irradiance obtained by integrating the spectral irradiance weighted by the CIE action spectrum. The required protection at a given UV-index depends upon the skin type and behaviour, but generally some protection is required for UV-index above 3 and a high degree of protection (seek shade, avoid beeing outside during midday hours, sunscreen etc.) is required for UV-index above 8.

The UV maps

The maps represent clear sky UV-fields valid for local noon. UV-index contours are colour coded from 0 to 15 in steps of 1 dex. Black areas (UVI = 0) represent either grid points with no ozone data or locations where the SZA at noon is larger than 95 degrees (polar regions, local winter season).

Calculations:

The radiative transfer model employed is the widely used UVSPEC (Kylling, 1995) which is based on the discrete ordinate method DISTORT (Stamnes, 1989) and has been thoroughly tested for stability. The calculations are performed with total ozone as the only dynamical input parameter whereas climatological parameters are used for all other atmospherical input data as well as surface albedo.

Input ozone data:

• The first priority is the GOME Assimilated Total Ozone (ATO) produced by KNMI and delivered every night.

• The second priority, in case of no ATO delivery or corrupted ATO data, is the assimilated total ozone product produced by ECMWF.

• In case both ozone products above fail, a total ozone climatology based upon TOMS ozone data is applied

The NRT UVI processor includes the following parameters in the calculations:

• Current total ozone (Assimilated GOME)

• Astronomical parameters (solar zenith angle (SZA) and sun-earth distance)

• Ozone profile, climatology from AFGL (Anderson 1987)

• Albedo, climatology (Tanskanen, 2004)

• Aerosols, climatology from the Global Aerosol Data Set (GADS). A summer and winter data set are available (Kopke, 1997)

• Topography (ETOPO5 and GTOPO30)

The NRT UVI processor is based on look-up tables. In the look-up tables pre-calculated UV index values have been tabulated for various solar zenith angles, ozone amounts, albedo and ozone profiles. Five look-up tables have been produced for model atmospheres representative of conditions at different geographical latitudes and seasons. UV-indices are determined from the the tables by interpolating subsequently in ozone values, SZA and albedo. Finally corrections for sun-earth distance, aerosols and altitude are applied:

UVI = UVI_interpolated * K_sun-earth * K_AOD * K_altitude

where:

UVI_interpolated is the UV-index from the interpolations

K_sun-earth is the correction factor for actual sun-earth distance

K_AOD is the correction factor for Aerosol Optical Depth, which is parameterized by: K_AOD = exp(-0.5*AOD).

K_altitude corrects for the effect of altitude: K_altitude 1 + 0.05*altitude [km]

References

Anderson, G.P. et al. AFGL Atmospherical Constituent Profiles (0-120km). AFGL-TR-86-0110 (OPI), Hanscomb AFB, MA 01736, 1987.

ETOPO5, Data Announcement 88-MGG-02, Digital relief of the Surface of the Earth. NOAA, National Geophysical Data Center, Boulder, Colorado, 1988.

Tanskanen, A., Lambertian Surface Albedo Climatology at 360 nm from TOMS Data using Moving Time-Window Technique. In: Proceedings of the XX Quadrennial Symposium, 1-8 June 2004, Kos, Greece

Kopke P. et al. Global Aerosol Data Set. Report no. 243. Max Planck Institute fur Meteorologie. Hamburg, September 1997

GTOPO30, Global Digital Elevation Model, U.S. Geological Survey, EROS Data Center, South Dakota, 1993

Last modified: April 2008