The forecasting of the downwelling surface solar irradiances is required by the operators of solar powered plants to predict the power output of the plant. The purpose of this study is to use aerosol optical depth and columnar water vapor forecasts by the chemistry transport model CHIMERE, coupled with the Weather Research and Forecast (WRF) model, as inputs in the REST2 model to predict the day-ahead clear-sky global horizontal, direct normal and diffuse horizontal irradiances at hourly steps. A dataset of quality-assured and cloud-screened surface solar irradiance measurements collected from 44 sites scattered throughout Saudi Arabia was used to validate the predicted surface solar irradiances. The validation for the day-ahead forecasting of the clear-sky global horizontal irradiance for the data of all 44 stations combined exhibits a bias (relative to the mean reference value) of 0%, a relative root mean square error of 4% and a correlation coefficient of 0.993. Respectively, the clear-sky direct normal irradiance exhibits values of 0%, 15% and 0.769, while the clear-sky diffuse horizontal irradiance values are 0%, 35% and 0.542. The errors in the inputs are also investigated. The results are satisfactory and pave the way to the inclusion of the attenuation due to clouds with the final aim of predicting the surface solar irradiance under all-sky conditions.

Routine measurements of the broadband beam irradiance at normal incidence by means of pyrheliometers or equivalent pyranometric systems include unknown contributions from the irradiance originating from within the extent of the solar disc B-n(sun), and that from a larger circumsolar region defined by its solid angle aperture, called the circumsolar normal irradiance CSn. This article describes a fast and simple parametric model that estimates the beam and circumsolar radiation, for opening half-angles in the interval [0.4 degrees, 5 degrees], under cloud-free conditions in a desert environment. 1 min measurements of the beam normal B-n, global G and diffuse D horizontal irradiances at Solar Village, Saudi Arabia, and Tamanrasset, Algeria, were used for calibration and validation. Using AERONET measurements as inputs to the radiative transfer code libRadtran, it has been checked-through an 'indirect' validation with the ground measured B-n-that the modelled B-n(sun) and CSn are accurate. Accordingly, a library of B(n)(sun )and CSn modelled by libRadtran for varying solid angle apertures was generated. Building on this library, a fast parametric model was developed to estimate B-n(sun) and CSn using G, D and B-n, as inputs. The coefficients of the model were fitted to a training set of measurements and then validated twice: once at their respective site, and once at the other site. When using the coefficients for their own site for both Solar Village and Tamanrasset for CSn, the relative bias is respectively -2.7% and -1.5%, the relative root mean square error (RMSE) is 19.9% and 19.6%, and the correlation coefficient is 0.871 and 0.935. As for B-n(sun), the relative bias is -2.0% and -2.2%, the RMSE is 2.7% and 3.6%, and the correlation coefficient is 0.999 and 0.998. Applying the coefficients of one site to the other site yields satisfactory results. It is recommended to use the coefficients of Tamanrasset for desert sites exhibiting frequent clear skies, and those of Solar Village for sites exhibiting frequent turbid skies. The coefficients have also been fitted using data from both sites, for a combined model.

Measurements of the global surface solar irradiation and its direct and diffuse components performed at three Egyptian sites (Aswan, Cairo, and Port Said) are used to test the ability of two published decomposition models to estimate the hourly direct normal irradiance from the measured global horizontal one in cloud-free conditions. The tested models failed to reproduce the temporal variability of the measurements, which we show to be partly induced by the large variability of the atmospheric content in aerosols. We propose a revised formulation of the decomposition models that takes into account the aerosol optical depth (AOD) at 1000 nm derived from onsite measurements. It leads to a significant reduction of the bias and root mean square deviation of the original models and this at the three Egyptian sites. However, because the AOD is rarely measured at the meteorological stations, we also quantify the performance of the revised models when the AOD is either derived from the MODIS observations or obtained by the products from Copernicus Atmospheric Monitoring Service (CAMS). Probably because of their finer temporal resolution that makes them more apt to reproduce the rapid variations of the AOD, the best results are obtained with the CAMS products. Therefore, we recommend using a combination of the revised decomposition models and these CAMS products to estimate the hourly direct normal irradiance in areas such as Egypt where aerosols are ubiquitous. Note that the improved decomposition models are generally applicable in all-sky conditions, although their benefit has been demonstrated to be significant, and probably limited to, cloud-free conditions.

A database containing the global and diffuse components of the surface solar hourly irradiation measured from 1 January 2004 to 31 December 2010 at eight stations of the Egyptian Meteorological Authority is presented. For three of these sites (Cairo, Aswan and El-Farafra), the direct component is also available. In addition, a series of meteorological variables including surface pressure, relative humidity, temperature, wind speed and direction is provided at the same hourly resolution at all stations. The details of the experimental sites and instruments used for the acquisition are given. Special attention is paid to the quality of the data and the procedure applied to flag suspicious or erroneous measurements is described in detail. Between 88 and 99 % of the daytime measurements are validated by this quality control. Except at Barrani where the number is lower (13 500), between 20 000 and 29 000 measurements of global and diffuse hourly irradiation are available at all sites for the 7-year period. Similarly, from 9000 to 13 000 measurements of direct hourly irradiation values are provided for the three sites where this component is measured. With its high temporal resolution this consistent irradiation and meteorological database constitutes a reliable source to estimate the potential of solar energy in Egypt. It is also adapted to the study of high-frequency atmospheric processes such as the impact of aerosols on atmospheric radiative transfer. It is planned to update regularly the current 2004–2010 database, which has been placed on the PANGAEA repository (doi:10.1594/PANGAEA.848804) and contains the individual meteorological and irradiation data files of the eight stations.

Can AERONET data be used to accurately model the monochromatic beam and circumsolar irradiances under cloud-free conditions in desert environment? Y. Eissa, P. Blanc, L. Wald, and H. Ghedira MINES ParisTech, PSL Research University, O.I.E. – Centre Observation, Impacts, Energy, CS 10207 – 06904 Sophia Antipolis CEDEX, France Masdar Institute, Research Center for Renewable Energy Mapping and Assessment, P.O. Box 54224, Abu Dhabi, United Arab Emirates

Routine measurements of the broadband direct normal irradiance (DNI), i.e. beam irradiance at normal incidence, by means of pyrheliometers or equivalent pyranometric systems include the irradiance originating from within the extent of the solar disc (DNIS) and that from a larger circumsolar region, called the circumsolar normal irradiance (CSNI). Such instruments have equivalent aperture half-angles between 2.5° and 5° which are one order of magnitude greater than the angular radius of the solar disc. The equivalent aperture half-angles of the concentrated solar powered systems are greater than the angular radius of the solar disc, but smaller than that of the measuring systems. Therefore, information on the CSNI should be provided for an improved assessment of the DNI. The objective of this PhD thesis is to contribute to an improved assessment of the beam and circumsolar radiation under cloud-free conditions in a desert environment. After selecting the aerosol optical properties of significance, the radiative transfer model libRadtran was used to model the CSNI and DNIS. A fast and simple parametric model which mimics the libRadtran values is proposed. This model uses standard measurements of the DNI and the diffuse horizontal irradiance as inputs to estimate the circumsolar ratio (CSR) for any aperture half-angle between 0.4° and 5°. The CSR is the ratio of the CSNI to the sum of the CSNI and the DNIS. Knowing the CSR and having the measured DNI, the CSNI and the DNIS can be computed.

McClear, a fast model based on a radiative transfer solver, exploits the atmospheric properties provided by the EU-funded MACC project (Monitoring Atmospheric Composition and Climate) to estimate the surface downwelling solar irradiances for cloud-free instances. This article presents the first validation of the McClear model for the specific climate of the United Arab Emirates where skies are frequently cloud-free but turbid. McClear accurately estimates the global horizontal irradiance measured every 10 min at seven sites. The bias ranges from -9 W m(-2) (-1% of the mean observed irradiance) to +35 W m(-2) (+6%). The root mean square error (RMSE) ranges from 22 W m(-2) (4%) to 47 W m(-2) (8%) and the coefficient of determination ranges from 0.980 to 0.990. Estimates of the direct irradiance at normal incidence exhibit an underestimation that is attributed to the overestimation of the aerosol optical depth in the MACC data set and not accounting for the circumsolar radiation in McClear. The corresponding bias ranges from -57 W m(-2) (-8%) to +6W m(-2) (+1%). The RMSE ranges from 62W m(-2) (9%) to 87W m(-2) (13%) and the coefficient of determination ranges from 0.830 to 0.863. When compared to two other models in the literature, McClear is better able to capture the temporal variability of the direct irradiance at normal incidence. The validation results remain comparable for the global horizontal irradiance. (C) 2015 The Authors. Published by Elsevier Ltd.

Abstract. Routine measurements of the beam irradiance at normal incidence include the irradiance originating from within the extent of the solar disc only (DNIS), whose angular extent is 0.266° ± 1.7 %, and from a larger circumsolar region, called the circumsolar normal irradiance (CSNI). This study investigates whether the spectral aerosol optical properties of the AERONET stations are sufficient for an accurate modelling of the monochromatic DNIS and CSNI under cloud-free conditions in a desert environment. The data from an AERONET station in Abu Dhabi, United Arab Emirates, and the collocated Sun and Aureole Measurement instrument which offers reference measurements of the monochromatic profile of solar radiance were exploited. Using the AERONET data both the radiative transfer models libRadtran and SMARTS offer an accurate estimate of the monochromatic DNIS, with a relative root mean square error (RMSE) of 6 % and a coefficient of determination greater than 0.96. The observed relative bias obtained with libRadtran is +2 %, while that obtained with SMARTS is −1 %. After testing two configurations in SMARTS and three in libRadtran for modelling the monochromatic CSNI, libRadtran exhibits the most accurate results when the AERONET aerosol phase function is presented as a two-term Henyey–Greenstein phase function. In this case libRadtran exhibited a relative RMSE and a bias of respectively 27 and −24 % and a coefficient of determination of 0.882. Therefore, AERONET data may very well be used to model the monochromatic DNIS and the monochromatic CSNI. The results are promising and pave the way towards reporting the contribution of the broadband circumsolar irradiance to standard measurements of the beam irradiance.

Les mesures de l'eclairement solaire direct recu au sol en incidence normale (DNI) par des pyrheliometres ou instruments equivalents incluent l'eclairement provenant de l'angle solide du disque solaire (DNIS) et celui provenant d'une region angulaire circumsolaire plus large, appele eclairement circumsolaire (CSNI). Les instruments ont des demi-angles d'ouverture equivalents variant entre 2,5° et 5°, soit un ordre de grandeur plus grand que le demi-angle du disque solaire. Quant aux demi-angles des systemes de production d'energie concentrant les rayons solaires, ils sont plus grands que le demi-angle du disque solaire, et plus petits que ceux des instruments. Par consequent, le CSNI doit etre connu pour une estimation precise du DNI. Cette these contribue a la connaissance et a la modelisation des eclairements direct et circumsolaire en milieu desertique par conditions de ciel clair. Apres avoir determine les propietes optiques des aerosols les plus influentes, le modele numerique de transfert radiatif libRadtran a ete utilise pour modeliser le CSNI et le DNIS. Un modele parametrique simplifie et tres rapide a ete developpe qui reproduit les resultats de libRadtran. Il estime le ratio circumsolaire (CSR), soit le rapport entre le CSNI et la somme du CSNI et du DNIS, a partir de mesures standards du DNI et de l'eclairement diffus. A partir du DNI mesure et de CSR modelise, le CSNI et le DNIS peuvent etre estimes pour tout demi-angle entre 0,4° et 5°. Le modele a ete valide pour deux stations de mesure, dans les Emirats Arabes Unis et en Algerie.

HelioClim-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 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.

Increasing interest is devoted to the development of solar powered plants in the countries of the Arabian Peninsula. However, the unique climatological conditions of the region, characterized by high dust concentrations, high humidity and modest cloud coverage, cause a challenge in solar radiation estimations. This study presents and compares yearly direct normal, diffuse horizontal and global horizontal irradiation maps for the years 2008 to 2010 over the United Arab Emirates using an ensemble artificial neural network model and a cloudiness index corrected semi-empirical model, both of which were derived from satellite images. A validation against ground measurements from one station is also performed.

Earth observation data and in situ measurements were used to derive a method for the estimation of Air Temperature(AirT) and thereby produce maps. The methodology was developed and validated using data acquired at five stations located in the United Arab Emirates(UAE) during the year 2012. In the first step, the effect of different variables on the estimation of AirT values was investigated and the variables of high importance were used to build the final model. Land surface temperature (LST), relative humidity, global horizontal irradiance, direct normal irradiance, and diffuse horizontal irradiance were identified as the most important inputs for Air Testimation. Models were developed separately for four different cases based on the seasons (Winter/Summer) and the time of the day (Day/Night). The models were evaluated using jackknife validation on the stations resulting in root mean square errors of 2.25°C(Winter/Day), 2.24°C (Winter/Night), 2.56°C (Summer/Day), and 2.69°C (Summer/Night) and an overall average accuracy of 2.44°C. Finally, the model has been applied on a larger scale by assimilating the relevant Earth observation data and in situ measurements for the creation of AirT maps for the entire country. The resulting AirT maps were generated in near real time at a temporal resolution of 15 min from METEOSAT/SEVIRI LST and the above-mentioned in situ measurements. These maps can be considered as important resources for several applications (e.g, climate studies, solar energy applications, thermal comfort studies, etc.).

Accurate solar resource assessments are necessary for numerous applications utilizing solar energy. The global tilt irradiance, in particular, is important for the designs and performance assessments of fixed-tilt flat-plate collector systems. In this study, the global tilt irradiance computed using isotropic and anisotropic models was validated against ground measurements. Then using solar irradiance maps derived over the UAE using a satellite-based model, the global tilt irradiance was computed using both models (assuming a south facing surface with a tilt equal to the latitude). The global tilt irradiation map of the United Arab Emirates for the year 2010 was derived and is presented.

Routine measurements of the direct normal irradiance (DNI) are not sufficient for optimal design of concentrating solar technologies. Due to a generally larger aperture angle of pyrheliometers or equivalent pyranometric systems when compared to that of concentrating collectors, the measured irradiance is overestimated as it includes the irradiance from the solar disc and a larger circumsolar region. The angular distribution of the circumsolar radiances, i.e. the sunshape, can have a significant effect on the performance of concentrating collectors. Therefore, optimal design of concentrating solar technologies requires accurate measurements or estimations of the DNI and the sunshape. Published models are available for reproducing the representative sunshape for a given circumsolar ratio (CSR), i.e. the ratio between the circumsolar irradiance and the sum of the circumsolar and solar disc irradiances. The objective of this study is to estimate the CSR over a cloudless turbid atmosphere using a published sky radiance model and a Radiative Transfer Model (RTM). Using 10 months of solar irradiance and aerosol optical depth measurements, results show that there is an underestimation in the CSR computed by means of the sky radiance model when compared to that computed by the RTM. Also, a high correlation coefficient of 0.87 was found between the CSR estimated from both models, implying that modifications to the sky radiance model are possible to accurately estimate the CSR.

Air Temperature (AirT) is a fundamental parameter in a wide range of applications such as climate change studies, weather forecast, energy balance modeling, efficiency of Photovoltaic (PV) solar cells, etc. Air temperature data are generally obtained through regular measurements from meteorological stations. The distribution of these stations is normally sparse, so the spatial pattern of this parameter cannot be accurately estimated by interpolation methods. This work investigated the relationship between Air Temperature measured at meteorological stations and spatially contiguous measurements derived from Remote Sensing techniques, such as Land Surface Temperature (LST) maps, emissivity maps and shortwave radiation maps with the aim of creating a continuous map of AirT. For LST and emissivity, MSG-SEVIRI LST product from Land Surface Analysis Satellite Applications Facility (LSA-SAF) has been used. For shortwave radiation maps, an Artificial Neural Networks ensemble model has been developed and previously tested to create continuous maps from Global Horizontal Irradiance (GHI) point measurements, utilizing six thermal channels of MSG-SEVIRI. The testing sites corresponded to three meteorological stations located in the United Arab Emirates (UAE), where in situ measurements of Air Temperature were available. From the starting parameters, energy fluxes and net radiation have been calculated, in order to have information on the incoming and outgoing long-wave radiation and the incoming short-wave radiation. The preliminary analysis (day and Night measurements, cloud free) showed a strong negative correlation (0.92) between Outgoing long-wave radiation - GHI and LST- AirT, with a RMSE of 1.84 K in the AirT estimation from the initial parameters. Regression coefficients have been determined and tested on all the ground stations. The analysis also demonstrated the predominant impact of the incoming short-wave radiation in the AirT hourly variation, while the incoming long-wave radiation remains almost constant during the testing period. To conclude, the final AirT maps have been used to calculate continuous maps of Net Radiation, showing an important application of the output of this work for surface energy balance retrieval.

In desert regions, dust is the most critical atmospheric parameter for irradiance assessment and solar resource conversion. Nowadays, atmospheric communities – chemical transport and re-analysis models – provide aerosols information, including dust, but with a spatial resolution greater than 100 km. This low spatial resolution makes the prediction of aerosol loads in a small scale very difficult, considering the possible high spatial variability of this aerosol loads in such desert regions. It has been indeed established in the United Arab Emirates that the variation of aerosol optical depth (AOD) within 100 km can lead to 18% deviation on Direct Normal Irradiance (DNI) estimations. Therefore, the MACC AOD, which is provided at a spatial resolution of 125 km, has to be corrected before being used for DNI estimation, as well as any other publicly available AOD database. In this work, images from the High Resolution Visible channel data of the SEVIRI instrument on board Meteosat Second Generation satellite are used to downscale the MACC AOD. The first results of the downscaling approach are tested with one year datasets of AOD from two AERONET ground stations showing that this downscaling leads to a decrease of the mean absolute error on AOD and on the corresponding estimated DNIs.

A statistical model for the prediction of the solar irradiance components, utilizing six thermal channels of the SEVIRI instrument (onboard Meteosat Second Generation satellite), is presented. Additional inputs to the model include the solar zenith angle, solar time, day number and eccentricity correction. Treating the cloud-free and cloudy observations separately, the model employs two trained artificial neural network ensembles, one for estimating the direct normal irradiance and the other for estimating the diffuse horizontal irradiance. The global horizontal irradiance is then computed from the model’s outputs. The model has been trained using reference data from three ground measurement stations for the full year of 2010 and tested over two independent stations for the full year of 2009. Over the two independent stations for all sky conditions, the relative root mean square errors for the direct, diffuse and global components are 26.1%, 25.6% and 12.4%, respectively, while the relative mean bias errors are −6%, +3.6% and −2.9%, respectively. The temporal and spatial variations of the direct, diffuse and global components are also presented for three days exhibiting different sky conditions in the year 2009.

This study proposes the use of an artificial neural network approach to estimate the direct normal irradiance (DNI), diffuse horizontal irradiance (DHI) and global horizontal irradiance (GHI) at temporal and spatial resolutions of 15min and 3km, respectively. Inputs to the models are six thermal channels of the SEVIRI instrument, onboard Meteosat Second Generation, along with solar zenith angle, latitude, longitude, solar time, day number and eccentricity correction. The study will show the generalization of the results when using an ensemble approach as opposed to a single network. For all sky conditions the testing dataset for DNI estimations have relative root mean square error (rRMSE) and relative mean bias error (rMBE) values of 17.8% and -3%, respectively. Results for DHI estimations are 13.4% and +1.6%, respectively, and finally GHI estimation results show error values of 7.3% and -1.7%, respectively.