Assessment of Heliosat-4 surface solar irradiance derived on the basis of SEVIRI-APOLLO cloud products

International audienceThe Heliosat-4 method developed by the MINES ParisTech and the German Aerospace Center (DLR), aims at estimating surface downwelling solar irradiance (SSI). It benefits from advanced products derived from recent Earth Observation missions, among which the cloud products are crucial for the assessment of SSI. The APOLLO cloud product provided by DLR includes abundant information about the cloud physical and optical properties. The performances of Heliosat-4 when using APOLLO product are evaluated for the period of 2004-2009. The estimated SSIs are compared to measurements made at six stations within the Baseline Surface Radiation Network. Extensive analysis of the discrepancies offers an in-depth view of the performance of Heliosat-4/APOLLO, an understanding of the advantages of this combination Heliosat-4/APOLLO when compared to existing methods and the identification of restrictions in both Heliosat-4 and the APOLLO product for future improvements

Use of OCA and APOLLO in Heliosat-4 method for the assessment of surface downwelling solar irradiance

International audienceWe test two cloud products: Optimal Cloud Analysis (OCA) of EUMETSAT, and APOLLO from the German Aerospace Center (DLR), for the assessment of surface downwelling solar irradiance (SSI). Each product is input to the Heliosat-4 method, and the SSI estimates are compared to accurate measurements performed in the Baseline Radiation Network (BSRN). The performances obtained by the two products are compared. The overall performance of Heliosat-4 method by using different cloud products is given and conclusions on the benefit of each product for an operational Heliosat-4 are drawn

Solar surface irradiance from new meteorological satellite data

International audienceThis paper presents the first prototype of a new method, for assessing solar surface irradiance, benefiting from advanced products derived from recent Earth Observation missions. This method -called Heliosat-4- is based on the radiative transfer model libRadtran and will provide direct, diffuse components and spectral distribution of solar surface irradiance every 3 km and ¼ h over Europe and Africa. The advantage of the Heliosat-4 method is the simultaneous computation of direct and diffuse irradiances. The outcomes of this prototype of Heliosat-4 method are compared to ground measurements, of direct and global irradiances, made at 4 stations in Europe and Northern Africa. The results show that standard deviation attained by the Heliosat-4 method for global irradiance is fairly similar to that attained by current methods. A significant bias is actually observed and discussed

Description of an operational tool for determining global solar radiation at ground using geostationary satellite images

International audienceAn operational tool for the fine-scale mapping of the incident solar radiation at ground is presented. This tool called "Heliosat station" makes use of image acquired in the visible spectral range by meteorological geostationary satellite to produce accurate maps of global radiation. Satellite data are directly received at ground by a cheap HF receiver and processed by a personal computer IBM-PC compatible using the already known Heliosat method. This method has been successfully tested during 30 consecutive months beginning January 1983. The first prototype of the Heliosat station was realized in 1985. Now a Heliosat station is routinely operated by Agence Française pour la Maîtrise de l'Energie since January 1987 for mapping solar radiation over Europe

Toward a solar climatological database: the HelioClim project

International audienceThe HelioClim project aims at producing temporal series of solar radiation maps at ground level over Europe, Africa and Atlantic Ocean from 1985 up to now. Meteosat satellites images in visible channel and in the reduced resolution B2 format are used to produce this climatological database. The Heliosat II method was selected to convert these images into maps of the global irradiation at ground level. It is an improved version of the original Heliosat method which estimates the global irradiation by comparison of what is observed by the sensor to what should be over that pixel if the sky were clear. The improvements contained in the Heliosat 11 method consist mostly in a use of a more accurate clear sky model and explicit formulations of the radiance and the transmittance instead of empirical values. A comparison was performed between irradiation values retrieved by this method and measured by ground stations in Europe. A measuring station is not necessarily contained within a B2 pixel and an interpolation method is needed. The accuracy is less than that observed for the high-resolution images because of the smaller number of images and the interpolation process. Nevertheless, the observed relative root mean square error is around 20 %, a very satisfactory result. We conclude that B2 images processed by Heliosat 11 method gives good results especially for the estimation of daily irradiation or for the irradiation of larger period. They are also able to provide useful data to create a more than fifteen years solar climatological database

Validation of the surface downwelling solar irradiance estimates of the HelioClim-3 database in Egypt

International audienceHelioClim-3 (HC3) is a database providing time series of the surface downwelling solar irradiance that are computed from images of the Meteosat satellites. This paper presents the validation results of the hourly global horizontal irradiance (GHI) and direct normal irradiance (DNI), i.e., beam irradiance at normal incidence, of versions four and five of HC3 at seven Egyptian sites. The validation is performed for all-sky conditions, as well as cloud-free conditions. Both versions of HC3 provide similar OPEN ACCESS Remote Sens. 2015, 7 9270 performances whatever the conditions. Another comparison is made with the estimates provided by the McClear database that is restricted to cloud-free conditions. All databases capture well the temporal variability of the GHI in all conditions, McClear being superior for cloud-free cases. In cloud-free conditions for the GHI, the relative root mean square error (RMSE) are fairly similar, ranging from 6% to 15%; both HC3 databases exhibit a smaller bias than McClear. McClear offers an overall better performance for the cloud-free DNI estimates. For all-sky conditions, the relative RMSE for GHI ranges from 10% to 22%, except one station, while, for the DNI, the results are not so good for the two stations with DNI measurements

Analysis of the influences of uncertainties in input variables on the outcomes of the Heliosat-2 method

International audienceThe Heliosat-2 method, which employs satellite images to assess solar irradiance at ground level, is one of the most accurate among the available operational methods. Its input variables have uncertainties which impact on the final result. The General Law of Uncertainty Propagation is employed to analyze the impact of these uncertainties on a single pixel with Meteosat-7 inputs in various stages, beginning with the sensitivity coefficients and the changes induced in the clear-sky index (KC) by each independent variable. Once these coefficients are known, the partial combined standard uncertainty (CSU) is calculated for KC from each independent variable and albedo. Finally, the total CSU of KC is calculated. All of the results are in agreement and show that the most influential variables in the uncertainty of estimation of cloudy skies are, in this order, the Linke turbidity factor (54% of KC value), terrain elevation (33%), the calibration coefficient of the satellite sensor (13%) and the ground albedo (5%). What causes the initial uncertainty in the ground albedo is its variation over time and the difficulty in assessing it from a reflectance time-series for mixed clear and cloudy skies. The Linke turbidity factor is the most influential variable on the width of the uncertainty interval, not only because of its own uncertainty (17% in this study), but because it is also used in numerous intermediate calculations. For clear skies, the partial CSUs are considerably lower, except for ground albedo (5% also)

The HelioClim Project: Surface Solar Irradiance Data for Climate Applications

URL : http://www.mdpi.com/2072-4292/3/2/343/International audienceMeteosat 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

The HelioClim Project: from satellite images to solar radiation maps

International audienceThe presentation will address the present status of the HelioClim Project. After a short description of the Project, the improvements already brought to the Heliosat method will be discussed: calibration, clear-sky model, relationship between clear-sky index and the cloud index. Their influence upon the quality of the assessment of the solar radiation will be shown. The conclusion will include a presentation of the further steps of HelioClim