Global 1-day UV index forecast

Description

Global one-day forecast of UV index both under clear-sky conditions and for the expected cloud cover with one hour time resolution.

Type

Forecast. The product is disseminated daily at 04:00 UTC at the latest.

Theoretical basis

The product is based on GOME-2/Metop-B and GOME-2/Metop-C assimilated total ozone (ATO) delivered by KNMI. Clear-sky UV index is calculated from ATO on 1-hour time steps, applying climatologies for surface albedo, aerosol optical depth etc. Forecast of cloud cover coverage for low , medium, and high altitude clouds are taken from the ECMWF Deterministic Atmospheric Model. An empirically established algorithm for the attenuation of the clear-sky UV index through the actual cloud cover is then applied and the result is the all-sky UV index.

Processing

Processing is performed at DMI. For each ATO grid point the UV index is calculated in 1-hour time steps, assuming the ATO value is valid for the whole day. ECMWF cloud cover forecast is spatially interpolated to the UV index grid, the algorithm for cloud cover correction applied, and the final output is a UV index grid where the time stamp for each grid point is shifted to local solar time.

Units and spatial resolution

UV index is a unit of measure of UV levels relevant to the effects on human skin. It is expressed numerically as the equivalent of multiplying the time weighted average effective irradiance (W/m²) by 40. The UV index maps are calculated in the same grid size as the input ATO field, with a sub-pixel altitude correction resolution of 0.25° x 0.25°.

Validation

This product is validated by the DMI. Validation is based on detailed comparison with ground-based measurements.

Online quality monitoring

The product quality is constantly monitored by DMI. On the NUV website various quality plots and files are updated daily.

Dissemination

See NRT access information.

File format

The all-sky (cloud cover corrected) UV index and clear-sky UV index are available at NUV website as NetCDF-4 files, containing the daily global grid. Other formats, subsets of the global grid etc. can be requested by registered users, see also NRT access information.

Algorithm version information

Time period	GOME-2A	GOME-2B	GOME-2C
11/02/2009 – 01/06/2010	1.4	N.A.	N.A.
02/06/2010 - 28/08/2012	3.1	N.A.	N.A.
29/08/2012 - 13/07/2013	3.2	N.A.	N.A.
14/07/2014 - 15/07/2020	N.A.	3.3	N.A.
16/07/2020 - 05/05/2025	N.A.	3.3	3.3
06/05/2025 onwards	N.A.	3.5	3.5