Mireille Lefèvre

The HelioClim Project: Surface Solar Irradiance Data for Climate Applications

Abstract: Meteosat satellite images are processed to yield values of the incoming surface solar irradiance (SSI), one of the Essential Climate Variables. Two HelioClim databases, HC-1 and HC-3, were constructed covering Europe, Africa and the Atlantic Ocean, and contain daily and monthly means of SSI. The HC-1 database spans from 1985 to 2005; HC-3 began in 2004 and is updated daily. Their quality and limitations in retrieving monthly means of SSI have been studied by a comparison between eleven stations offering long time-series of measurements. A good agreement was observed for each site: bias was less than 10 W/m² in absolute value (5% in relative value) for HC-3. HC-1 offers a similar quality, though it underestimates the SSI for latitudes greater than 45° and less than −45°. Time-series running from 1985 to date can be created by concatenating the HC-1 and HC-3 values and could help in assessing SSI and its changes. Keywords: solar radiation; solar irradiance; solar exposure; climate; Africa; Europe; Atlantic Ocean; remote sensing; long-term analysis; Meteosat

Assessing surface solar irradiance and its long-term variations in the northern Africa desert climate using Meteosat images

Two databases of solar surface irradiance (SSI) derived from satellites were compared to ground measurements in Algeria, Egypt, Libya and Tunisia. We found that it was possible to accurately derive the SSI from geostationary meteorological satellites, even with a coarse spatial resolution. The two databases HelioClim-1 (HC1) and SSE exhibited similar and good performances. The bias was generally lower for SSE than for HC1; however, HC1 exhibited a smaller scattering of data compared to ground measurements (smaller standard deviation) than the SSE, allowing better performance when mapping the long-term variations in SSI. The long-term variations in SSI from 1985 to 2005 show that these four countries as a whole experienced dimming. Detailed analyses of the range of dimming at sites with long-term records and of its spatial distribution were performed. We found that the analysis of SSI from HC1 supports the findings for the individual sites. Dimming may be explained by: (1) transportation of sand dust northwards from the Sahel, (2) an increase in urbanization, and (3) an increase in cloud cover and aerosol loading.

The Operational Calibration of Images Taken in the Visible Channel of the Meteosat Series of Satellites

A method and its implementation are presented for the automatic calibration of the images taken in the visible channel of the Meteosat series of satellites. The method performs on a daily basis and is based on a statistical analysis of two images: one when the sun illuminates the entire field of view of Meteosat, the other during nighttime. This approach does not require any information about atmospheric and surface parameters, and therefore can be easily performed either on archived data, or in quasi-real time when receiving the images. Daily calibration coefficients were obtained since 1985 and are available on the Internet. The results are fully consistent with previous studies.

An automatic method for the calibration of time-series of Meteosat images

A method is described for the automatic calibration of the successive operational Meteosat sensors in the visible range since 1985. As a starting point, the absolute vicarious calibration for only one image in the whole time-series is needed, which can be provided by previous published works. The automatic method performs on a daily basis and is based on a statistical analysis of two images, one when the Sun illuminates the entire field of view of Meteosat, the other during night-time. This approach does not require any information about atmospheric and surface parameters, and therefore can be easily performed either on archived data, or in quasi-real time when receiving the images. Our results are fully consistent with previous studies. Several tests have been performed to validate the method and check its robustness and reliability.

The HelioClim-1 Database of Daily Solar Radiation at Earth Surface: An Example of the Benefits of GEOSS Data-CORE

The HelioClim-1 database contains daily values of the solar radiation reaching the ground. This GEOSS (Global Earth Observation System of Systems) Data Collection of Open Resources for Everyone (Data-CORE) covers Europe, Africa and the Atlantic Ocean, from 1985 to 2005. It is freely accessible at no cost through the SoDa Service (www.soda-is.com). Several assessments of the HelioClim-1 data against measurements made in meteorological networks reveal that the HelioClim-1 database offers a reliable and accurate knowledge of the solar radiation and its daily, seasonal and annual variations over recent years. The HelioClim-1 data may help in qualifying in situ measurements and may supplement them, thus offering 21 years of accurate daily means of surface solar irradiance. Several published works benefited from openness, availability and accuracy of the HelioClim-1 database in various domains: oceanography, climate, energy production, life cycle analysis, agriculture, forestry, architecture, health and air quality. This demonstration of the benefit of the HelioClim-1 database draws attention to resources open to everyone such as those labeled GEOSS Data-CORE.

Twelve monthly maps of ground Albedo parameters derived from MODIS data sets

The National Aeronautics and Space Administration (NASA) of the USA is making available to anyone worldwide maps of ground albedo parameters that are derived from the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument. The first parameter fiso describes the isotropic part of the bidirectional reflectance distribution function (BRDF); the two other parameters fvol and fgeo are linked to the viewing and illuminating geometry and describe the anisotropic part of the BRDF. These maps exhibit missing values. This communication describes a series of 12 monthly maps of the BRDF parameters with no missing values available on the Web. In addition, a method for computing the ground albedo is proposed that does not require the a priori knowledge of the ratio of the direct to the global irradiance.

Satellite-based estimation of surface solar irradiance

Databases of solar radiation values measured since 1985, which are of high interest in domains such as renewable energy, health, or architecture, are now freely available online.