Abstract
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.
Highlights
Monitoring of UV solar radiation at the surface is a necessary and important task to characterize the impact of atmospheric composition change, which is the goal, for example, of the Network for the Detection of Atmospheric Composition Change (NDACC) and of the Global Atmosphere Watch Programme (GAW)
Comparisons are conducted at noontime, and the cloudiness measurements used in OMI and GOME-2 algorithms are not actual values at noontime
For all sky conditions (AS), GB UV index (UVI) measurements have been averaged over a time interval around noontime
Summary
Monitoring of UV solar radiation at the surface is a necessary and important task to characterize the impact of atmospheric composition change, which is the goal, for example, of the Network for the Detection of Atmospheric Composition Change (NDACC) and of the Global Atmosphere Watch Programme (GAW). Differences between the data of the two satellite instruments that will be used in this work (OMI, the Ozone Monitoring Instrument, and GOME-2, the second Global Ozone Monitoring Experiment) are detailed below Despite their extensive geographical coverage, satellitebased (SB) data products are affected by measurement uncertainties, as are ground-based (GB) products. At sites with significant influence from absorbing aerosols, OMI surface UVI show a larger positive bias of up to 50 % All these OMI validations, apart from Buntoung and Webb (2010), were conducted using data collected at the time of the satellite overpass. The two other metropolitan sites are characterized by the presence of absorbing aerosols, on average in larger quantity at Villeneuve d’Ascq than at Observatoire de Haute-Provence, but less absorbing Their midlatitude situation implies lower UV radiation levels than in the tropics (lower sun elevation in summer and larger total ozone column).
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