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Volume 16, issue 23 | Copyright

Special issue: Twenty-five years of operations of the Network for the Detection...

Atmos. Chem. Phys., 16, 15049-15074, 2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 06 Dec 2016

Research article | 06 Dec 2016

Validation of satellite-based noontime UVI with NDACC ground-based instruments: influence of topography, environment and satellite overpass time

Colette Brogniez1, Frédérique Auriol1, Christine Deroo1, Antti Arola2, Jukka Kujanpää3, Béatrice Sauvage1,a, Niilo Kalakoski3, Mikko Riku Aleksi Pitkänen2,4, Maxime Catalfamo1, Jean-Marc Metzger5, Guy Tournois6, and Pierre Da Conceicao6 Colette Brogniez et al.
  • 1Laboratoire d'Optique Atmosphérique, Université Lille 1 Sciences et Technologies, Villeneuve d'Ascq, France
  • 2Finnish Meteorological Institute, Yliopistonranta 1 F, P.O. Box 1627, 70211 Kuopio, Finland
  • 3Finnish Meteorological Institute, Earth Observation Unit, P.O. Box 503, 00101 Helsinki, Finland
  • 4Department of Applied Physics, University of Eastern Finland, Yliopistonranta 1 F, 70210 Kuopio, Finland
  • 5UMS 3365 – OSU Réunion, Université de La Réunion, Saint-Denis, Réunion, France
  • 6UMS 3470 – OSU Pytheas, Observatoire de Haute-Provence, Saint-Michel l'Observatoire, France
  • aformerly at: Laboratoire d'Optique Atmosphérique, Université Lille 1 Sciences et Technologies, Villeneuve d'Ascq, France

Abstract. Spectral solar UV radiation measurements are performed in France using three spectroradiometers located at very different sites. One is installed in Villeneuve d'Ascq, in the north of France (VDA). It is an urban site in a topographically flat region. Another instrument is installed in Observatoire de Haute-Provence, located in the southern French Alps (OHP). It is a rural mountainous site. The third instrument is installed in Saint-Denis, Réunion Island (SDR). It is a coastal urban site on a small mountainous island in the southern tropics. The three instruments are affiliated with the Network for the Detection of Atmospheric Composition Change (NDACC) and carry out routine measurements to monitor the spectral solar UV radiation and enable derivation of UV index (UVI). The ground-based UVI values observed at solar noon are compared to similar quantities derived from the Ozone Monitoring Instrument (OMI, onboard the Aura satellite) and the second Global Ozone Monitoring Experiment (GOME-2, onboard the Metop-A satellite) measurements for validation of these satellite-based products. The present study concerns the period 2009–September 2012, date of the implementation of a new OMI processing tool. The new version (v1.3) introduces a correction for absorbing aerosols that were not considered in the old version (v1.2). Both versions of the OMI UVI products were available before September 2012 and are used to assess the improvement of the new processing tool. On average, estimates from satellite instruments always overestimate surface UVI at solar noon. Under cloudless conditions, the satellite-derived estimates of UVI compare satisfactorily with ground-based data: the median relative bias is less than 8% at VDA and 4% at SDR for both OMI v1.3 and GOME-2, and about 6% for OMI v1.3 and 2% for GOME-2 at OHP. The correlation between satellite-based and ground-based data is better at VDA and OHP (about 0.99) than at SDR (0.96) for both space-borne instruments. For all sky conditions, the median relative biases are much larger, with large dispersion for both instruments at all sites (VDA: about 12%; OHP: 9%; SDR: 11%). Correlation between satellite-based and ground-based data is still better at VDA and OHP (about 0.95) than at SDR (about 0.73) for both satellite instruments. These results are explained considering the time of overpass of the two satellites, which is far from solar noon, preventing a good estimation of the cloud cover necessary for a good modelling of the UVI. Site topography and environment are shown to have a non-significant influence. At VDA and OHP, OMI v1.3 shows a significant improvement with respect to v1.2, which did not account for absorbing aerosols.

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The atmospheric ozone layer is changing, thus the UV radiation at the surface is changing. Due to both beneficial and adverse effects of UV on the biosphere, monitoring of UV is essential. Satellite sensors provide estimates of UV at the surface with a global coverage. Validation of satellite-sensor UV is therefore needed and this can be done by comparison with ground-based measurements. The present validation in three sites (midlatitude, tropical) shows that OMI and GOME-2 provide reliable UV.
The atmospheric ozone layer is changing, thus the UV radiation at the surface is changing. Due...