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Aerosols and clouds are the most important constituents in the atmosphere that affect the incoming solar radiation, either directly through absorbing and scattering processes or indirectly by changing the optical properties and lifetime of clouds. Under clear skies, aerosols become the dominant factor that affect the intensity of solar irradiance reaching the ground. Under cloudy skies, the high temporal and spatial variability of cloudiness is the key factor for the estimation of solar irradiance. In this study, recent research activities related to the climatology and the prediction of solar energy in Greece are presented with emphasis on new challenges in the climatology of global horizontal irradiance (GHI) and direct normal irradiance (DNI), the changes of DNI due to the decreasing aerosol optical depth and the short-term (15–240 min) forecasts of solar irradiance with the collaborative use of neural networks and satellite images.

An all-sky imaging system is deployed to estimate the effect of clouds on incident solar irradiance, for a 2-year period over the city of Thessaloniki, Greece. The minutely cloud radiative effect (CRE) is examined in relevance to the cloud cover and type as well as the percentage of the solar disk covered by clouds and the relative position of Sun and clouds in the sky. CRE increases with the cloud cover and decreases with the solar zenith angle (SZA). The minimum instantaneous values can reach −900 W m −2 while enhancement events are found to reach up to +200 W m −2 . The greatest cooling effects are caused by thick cumulus clouds, in cases where obstruction of the solar disk is visible and by stratocumulus, stratus-altostratus and cumulonimbus-nimbostratus when accompanied by high values of cloud cover. The enhancement events are mostly found when the clouds are in the vicinity of the Sun and when the clouds are accumulated at the upper part of the sky but the Sun is in a lower position.

Cloud properties derived from the SEVIRI (Spinning Enhanced Visible and Infrared Imager) instrument, on board the MSG (Meteosat Second Generation) satellite, have been used to retrieve the surface global solar irradiance incident on a horizontal surface GHI (global horizontal irradiance) and the DNI (direct normal irradiance) with a temporal resolution of 15 min. The daily amount of solar energy as well as monthly and annual sums, are estimated. Based on a 6-year study period (2008–2013), a monthly climatology is derived. Results are compared with ground-based measurements in Greece. Comparison shows a general good agreement between satellite and ground data, with the highest differences occurring in cases of broken or very thick cloudiness. The highest collected monthly solar energy values are found during summer months, in Southern Peloponnese, Crete and Cyclades islands, and exceed 250 kWh m −2 . The annual average energy for GHI is 1400–1500 kWh m −2 in Northern Greece and up to 1900 kWh m −2 in Southern Peloponnese, Crete and the islands. For DNI, values increase to about 9% in Northern Greece and around 15% in Southern Greece.

Aerosol optical depth is a key atmospheric constituent for direct normal irradiance calculations at concentrating solar power plants. However, aerosol optical depth is typically not measured at the solar plants for financial reasons. With the recent introduction of all-sky imagers for the nowcasting of direct normal irradiance at the plants a new instrument is available which can be used for the determination of aerosol optical depth at different wavelengths. In this study, we are based on Red, Green and Blue intensities/radiances and calculations of the saturated area around the Sun, both derived from all-sky images taken with a low-cost surveillance camera at the Plataforma Solar de Almeria, Spain. The aerosol optical depth at 440, 500 and 675nm is calculated. The results are compared with collocated aerosol optical measurements and the mean/median difference and standard deviation are less than 0.01 and 0.03 respectively at all wavelengths.

The effect of spatial and temporal variability of aerosol optical depth (AOD) on direct normal irradiance (DNI) under clear skies is studied, with the synergetic use of satellite and ground-based data as well as calculations from a radiative transfer model. The area of interest is Europe; data from May to September during 13 years (2000–2012) are analyzed. The aerosol effect on DNI is high in areas influenced by desert dust intrusions and intense anthropogenic activities, such as the Mediterranean basin and the Po Valley in Italy. In May, the attenuation of DNI from aerosols, over these areas, can reach values up to 35% and 45% respectively, which corresponds to 4 and 6 kWh m−2 per day. In most areas, even for periods with lower values of AOD, the attenuation of DNI is found to be around 20%, which corresponds to about 2–3 kWh m−2 less received DNI per day, compared to the corresponding value on an aerosol clean day. However, the DNI has increased during the recent years, due to the decreasing tendency of AOD over most areas of Europe. The increase is around 6–12%, which corresponds to an amount of 0.5–1.25 more kWh m−2 received per day, compared to a clean day. The percentage differences of daily DNI from the corresponding monthly climatological value reveals that day-to-day differences (due to AOD changes) from the monthly mean, by ±20%, can occur. The significance of the aerosol changes in Europe reveals the necessity for near real-time measurements or forecasts of AOD when reliable estimations of DNI are required.

Methods applied on efficient planning of ground-based monitoring networks of surface solar irradiance could provide valuable scientific results and be useful for accurate monitoring and efficient planning of solar energy applications. Based on the dominance of cloud effect on solar irradiance and the advantage of the high spatial resolution of a geostationary satellite, a novel method is presented for optimizing the location of measuring sites for the newly built Hellenic Network of Solar Energy (www.helionet.gr). The k-means algorithm is used for cluster analysis and the validation of the clustering method reveals that the variability of surface solar irradiance due to cloudiness over Greece could be sufficiently monitored with the establishment of 22 ground-based instruments. The spatial representativeness of the proposed sites is also assessed. The proposed number of stations could be considered as the basis to build the climatology of surface solar irradiance over Greece.

The measurements of aerosol optical depth, total ozone and UV irradiance from a Brewer spectrophotometer located at Uccle, Belgium, were used to estimate, for the first time at a typical site in Western Europe, the aerosol radiative forcing efficiency (the forcing performed per unit of aerosol optical depth). The study was performed at selected solar zenith angles during the period July 2006–May 2010. In the 300–360 nm spectral region, the highest values were revealed at 30° (−6.9 ± 0.9 W m−2), while at 60° the RFE was almost 2.5 times lower (−2.7 ± 0.1 W m−2). In the UV-B region (300–315 nm), the RFE value at 60° (−0.069 ± 0.005 W m−2) was 5 times lower than the corresponding value at 30° (−0.35 ± 0.04 W m−2). Extending previous studies for the estimation of aerosol single scattering albedo in UV-A wavelengths down to 340 nm, an attempt was made, taking advantage of the Brewer measurements, to provide estimates at low UV-A wavelengths and in the UV-B region. The estimated monthly averages of the Brewer single scattering albedo at 320 nm are in very close agreement (within ±0.01) with measurements at 440 nm from a collocated CIMEL sunphotometer. Due to increased measurement uncertainties and the effect of ozone absorption, large differences between the two instruments were found at 306.5 nm. For the rest of wavelengths, average differences up to 0.03 were revealed.

Aerosols and clouds are the most important constituents in the atmosphere that attenuate the incoming solar radiation. Under clear skies, aerosols become the dominant factor that affect the intensity of solar irradiance reaching the ground with the variability in direct normal irradiance (DNI) being more important than the one induced in global horizontal irradiance (GHI). For cloudy skies, the spatial and temporal variability of clouds is a challenging issue on solar irradiance resource and forecasting. In recent years, all-sky imagers are used for the detection of aerosol optical properties, cloud coverage, type and velocity in a bouquet of applications including meteorological observations and solar energy control systems. The developed algorithms can provide also sufficient information about the reconstruction the three-dimensional nature of clouds, the real-time resource and short-time (0–30 min) forecast of solar irradiance. In this study, a brief overview of the current bouquet of methodologies—named as “Sky Analyzer”—developed in the Laboratory of Atmospheric Physics, University of Patras for all-sky camera applications in meteorology and solar energy forecasting is presented.

An all-sky imaging system has been deployed to derive data regarding cloud cover and cloud type, for the city of Thessaloniki, Greece. A 2-year (2006–2007) period is studied in order to estimate the effect of clouds on incident solar radiation. The method is based on irradiance measurements and modeled clear-sky irradiances, corresponding to the measurements’ conditions. The cloud effect is studied based on the cloud type and the percentage of visibility of the solar disk. Cloud radiative forcing shows a dependence on solar zenith angle with the highest values being found at the highest sun elevations, while it increases with total cloud cover. The effect is mainly cooling at the surface level, with the maximum instantaneous values reaching down to −900 Wm−2. Positive values are found, up to +200 Wm−2, caused by enhancements from broken clouds. The greatest cooling effects are caused by thick cumulus and cirrocumulus–altocumulus clouds, in cases where obstruction of the solar disk is present and by stratus, stratocumulus and cumulonimbus, when accompanied by high values of total cloud cover.

The Hellenic Network for Solar Energy is a network dedicated to provide information regarding the availability of solar energy in Greece with the purpose of supporting solar installations. The Network comprises the use of satellite products and calculations from radiative transfer and prognostic models. The results have been validated using a ground-based network of 14 stations, located in various areas of Greece that provide surface solar irradiance measurements. Cloud data derived from the Meteosat Second Generation satellites are used, along with radiative transfer models, to calculate the available solar energy. As a result, global horizontal and direct normal irradiances are calculated with a high spatial and temporal resolution (5 km and 15 min respectively) from 2002 and onwards. Based on data from the period 2002 to 2012, a climatology of the solar energy field in Greece, has been produced. Solar maps are provided, presenting the climatological annual amounts of global horizontal and direct normal irradiances, for the whole country as well as for each geographic region in particular. The maps are freely available at: http://atmosphere-upatras.gr/en/solarmaps.

The variability of aerosol optical properties over the Mediterranean basin, in the ultraviolet (UV) and visible (VIS) spectral regions, is examined, using ground-based data from eight stations of the AErosol RObotic NETwork (AERONET), climatological values from the AeroCom (Aerosol Comparisons between Observations and Models) project and data from the MODerate resolution Imaging Spectroradiometer (MODIS) on-board the Terra and Aqua satellites. The aerosol optical properties from the three datasets are used as input data in a radiative transfer model, in order to estimate the differences in irradiance at the ground under cloud-free skies. The MODIS aerosol optical depth climatology shows better agreement with AERONET data. The highest difference in the monthly average values is equal to 0.09 at 550 nm, while the differences between the AERONET and the AeroCom climatologies reach 0.25 and 0.15 in the UV and VIS wavelengths, respectively. As a result, the AERONET modelled VIS and UV irradiances are closer to MODIS, with the absolute differences in average values reaching 15 (6%) and 2 W m–2 (6%), respectively, while absolute differences with AeroCom irradiances can reach up to 30 (12%) and 4.5 W m–2 (12%). The differences are higher in FORTH Crete, Nes Ziona, Sede Boker and Blida, which are influenced by desert dust aerosols. The separate effect on irradiances due to differences in aerosol optical depth and single scattering albedo from the three datasets has been also examined; according to results, the effect of aerosol optical depth is dominant. At this stage, the use of MODIS aerosol climatology would be recommended for calculations of UV and VIS irradiance in the Mediterranean basin. However, the AeroCom climatology, with some further improvements in the dust cycle, could be a valuable tool in future studies in this area, due to the provision of natural and manmade aerosol optical properties with higher spectral resolution.

Methods applied on efficient planning of ground-based monitoring networks of surface solar ultraviolet (UV) irradiance could provide valuable scientific results and be useful for accurate monitoring and assessment of climatic values for UV-health studies. Based on the dominance of cloud effect on solar irradiance and the advantage of the high spatial resolution of a geostationary satellite, a novel method is presented for optimizing the location of UV measuring sites. The k-means algorithm is used for cluster analysis and the validation of the clustering method reveals that the variability of surface solar UV irradiance due to cloudiness over Greece could be sufficiently monitored with the establishment of 23-28 ground-based instruments.

This study investigates the influences of aerosols optical properties on the UV solar irradiance reaching the ground. It does so using ground-based spectroradiometric measurements taken over the Uccle atmosphere during July 2006 to May 2010. Calculations with the radiative transfer code UVSPEC (LibRadtran package) are used in order to estimate the aerosol effect in the UV and the single scattering albedo. The results are compared with those from a collocated Cimel instrument, part of the AErosol RObotic NETwork (AERONET). The accuracy of the single scattering albedo estimation depends on the aerosol optical depth (AOD) and the solar zenith angle (SZA) at the time of the measurement and is higher for high AOD.

We introduce the “Hellenic Network for Solar Energy”, a system for supporting applications of solar energy with the combined use of measurements from the ground, satellite images, theoretical calculations with radiative transfer models and predictions. This study presents the design of the Network which is currently under development, the supporting networking applications, and preliminary results from basic methodologies that have been developed relating to the assessment of solar energy in near real-time using satellite images, and the forecast of solar energy using a meteorological forecasting model. In addition, preliminary results of the evaluation of these methods by comparison with ground based measurements from the stations of the Network are shown.

Clouds are one of the basic factors that influence Earth’s climate and energy balance. Depending on their type and position they can reflect the incoming solar radiation or absorb the infrared radiation that is emitted by Earth. Their role in the climatic changes can be studied if their characteristics and variations are known with high temporal and spatial resolution. In this study, the cloud modification factor (CMF) is studied for the city of Thessaloniki, based on ground and satellite data. Given the radiation measurements at the ground station of Thessaloniki and model calculated values under cloudless conditions, the ground-based values of CMF are estimated. These are compared with data from the Meteosat Second Generation (MSG) satellite, provided every 15 min and from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument, on board the Terra and Aqua satellites which provides the data twice a day. The comparison covers the period of 2007–2010. The linear regression between the monthly values of MSG and ground CMF has a slope of 0.81, while MODIS data are in better agreement with the ground measurements (slope = 0.94). The higher differences between the MSG and ground data correspond to cases of broken or very thick clouds.