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Anton M.,University of Extremadura | Anton M.,University of Evora | Serrano A.,University of Extremadura | Serrano A.,University of Evora | And 3 more authors.
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2010

The ground track of the annular eclipse of 3 October 2005 crossed the Iberian Peninsula. The main objective of this work was to analyze the variability of the solar irradiance and the total ozone column during the course of this event at El Arenosillo (Southwestern Spain). For achieving this goal, two Kipp & Zonen broadband radiometers (one for measuring total solar irradiance and other for measuring ultraviolet erythemal solar irradiance), one NILU-UV multi-band instrument and one Brewer spectroradiometer were used in this work. Total irradiance (310-2800nm), and ultraviolet erythemal radiation (UVER) were recorded at a high frequency of 5s, showing a strong reduction (higher than 80%) of the irradiance at the maximum solar obscuration which was of 79.6%. The irradiance decrease during the course of the eclipse was positively correlated with the percentage of eclipse obscuration, showing a very high agreement (R20.99). The irradiance recorded at selected wavelengths from the NILU-UV instrument shows a more pronounced decrease in the UV irradiance at the lower wavelengths during the solar eclipse. Finally, the evolution of the total ozone column (TOC) derived from Brewer and NILU instruments during the eclipse presented an opposite behavior: while the Brewer derived TOC values increase about 15DU, the NILU derived TOC values decrease about 11DU. This opposite behavior is mainly related to an artifact in the spectral irradiances recorded by the two instruments. © 2010 Elsevier Ltd. Source


Anton M.,University of Granada | Loyola D.,German Aerospace Center | Clerbaux C.,University Pierre and Marie Curie | Lopez M.,Agencia Estatal de Meteorologia AEMET | And 9 more authors.
Remote Sensing of Environment | Year: 2011

One of the most important atmospheric composition products derived from the first EUMETSAT Meteorological Operational satellite (MetOp-A) is the total ozone column (TOC). For this purpose, MetOp-A has two instruments on board: the Global Ozone Monitoring Experiment 2 (GOME-2) that retrieves the TOC data from the backscattered solar ultraviolet-visible (UV-Vis) radiance, and the Infrared Atmospheric Sounding Interferometer (IASI) that uses the thermal infrared radiance to derive TOC data. This paper focuses on the simultaneous validation of the TOC data provided by these two MetOp-A instruments using the measurements recorded by five well-calibrated Brewer UV spectrophotometers located at the Iberian Peninsula during the complete 2009. The results show an excellent correlation between the ground-based data and the GOME-2 and IASI satellite observations (R2 higher than 0.91). Differences between the ground-based and satellite TOC data show that the IASI instrument significantly overestimates the Brewer measurements (about 4.4% when all five ground-based stations are jointly used). In contrast, the GOME-2 instrument shows a slight underestimation (~1.6%). In addition, the absolute relative differences between the Brewer and GOME-2 data are quite smaller (about a factor higher than 2) than the Brewer-IASI absolute differences. The satellite viewing geometry (solar zenith angle and the view zenith angle) has no significant influence on the Brewer-satellite relative differences. Moreover, the analysis of these relative differences with respect to the ground-based TOC data indicates that GOME-2 instrument presents a slight underestimation for high TOC values. Finally, the IASI-GOME-2 correlation is high (R2~0.92), but with a mean relative difference of about ±6% which could be associated with the bias between UV-Vis and infrared spectroscopy used in the retrieval processes. © 2011 Elsevier Inc. Source


Costa M.J.,University of Evora | Bortoli D.,University of Evora | Pereira S.,University of Evora | Salgueiro V.,University of Evora | And 9 more authors.
AIP Conference Proceedings | Year: 2013

Measurements of both UV B (280 - 315 nm) and UV AB (280 - 400 nm) irradiances taken during about seven years are presented and analyzed here. The UV irradiance data were measured with MACAM radiometers, which are installed in the Atmospheric Physics Observatory of the University of Èvora Geophysics Center - CGE (38°34'N, 7°54'W, 300 m above mean sea level) since 2004. Special attention is devoted to calibration issues, since the radiometers were calibrated in Spain, at "El Arenosillo" ESAt/INTA laboratory in 2009 and a methodology based on radiative transfer calculations combined with observations from atmospheric quantities, was developed and applied to retrocalibrate the data from 2009 to 2004. The comparison between the radiative transfer based and the laboratory calibration methodologies yielded a quite promising outcome, with normalized root mean square errors lower than 3% and mean absolute percentage errors lower than 2%. Cloud optical thickness values derived from ground-based spectral irradiance measurements taken at the CGE observatory, are also used in order to investigate the influence of this cloud quantity on UV irradiances. © 2013 AIP Publishing LLC. Source


Antan M.,University of Granada | Antan M.,University of Evora | Gil J.E.,University of Granada | Cazorla A.,University of Granada | And 4 more authors.
Atmospheric Measurement Techniques | Year: 2011

The ultraviolet index (UVI) is the most commonly used variable to inform about the level and potential harmful effect of ultraviolet (UV) radiation reaching the Earth's surface. This variable is derived from the output signal of UV radiometers applying conversion factors from calibration methods. This paper focused on the influence of the use of two of these methods (called one-step and two-steps methods) on the experimental UVI measured by a YES UVB-1 radiometer located in a midlatitude station, Granada (Spain) for the period 2006ĝ€"2009. In addition, it also analyzes the deviation from the UVI values obtained when the manufacturer's calibration factors are applied. For this goal, a detailed characterization of the UVB-1 radiometer from the first Spanish calibration campaign of broadband UV radiometers at the "El Arenosillo" INTA station in 2007 was used. In addition, modeled UVI data derived from the LibRadtran/UVSPEC radiative transfer code are compared with the experimental values recorded at Granada for cloud-free conditions.

Absolute mean differences between measured and modeled UVI data at Granada were around 5% using the one-step and two-steps calibration methods, indicating an excellent performance of these two techniques for obtaining UVI data from the UVB-1 radiometer. Conversely, the application of the manufacture's calibration factor produced a large overestimation (∼14%) of the UVI values, generating unreliable alarming high UVI data in summer. Thus, the number of days with an extreme erythemal risk (UVI higher than 10) increased up to 46% between May and September at Granada. This percentage reduced to a more reliable value of 3% when the conversion factors obtained with the two-steps calibration method are used. These results evidence the need for a sound calibration of the broadband UV instruments in order to obtain reliable measurements. © Author(s) 2011. Source


Anton M.,University of Extremadura | Anton M.,University of Evora | Vilaplana J.M.,Estacion de Sondeos Atmosferico El Arenosillo | Kroon M.,Royal Netherlands Meteorological Institute | And 5 more authors.
IEEE Transactions on Geoscience and Remote Sensing | Year: 2010

This paper focuses on the validation of the empirically corrected total ozone column (TOC) data provided by the Earth Probe Total Ozone Mapping Spectrometer (EP-TOMS) using ground-based measurements recorded by a well-calibrated Brewer spectroradiometer located at El Arenosillo (Spain). In addition, satellite TOC observations derived from the Ozone Monitoring Instrument (OMI) with the TOMS algorithm are also used in this paper. The agreement between EP-TOMS TOC data and Brewer measurements is excellent (R 2 ∼ 0.92) even for the period 20002005 when a higher EP-TOMS instrument degradation occurred. Despite its low magnitude, the EP-TOMSBrewer relative differences depend on the solar zenith angle (SZA), showing a clear seasonal cycle with amplitude between ± 2% and ±4%. Conversely, OMIBrewer relative differences show a constant negative value around -1% with no significant dependence on SZA. No significant dependence on the ground-based to satellite-based differences with respect to the EP-TOMS scene or to the OMI crosstrack position is observed for either satellite retrieval algorithm. Finally, TOC, estimated by the two satellite instruments, have also been compared, showing a good agreement (R2 ∼ 0.88). Overall, we conclude that the empirical correction of the EP-TOMS data record provides a reprocessed set of high quality. However, EP-TOMS data after year 2000 should not be used in calculations of global-ozone trending due to remaining errors in the data set and because it is no longer an independent data set. © 2006 IEEE. Source

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