The fifth North American Intercomparison of Ultraviolet Monitoring Spectroradiometers was held June 13 to 21, 2003 at Table Mountain outside of Boulder, Colorado, USA. The main purpose of the Intercomparison was to assess the ability of spectroradiometers to accurately measure solar ultraviolet irradiance, and to compare the results between instruments of different monitoring networks. This Intercomparison was coordinated by NOAA and included participants from six national and international agencies. The UV measuring instruments included scanning spectroradiometers, spectrographs, and multi-filter radiometers. Synchronized spectral scans of the solar irradiance were performed between June 16 and 20, 2003. The spectral responsivities were determined for each instrument using the participants' lamps and calibration procedures and with NOAA/CUCF standard lamps. This paper covers the scanning spectroradiometers and the one spectrograph. The solar irradiance measurements from the different instruments were deconvolved using a high resolution extraterrestrial solar irradiance and reconvolved with a 1-nm triangular band-pass to account for differences in the bandwidths of the instruments. The measured solar irradiance from the spectroradiometers using the rivmSHIC algorithm on a clear-sky day on DOY 172 at 17.0 UTC (SZA = 30o) had a relative 1- standard deviation of +/-2.6 to 3.4% for 300- to 360-nm using the participants' calibration.
Ship-borne measurements of spectral as well as biologically effective UV irradiance have been performed on the German research vessel Polarstern during the Atlantic transect from Bremerhaven, Germany, to Cape Town, south Africa, from 13 October to 17 November 2005. Such measurements are required to study UV effects on marine organisms. They are also necessary to validate satellite-derived surface UV irradiance. Cloud free radiative transfer calculations support the investigation of this latitudinal dependence. Input parameters, such as total ozone column and aerosol optical depth have been measured on board as well. The maximum daily dose of erythemal irradiance with 5420 J/m<sup>2</sup> was observed on 14 November 2005, when the ship was in the tropical Atlantic south of the equator. The expected UV maximum should have been observed with the Sun in the zenith during local noon (11 November). Stratiform clouds reduced the dose to 3835 J/m<sup>2</sup>. In comparison, the daily erythemal doses in the mid-latitudinal Bay of Biscay only reached values between 410 and 980 J/m<sup>2</sup> depending on cloud conditions. The deviation in daily erythemal dose derived from different instruments is around 5%. The feasibility to perform ship-borne measurements of spectral UV irradiance is demonstrated.
We demonstrate the importance of surface reflectivity for the radiation field in polar regions by a combination of
measurements and radiative transfer calculations. Results from measurements of spectral albedo, radiance and irradiance
from 280 to 1050 nm at German Neumayer Station in Antarctica in summer 2003/2004 as well as measurements of UV
irradiance during summer 1997/1998 at Australian Davis Station, Antarctica are presented. The impact of surface albedo
inhomogeneity is investigated by 3-D Monte Carlo modelling. We found that high surface reflectivity in the ultraviolet
and visible parts of the spectrum due to the snow covered surface in Antarctica modifies the radiation field considerably
compared to mid-latitudes. A change of the spectral reflectivity, which happens as a consequence of climate change will
have a large impact in the radiation properties in polar regions and vice versa.
During the austral summer 2003/04 the Institute for Meteorology and Climatology, University of Hannover, Germany, has deployed a newly developed spectroradiometric system at the permanent German Antarctic Neumayer Station (70° 39' S, 8° 15' W). Aim of this campaign was to characterize the solar radiation conditions in an Antarctic environment. These are different from other areas of the Earth due to extremely high reflection of the ground (albedo). Relatively low cloud optical depths and ozone depletion further contribute to rather different radiation conditions compared to mid-latitudes. The investigation of these conditions will improve the understanding of the impact of climate change and ozone depletion in polar regions.
Spectral irradiance and radiance as well as luminance and spectral albedo have been measured in a wavelength range from 280 to 1050 nm. With this set of radiation parameters it is assured that directional information of incident radiation parameters as well as the impact of surface albedo can be investigated.
Monitoring of radiation parameters is carried out by the Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany. Spectral irradiance from 290 to 400 nm has been measured since January 2001. UVB monitoring started in 1997. Broadband radiation parameters have been detected since 1981. Furthermore, the station participates in international networks initiated by the Word Climate Research Programme such as the Baseline Surface Radiation Network (BSRN) and the Global Atmospheric Watch (GAW). It is also a complementary site of the Network for the Detection of Stratospheric Change (NDSC).
QASUME is a European Commission funded project that aims to develop and test a transportable unit for providing quality assurance to UV spectroradiometric measurements conducted in Europe. The comparisons will be performed at the home sites of the instruments, thus avoiding the risk of transporting instruments to participate in intercomparison campaigns. Spectral measurements obtained at each of the stations will be compared, following detailed and objective comparison protocols, against collocated measurements performed by a thoroughly tested and validated travelling unit. The transportable unit comprises a spectroradiometer, its calibrator with a set of calibration lamps traceable to the sources of different Standards Laboratories, and devices for determining the slit function and the angular response of the local spectroradiometers. The unit will be transported by road to about 25 UV stations over a period of about two years. The spectroradiometer of the transportable unit is compared in an intercomparison campaign with six instruments to establish a relation, which would then be used as a reference for its calibration over the period of its regular operation at the European stations. Different weather patterns (from clear skies to heavy rain) were present during the campaign, allowing the performance of the spectroradiometers to be evaluated under unfavourable conditions (as may be experienced at home sites) as well as the more desirable dry conditions. Measurements in the laboratory revealed that the calibration standards of the spectroradiometers differ by up to 10%. The evaluation is completed through comparisons with the same six instruments at their homes sites.