The surface ultraviolet (UV) radiation product, version 1.20,
generated operationally in the framework of the Satellite
Application Facility on Ozone and Atmospheric Chemistry Monitoring
(O3M SAF) of the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) is described. The product is
based on the total ozone column derived from the measurements of the
second Global Ozone Monitoring Experiment (GOME-2) instrument aboard
EUMETSAT's polar orbiting meteorological operational (Metop)
satellites. Cloud cover is taken into account by retrieving
cloud optical depth from the channel 1 reflectance of the third
Advanced Very High-Resolution Radiometer (AVHRR/3) instrument aboard
both Metop in the morning orbit and Polar Orbiting Environmental Satellites (POES) of the National Oceanic and Atmospheric
Administration (NOAA) in the afternoon orbit. In addition,
more overpasses are used at high latitudes where the
swaths of consecutive orbits overlap. The input satellite data are
received from EUMETSAT's Multicast Distribution System (EUMETCast).
The surface UV product includes daily
maximum dose rates and integrated daily doses with different
biological weighting functions, integrated ultraviolet B (UVB) and ultraviolet A (UVA) radiation,
solar noon UV index and daily maximum photolysis frequencies of
ozone and nitrogen dioxide at the surface level. The quantities are
computed in a
Sunlight covers a wide spectral range of electromagnetic
radiation. Ultraviolet (UV) radiation relevant to life on Earth is in
the wavelength range 280–400
Surface UV radiation can be measured locally with ground-based
instruments, although an accurate measurement is a challenging
task
Description of the surface UV radiation quantities stored in the OUV product version 1.20. The
quantities and the biological weighting functions are defined in Sect.
Europe's first dedicated operational polar orbiting weather satellite program is the EUMETSAT Polar System (EPS), operated by the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT). The space segment of the EPS consists of a series of three polar orbiting meteorological operational (Metop) satellites. The first two Metop satellites, Metop-A and Metop-B, were launched on 19 October 2006 and 17 September 2012, respectively, with Metop-B taking over the prime service on 24 April 2013. The third, Metop-C, is due to be launched in the 2018 time frame. EPS is the European contribution to the Initial Joint Polar System Agreement (IJPS), an agreement between EUMETSAT and National Oceanic and Atmospheric Administration (NOAA). EUMETSAT is responsible for the local morning orbit (daytime descending node around 09:30 LT) while NOAA is responsible for the afternoon orbit (daytime ascending node around 14:30 LT), beginning with the NOAA-18, launched on 20 May 2005, and taken over by the current NOAA-19, launched on 6 February 2009, as the prime observer on 23 June 2009.
EUMETSAT ensures maximal benefit from its satellite programs by maintaining a network of Satellite Application Facilities (SAF) within its member states. The SAF on Ozone and Atmospheric Chemistry Monitoring (O3M SAF) focuses on the measurements of the second Global Ozone Monitoring Experiment (GOME-2) aboard the three Metop satellites. The products of the O3M SAF include ozone, trace gas, aerosol and surface UV radiation products. The surface UV radiation products include a near-real-time product based on assimilated total ozone and an offline product utilising level-2 total ozone columns. In this paper we describe the version 1.20 of the offline product.
An example product field. Solar noon UV index derived from
Metop-A and NOAA-19 data on 30 March 2011 during the Arctic ozone
hole episode when exceptionally large values of UV index for the
point in time were observed in the Arctic. In Sodankylä, northern
Finland, a solar noon UV index of 2.14 was measured at ground level
exceeding the climatological value by
100 %
The O3M SAF offline surface UV (OUV) radiation product is a global
product containing daily maximum dose rates and daily integrated doses
weighted with different biological weighting functions, integrated UVB
and UVA radiation, solar noon UV index and daily maximum photolysis
frequencies of ozone and nitrogen dioxide at the surface level. The
quantities stored in the product are listed in
Table
GOME-2 is a nadir-viewing scanning UV–VIS (visible) spectrometer measuring
back-scattered and reflected radiation from the Earth–atmosphere
system in a spectral range between 240 and
790
UV–VIS reflectance spectrum (solid line) together with
AVHRR/3 channel 1 spectral response function (dotted line)
illustrating the use of the total ozone column product derived from
325–335
GOME-2 continues the European contribution to long-term monitoring of
atmospheric ozone started by the GOME aboard the second European Remote Sensing satellite
(ERS-2;
launched in 1995) and the Scanning Imaging Absorption spectroMeter for
Atmospheric CartograpHY (SCIAMACHY) aboard the Environmental Satellite
(Envisat; launched in 2002). Total ozone columns are produced by the
German Aerospace Center (DLR) in the framework of the O3M SAF and
disseminated to the near-real-time users via EUMETSAT's Multicast
Distribution System (EUMETCast). The operational retrieval is based
on the GOME data processor (GDP) version 4 family of algorithms using
differential optical absorption spectroscopy
(DOAS)
Several enhancements to the basic algorithm were introduced in GDP
4.4: improved cloud retrieval algorithms including detection of Sun
glint effects, a correction for intracloud ozone, better treatment of
snow and ice conditions, accurate radiative transfer modelling for
large viewing angles and elimination of scan angle
dependencies
The UV bands of the GOME-2 instrument suffer from severe throughput
degradation
The AVHRR/3 aboard
the Metop satellites is a heritage instrument provided by NOAA. It is
the latest version of the series carried on the POES series of
satellites, beginning with a four-channel instrument aboard TIROS-N in
1978
Unfortunately, there are no onboard calibration devices for the
visible channels. The calibration coefficients determined prior to
launch are traceable to the radiance standards maintained at the
National Institute of Standards and Technology
(NIST)
The surface height is obtained from the US Geological Survey's Global 30 Arc-Second (GTOPO30) digital elevation model covering the
full extent of latitude from 90
In order to reduce dependencies on external inputs, the operational
algorithm relies on climatologies for the surface albedo and aerosol
optical depth. The surface albedo climatology for the
Left: biological weighting functions used in the
product. Right: absorption cross sections of ozone and nitrogen
dioxide (solid lines, left
Partially ice or snow covered grid cells are inhomogeneous with respect to surface albedo, especially near the edges of ice sheets and around small snow or ice covered islands where one part is water with a low UV reflectivity and the other part is ice or snow with a high UV reflectivity. It is assumed that in order for a grid cell to be homogeneous, the area defined by the current cell and its nearest neighbouring cells has to be homogeneous. The minimum and maximum values of surface albedo for this area are determined, and if the difference between the maximum and minimum is larger than a threshold value, currently set to 0.1, the current grid cell is quality flagged as inhomogeneous to indicate that the surface UV quantities vary significantly within the grid cell due to the surface albedo.
Obtaining reliable aerosol information from satellite measurements is
a complicated task
The calculation of dose rates is based on the hemispherical spectral
irradiance
In this paper, we call the weighted and integrated irradiance
The erythemal (CIE) weighting function
The weighting function for DNA damage
Node points of the cloud optical depth look-up table. A subset of the TOMS V7 ozone profile climatology is used together with the associated temperature profiles. M125, M325 and M575 refer to the middle latitude profiles for total ozone columns of 125, 325 and 575 DU, respectively.
Two important tropospheric photolysis reactions are driven by UV
radiation. For the formation of atomic oxygen in its exited
For the photolysis frequencies, the spherical spectral irradiance
(actinic flux)
The radiative transfer (RT) modelling involves building of model
atmospheres characterised by surface pressure, surface albedo, cloud
optical depth, aerosol optical depth and vertical profiles of
temperature and ozone number density (Sect.
The model atmosphere contains 30 homogeneous layers above
a horizontally homogeneous Lambertian surface. The layer thickness is
1 up to 15
Node points of the look-up table for the dose rates and photolysis frequencies. The full 26 profile set of the TOMS V7 climatology is used. L, M and H refer to the low, middle and high latitude profiles, respectively, while the numbers refer to total ozone columns in DU.
The attenuation of radiation through the homogeneous cloud layer in our model
is described by cloud optical depth
Typical dependence of the AVHRR/3 channel 1 reflectance on
cloud optical depth for different surface albedos (solid lines, left
The dose rates (Eq.
Similarly, the spherical irradiance (actinic flux) at the surface
level for unit solar irradiance, i.e. spherical transmittance, is
obtained from the output of the radiative transfer model in the same wavelength grid but extended
from 400 to 430
Typical dependence of the erythemal dose rate on cloud
optical depth for different surface albedos (solid lines, left
The product is processed at the Finnish Meteorological Institute (FMI)
in Helsinki, Finland, as part of the distributed O3M SAF data
processing network. The near-real-time total ozone column
product is produced by the DLR in
Oberpfaffenhofen, Germany. The AVHRR/3 level 1b data both from Metop
and NOAA satellites are processed at the EUMETSAT headquarters in
Darmstadt, Germany. The satellite data are transmitted between the
processing sites with the EUMETCast: a multi-service dissemination
system based on standard Digital Video Broadcast technology
utilising commercial telecommunication geostationary satellites. The
product is archived in the O3M SAF distributed archive at the
Sodankylä Satellite Data Centre, Finland. An online catalogue of
the products is maintained at the EUMETSAT Data Centre, and the
products can be ordered from there. The version 1.20 is available from
9 July 2013. The latest information is given at the website
Figure
Cloud optical depth is retrieved from the channel 1 reflectance of
AVHRR/3 aboard Metop and NOAA satellites. One satellite, nominally the
prime, from both programmes is used at any given
time. Figure
The cloud optical depth is retrieved in the lowest common spatial
resolution, i.e. the GAC resolution of the NOAA data, and therefore
the Metop LAC-resolution data are thinned and averaged to the GAC
resolution prior to processing. The
Overall processing algorithm. The radiative transfer modelling illustrated on the right side is performed offline and the results are stored in look-up tables (LUT). The input satellite data are received from the EUMETCast system. The GOME-2 total column ozone product is mapped to the processing grid and then used together with the climatologies in both deriving the intermediate cloud optical depths from the AVHRR/3 level 1b data and interpolating the output products from the look-up table.
Average number of cloud observations per day obtained from
AVHRR/3 using the operational combination of Metop-A with either NOAA-18 or
NOAA-19 during the period from 1 June 2007 to 31 December 2012. The
average is calculated from the intermediate gridded cloud optical
depth file (Fig.
The AVHRR/3 ground pixels at the GAC resolution are smaller than the
Intercomparison of the diurnal UV index extracted from the
products (reprocessed with the algorithm version 1.20)
and the corresponding ground-based SL-501
measurements (GB) in Sodankylä (67.37
The dependence of the erythemal dose rate on the channel 1
reflectance for different surface albedos derived from data shown in
Figs.
The sunlit part of the diurnal cycle is discretised with a half-hour
time steps from sunrise to sunset taken as the times when the solar
zenith angle is 88
Figure
A number of quality flags are set during the product processing to
indicate the expected quality for each grid cell. The flags related to
surface homogeneity were already discussed in
Sect.
Overall consistency of the latest processed product with the existing data record is verified by monitoring the global average value of the erythemal daily dose. The monitoring results are shown on an online quality monitoring page at the O3M SAF web site. An intercomparison of the product with traditional ground-based surface UV measurements will be presented in a separate paper.
The operational surface UV product of the O3M SAF was described. The
product requires information on clouds, ozone, aerosols and surface
albedo. In the operational version the cloud optical depth and total
column ozone are derived from measurements while the aerosol and
surface albedo information come from climatologies. Future product
developments aim at replacing these climatologies with measured
data. A number of suitable products derived from GOME-2 measurements
are evolving and new data sets have recently become available.
The O3M SAF Lambertian Equivalent Reflectivity (LER) data records
derived either from the GOME-2 main channels or from the PMD
measurements
Development of the O3M SAF surface UV product has been funded by EUMETSAT. The operational NOAA AVHRR/3 data were obtained via EUMETSAT within the IJPS programme and for offline use from the NOAA Comprehensive Large Array-data Stewardship System (CLASS). The GTOPO30 data were available from the US Geological Survey and the snow and ice data from the National Snow and Ice Data Center (NSIDC), University of Colorado, Boulder. The GADS aerosol data were obtained from the Meteorological Institute of the University of Munich, Germany, and the MLER data were obtained from the NASA TOMS team. We also wish to thank Robert Spurr from the RT Solutions Inc. for providing the VLIDORT radiative transfer model and Stephan Kinne at MPI Hamburg for providing the aerosol climatology. Edited by: P. Stammes