Radiometric Stations Research Articles

We-P11:101 The prevalence of metabolic syndrome amongst patients presenting for elective surgery, and its association with post-operative complications

The accurate prediction of direct solar irradiance is essential in many solar energy applications, particularly those relying on concentrating solar technologies. The present paper is aimed at a detailed assessment of a large range of clear-sky solar radiation models under Algerian climate to select the more accurate one for estimating the performance of solar power projects where meteorological and radiometric measurement stations are not available. To this end, seventeen models have been reviewed and their performance compared to measured irradiance of Ghardaia (Southern Algeria). The validation methodology presented herein is very helpful for ranking the models. A new statistical accuracy indicator has been originally introduced to find out the most accurate ones.A thorough analysis of selected models has shown that the more complex models, that seem at first sight more sophisticated, are not necessary the most accurate; while simpler models depending on a limited number of parameters are more suitable. In other words, the suitability and accuracy of a model do not necessarily improve with an increase in the number of its parameters. This important finding is in good agreement with the previous published studies. This fact is important to take into account, in the case where measured data are not available, for the selection of the most suitable locations for the installation of the future concentrating solar power plants in Algeria or even in other countries.

Modelling of broad band ultraviolet clearness index distributions for Cordoba, Spain

The biological and photochemical effects of ultraviolet (UV) solar radiation and solar energy applications make it really important to characterize this component. In this report, for different ranges of optical air mass ( m r), we have described and modelled its behaviour, in terms of the UV clearness index. In addition, we have compared it to that of the horizontal solar broadband global irradiance ( G), in terms of the clearness index. We have used 5-min measurements registered at a radiometric station located in Cordoba (Spain) and analyzed the relative probability distributions as an approximation of the instantaneous ones. We may conclude that, although both have significant similarities, G distributions are bimodal while UV ones are not. According to that, we have modelled UV solar radiation noting that the fitting parameters depend on m r.

Estimate of the atmospheric turbidity from three broad-band solar radiation algorithms. A comparative study

Abstract. Atmospheric turbidity is an important parameter for assessing the air pollution in local areas, as well as being the main parameter controlling the attenuation of solar radiation reaching the Earth's surface under cloudless sky conditions. Among the different turbidity indices, the Ångström turbidity coefficient β is frequently used. In this work, we analyse the performance of three methods based on broad-band solar irradiance measurements in the estimation of β. The evaluation of the performance of the models was undertaken by graphical and statistical (root mean square errors and mean bias errors) means. The data sets used in this study comprise measurements of broad-band solar irradiance obtained at eight radiometric stations and aerosol optical thickness measurements obtained at one co-located radiometric station. Since all three methods require estimates of precipitable water content, three common methods for calculating atmospheric precipitable water content from surface air temperature and relative humidity are evaluated. Results show that these methods exhibit significant differences for low values of precipitable water. The effect of these differences in precipitable water estimates on turbidity algorithms is discussed. Differences in hourly turbidity estimates are later examined. The effects of random errors in pyranometer measurements and cloud interferences on the performance of the models are also presented. Examination of the annual cycle of monthly mean values of β for each location has shown that all three turbidity algorithms are suitable for analysing long-term trends and seasonal patterns.

Selection of input parameters to model direct solar irradiance by using artificial neural networks

A very important factor in the assessment of solar energy resources is the availability of direct irradiance data of high quality. However, this component of solar radiation is seldom measured and thus must be estimated from data of global solar irradiance, which is registered in most radiometric stations. In recent years, artificial neural networks (ANN) have shown to be a powerful tool for mapping complex and non-linear relationships. In this work, the Bayesian framework for ANN, named as automatic relevance determination method (ARD), was employed to obtain the relative relevance of a large set of atmospheric and radiometric variables used for estimating hourly direct solar irradiance. In addition, we analysed the viability of this novel technique applied to select the optimum input parameters to the neural network. For that, a multi-layer feedforward perceptron is trained on these data. The results reflect the relative importance of the inputs selected. Clearness index and relative air mass were found to be the more relevant input variables to the neural network, as it was expected, proving the reliability of the ARD method. Moreover, we show that this novel methodology can be used in unfavourable conditions, in terms of limited amount of available data, performing successful results.

Selection of input parameters to model direct solar irradiance by using artificial neural networks

A very important factor in the assessment of solar energy resources is the availability of direct irradiance data of high quality. However, this component of solar radiation is seldom measured and thus must be estimated from data of global solar irradiance, which is registered in most radiometric stations. In recent years, artificial neural networks (ANN) have shown to be a powerful tool for mapping complex and non-linear relationships. In this work, the Bayesian framework for ANN, named as automatic relevance determination method (ARD), was employed to obtain the relative relevance of a large set of atmospheric and radiometric variables used for estimating hourly direct solar irradiance. In addition, we analysed the viability of this novel technique applied to select the optimum input parameters to the neural network. For that, a multi-layer feedforward perceptron is trained on these data. The results reflect the relative importance of the inputs selected. Clearness index and relative air mass were found to be the more relevant input variables to the neural network, as it was expected, proving the reliability of the ARD method. Moreover, we show that this novel methodology can be used in unfavourable conditions, in terms of limited amount of available data, performing successful results.

Relationship between net radiation and solar radiation for semi-arid shrub-land

This paper presents the results obtained by analysing a set of measurements of surface radiation balance components at a semi-arid location in SE Spain. The relationships between net and surface shortwave radiation were explored by using 38 months of 5 min surface radiation. The study area is covered by sparse clumped shrub-land of different species, although close to the radiometric station Retama sphaerocarpa is dominant. After this study, it is evident that a single model based on surface shortwave radiation could provide appropriate estimates. The inclusion of albedo information provides a slight improvement on the estimation model. The use of seasonal models provides an appreciable improvement, while the benefits of including cloud information are only marginal.

Improved estimation of diffuse photosynthetically active radiation using two spectral models

The efficiency with which incoming photosynthetically active radiation (PAR) is intercepted by a canopy depends on the proportions of diffuse and direct radiation in the incoming PAR. In the present work, we developed a comparison between two spectral cloudless sky parameterization schemes in terms of their capability to provide accurate estimates of global, diffuse and direct incoming PAR. Both methods provide an estimation of the solar spectral irradiance that can be integrated spectrally within the limits of interest. For this purpose, data recorded at two radiometric stations one located at Granada, an inland location, and the other at Almerı́a, a coastal location, were used. After our study, it appears that the information concerning the aerosol radiative effects is fundamental to obtain a good estimation, especially for the direct and diffuse components. In this sense SMARTS2 model offers increased flexibility concerning the selection of different aerosol models included in the code. The original version of SPCTRAL2 code does not offer this flexibility but in this work a modified version that allows the selection between several aerosol models was considered. Our results show that a rather accurate estimation of the global component can be obtained for any of the aerosol models tested in this work, thus indicating the lower influence of the scattering processes in this term. In our analyses, we considered different aerosol models compatible with the features of each one of the analyzed locations. The a priori choice of the aerosol models dictated by the features of each place provided estimation of the different components of PAR with mean bias deviation (MBD) smaller than the experimental error. This is a clear advantage over the simpler broadband parametric models that need local information of all the relevant aerosol properties and some ad hoc modifications to provide accurate estimates of the PAR components.

Dependence of one-minute global irradiance probability density distributions on hourly irradiation

This paper analyzes the behavior of one minute global irradiance distributions as a function of hourly average solar global irradiance. For this purpose, we have used the clearness index k t which describes the atmospheric transmittance. Our interest is in characterizing the intrahourly variability of solar global irradiance and the behavior of the instantaneous values as a function of hourly values of solar global irradiance. The distributions are unimodal and show a marked symmetry around a central value that is close to the corresponding hourly average value. The probability density functions have been modeled using functions based on the Boltzmann statistic used in recent studies of the one minute distributions of k t conditioned to the optical air mass. These functions provide good fit of the distributions and are analytically integrable and can be inverted analytically. The one minute global irradiance data used in this study have been recorded during a three year period in a radiometric station located in south eastern Spain.

Estimation of hourly global photosynthetically active radiation using artificial neural network models

Photosynthetically active radiation (PAR) reaching the earth’s surface is a major parameter controlling many biological and physical processes related with the evolution of plant canopies, agricultural and environmental fields. Unfortunately, PAR is not often measured and therefore it must be estimated. The unavailability of measurements of global solar radiation at the place of interest and different factors affecting the linear relation between PAR and global solar radiation can preclude the estimation of PAR from global solar radiation. In this paper, a novel approach based on a simple multilayered feedforward perceptron has been used to analyse the non-linear relationships between PAR and different meteorological and radiometric variables in order to determine their relative relevance. An artificial neural network based model for the estimation of the hourly PAR involving hourly global irradiance as only measured variable has been successfully developed. The model was tested using data recorded at six radiometric stations covering a wide range of climates. The model performance has been compared with other existing empirical complex models showing important improvements. Next, a second artificial neural network based model involving only sunshine duration measurements has been developed and proved to be an acceptable alternative to calculate hourly PAR when radiometric information is not available.

On the use of a cloud modification factor for solar UV (290-385 nm) spectral range

Knowledge of ultraviolet radiation is necessary in different applications, in the absence of measurements, this radiometric flux must be estimated from available parameters. To compute this flux under all sky conditions one must consider the influence of clouds. Clouds are the largest modulators of the solar radiative flux reaching the Earth’s surface. The amount and type of cloud cover prevailing at a given time and location largely determines the amount and type of solar radiation received at the Earth’s surface. This cloud radiative effect is different for the different solar spectral bands. In this work, we analyse the cloud radiative effect over ultraviolet radiation (290–385 nm). This could be done by defining a cloud modification Factor. We have developed such cloud modification Factor considering two different types of clouds. The efficiency of the cloud radiative effect scheme has been tested in combination with a cloudless sky empirical model using independent data sets. The performance of the model has been tested in relation to its predictive capability of global ultraviolet radiation. For this purpose, data recorded at two radiometric stations are used. The first one is located at the University of Almeria, a seashore location (36.83° N, 2.41° W, 20 m a.m.s.l.), while the second one is located at Granada (37.18° N, 3.58° W, 660 m a.m.s.l.), an inland location. The database includes hourly values of the relevant variables that cover the years 1993–94 in Almeria and 1994–95 in Granada. Cloud cover information provided by the Spanish Meteorological Service has been include to compute the clouds radiative effect. After our study, it appears that the combination of an appropriate cloudless sky model with the cloud modification Factor scheme provides estimates of ultraviolet radiation with mean bias deviation of about 5% that is close to experimental errors. Comparisons with similar formulations of the cloud radiative effect over the whole solar spectrum provides evidence for the spectral dependency of the cloud radiative effect.

Improvement of the Technology of Radiometric Enrichment of Uranium Ores

Under the new economic conditions, the problem of decreasing substantially the cost of uranium in the final product obtained by processing mined ores has become acute. A search has shown that the problem can be successfully solved by closely combining hydrometallurgical conversion in plants with inexpensive processes, such as large-batch sorting in radiometric monitoring stations, radiometric piecewise separation, and mound leaching of low-grade ore. Radiometric enrichment produces tailings as well as average and rich concentrates. The rich concentrate will be processed using a separate scheme without a sorption process. A specific example of the combined scheme and the apparatus developed for implementing this scheme are examined. 1 figure, 3 tables, 10 references.

Estimating solar ultraviolet irradiance (290-385 nm) by means of the spectral parametric models: SPCTRAL2 and SMARTS2

Abstract. Since the discovery of the ozone depletion in Antarctic and the globally declining trend of stratospheric ozone concentration, public and scientific concern has been raised in the last decades. A very important consequence of this fact is the increased broadband and spectral UV radiation in the environment and the biological effects and heath risks that may take place in the near future. The absence of widespread measurements of this radiometric flux has lead to the development and use of alternative estimation procedures such as the parametric approaches. Parametric models compute the radiant energy using available atmospheric parameters. Some parametric models compute the global solar irradiance at surface level by addition of its direct beam and diffuse components. In the present work, we have developed a comparison between two cloudless sky parametrization schemes. Both methods provide an estimation of the solar spectral irradiance that can be integrated spectrally within the limits of interest. For this test we have used data recorded in a radiometric station located at Granada (37.180°N, 3.580°W, 660 m a.m.s.l.), an inland location. The database includes hourly values of the relevant variables covering the years 1994-95. The performance of the models has been tested in relation to their predictive capability of global solar irradiance in the UV range (290–385 nm). After our study, it appears that information concerning the aerosol radiative effects is fundamental in order to obtain a good estimation. The original version of SPCTRAL2 provides estimates of the experimental values with negligible mean bias deviation. This suggests not only the appropriateness of the model but also the convenience of the aerosol features fixed in it to Granada conditions. SMARTS2 model offers increased flexibility concerning the selection of different aerosol models included in the code and provides the best results when the selected models are those considered as urban. Although SMARTS2 provide slightly worse results, both models give estimates of solar ultraviolet irradiance with mean bias deviation below 5%, and root mean square deviation close to experimental errors.Key words: Atmospheric composition and structure (transmission and scattering of radiation) - Meteorology and atmospheric dynamics (radiative process)

Comparison of Cloudless Sky Parameterizations of Solar Irradianceat Various Spanish Midlatitude Locations

We have developed a comparison among several cloudless sky parameterization schemes of the solar irradiance incoming at surface level. The data set was recorded at various Spanish mid-latitude radiometric stations, covering different climatic regimes. It consists of hourly values of the relevant quantities covering periods greater than one year at each radiometric station. Diffuse irradiance data have been corrected for shadowband effect using a model developed by the authors and successfully tested at various radiometric stations. At a first stage, the models were run using all the available local information. After that, we have run the models that present the better performance and, considering that under some circumstances not all the input parameters are available replacing some parameters by their monthly values. As the study reveals, the Gueymard and the Iqbal C model, which require the availability of appropriate information on aerosols, perform best. The influence of precipitable water is of second order thus allowing for the use of estimates based on monthly mean values obtained from climatological records. On the other hand, the study performed on the influence of solar elevation angle on models performance reveals that the worst results correspond to lower solar elevations. This could be a result of limitations in the transmittance parameterization for these elevations.

Empirical modeling of hourly direct irradiance by means of hourly global irradiance

A very important factor in the assessment of solar energy resources is the availability of direct irradiance data of high quality. Nevertheless, this quantity is seldom measured and thus must be estimated from measures of global solar irradiance, a quantity that is registered in most radiometric stations. In this work we analyze the results provided by different models in the estimation of hourly direct irradiance values. We have selected several models proposed by Orgill and Hollands, Erbs et al., Reindl et al., Skarveit and Olseth, Maxwell, and Louche et al. With the exception of the model from Louche et al. that estimates direct irradiance values from direct transmittance values, all of the models estimate direct irradiance from the diffuse fraction. The data set used in this study comprises 25 000 hourly values of global and diffuse irradiance. These values were registered in six Spanish locations with different climatic conditions. The results provided by the model depend on the clearness index, k t, and the solar elevation. The best results are obtained for cloudless skies and higher solar elevation. In those conditions we can estimate the direct irradiance with a root square mean error close to 14% of the average measured value. We have estimated the direct irradiance under cloudless sky conditions using a parametric model proposed by Iqbal. In order to analyze the effect of turbidity on the estimation of direct irradiance we have compared the results obtained by the parametric model when using hourly values of the Angstrom turbidity parameter β with those obtained when using monthly means of hourly values of β.

Estimation of photosynthetically active radiation under cloudy conditions

Clouds are the largest modulators of the solar radiative flux reaching the Earth’s surface. The amount and type of cloud cover prevailing at a given time and location largely determines the amount and type of solar radiation received at the Earth’s surface. This cloud radiative forcing is different for the different solar spectral bands. In this work, we analysed the influence of cloud radiative forcing over the photosynthetically active radiation. Knowledge of the photosynthetically active radiation is necessary in different applications, but due to the absence of widespread measurements of this radiometric flux, it must be estimated from available variables. Cloudless sky parametric models compute the global photosynthetically active radiation at surface level by addition of its direct beam and diffuse components. To compute this flux under all sky conditions one must consider the influence of clouds. This could be done by defining a cloud transmittance function. We have developed such a cloud transmittance function considering three different types of clouds. The efficacy of the cloud radiative forcing scheme has been tested in combination with a cloudless sky parametric model using independent data sets. For this purpose, data recorded at two radiometric stations are used. The combination of an appropriate cloudless sky parametric model with the cloud transmittance scheme provides estimates of photosynthetically active radiation with mean bias deviation about 4% that is close to experimental errors. Comparisons with similar formulations of the cloud radiative effect over the whole solar spectrum shows the spectral dependency of the cloud radiative effect.

Parametric models to estimate photosynthetically active radiation in Spain

Different applications dealing with plant physiology, biomass production and natural illumination in greenhouses require knowledge of the photosynthetically active radiation. In absence of measurements of this flux, one must rely on parametric approaches. In this way, the radiant energy is computed using available atmospheric parameters. In the present work, we have developed a comparison among several cloudless sky parameterization schemes. For this purpose, data recorded at two radiometric stations are used. The first one is located at the University of Almerı́a, a seashore location, while the second one is located at Granada, an inland location. The performance of the models has been tested in relation to their predictive capability of direct, diffuse and global components of the photosynthetically active radiation. After our study, it appears that the information concerning the aerosol radiative effects is fundamental to obtain a good estimation, especially for the direct and diffuse components. In order to improve the fitting of the model to the experimental data, several modifications have been suggested. The modified version of the models provide estimates of direct and global components of the photosynthetically active radiation with mean bias deviation below 2%, and root mean square deviation close to experimental error, with slightly worse results for the diffuse component.

A comparison of ground level solar radiative effects of recent volcanic eruptions

The aerosols injected into the lower stratosphere by the El Chichón and Mt. Pinatubo eruptions have increased the scattering of solar radiation back to space, thus, leading to a decrease in the solar radiation input to the Earth System. In this work we study the evolution in time of the volcanic effects of El Chichón's eruption using broadband solar radiation measured at two radiometric stations located in northern mid and high latitudes: Madrid (40.42°N, 2.75°S) and Tiirikoja (58.87°N, 26.97°E). Further, we compare the radiative effects produced by the Mt. Pinatubo eruption at Madrid with those detected at a close radiometric station located in Almerı́a (36.83°N, 2.41°S) that has been a matter of a previous study. After the El Chichón eruption the aerosol optical depth presented a maximum relative anomaly about 11% in Madrid. The radiative effects of the Mt. Pinatubo are responsible of an increase in aerosol optical depth that reached 11% at Madrid and 12% at Almerı́a. The analysis of the time series of relative anomalies in aerosol optical depth indicates that the anomaly produced by the El Chichón eruption decreased with an average 1/e-decay time around 7±1 months. This e-folding decay is faster than that deduced by the authors in the analysis of the Mt. Pinatubo effect at Almerı́a (36.83°N, 2.41°S). Although the El Chichón and Mt. Pinatubo eruption produced similar radiative effects in the initial stages at different mid-latitude stations, the persistence of the radiative influence of Mt. Pinatubo is greater than that of El Chichón. Due to the increase in aerosol optical depth associated with the El Chichón eruption we find at Madrid a reduction in solar direct irradiance of about 10%, while solar diffuse irradiance presents an increase about 33%. Finally, the effect observed on the global solar radiation is a reduction about 3.6%, giving an indication of the radiative cooling provided by the volcanic aerosol.

ONE MINUTE kb AND kd PROBABILITY DENSITY DISTRIBUTIONS CONDITIONED TO THE OPTICAL AIR MASS

In this work we have analysed and modelled one-minute probability distribution function of solar direct and diffuse irradiance conditioned to the optical air mass. For this purpose, we have used one-minute data acquired in a radiometric station located in southeastern Spain (37.13° N, 3.63° W, 687 m a.m.s.l.). The study has been made over the dimensionless indices k b and k d. We have found marked bimodality in the k b distribution function and asymmetry associated with the unimodal function of one-minute k d values. In spite of these differences, we have modelled both distribution functions using a functional form based on Botlzmann's statistics. These functions have been used in a previous study devoted to modelling the clearness index for global radiation, k t. In order to model the asymmetry that is evident in the k d and k b distribution functions, the functional forms have been modified by the inclusion of an additional parameter. The adjustable parameters included in the model equations present a dependence on the optical air mass.

Direct and diffuse photosynthetically active radiation: measurements and modelling

Knowledge of photosynthetically active radiation is necessary in different applications dealing with plant physiology, biomass production and natural illumination in greenhouses. Nevertheless, due to the absence of widespread measurements of this radiometric flux it is often calculated as a constant ratio of broadband solar radiation. This ratio is affected by many parameters. In our previous work, we analysed the global horizontal component of this flux. This work describes the variations of the direct and diffuse ratios of photosynthetically active radiation to broadband solar radiation in sky conditions. The latter has been characterised by means of variables that could be accessible in common radiometric networks. For this purpose, data recorded at two radiometric stations are used. The first one is located at the University of Almerı́a, a seashore location (36.83°N, 2.41°W), while the second one is located at Granada (37.18°N, 3.58°W, 660 m a.m.s.l.), an inland location. The database includes hourly values of the relevant variables that cover the years 1993–1994 in Almerı́a and 1994–1995 in Granada. We have studied the variability of the ratios of photosynthetically active radiation to broadband solar radiation, both for the diffuse and direct components. We have explained this variability through the influence of sky conditions, represented by means of broadband solar radiation dimensionless ratios, solar zenith angle and dewpoint temperature. Additionally we have developed weather dependent functions of this ratio, to obtain an empirical model, which could be applied to estimate the photosynthetically active radiation in those radiometric networks that register solar broadband irradiance. The model validation shows that all the models estimate the experimental values with mean bias deviation (MBD), close to 1–2%. On the other hand the root mean square deviation (RMSD), presents values close to the experimental error. The regression analysis of measured versus estimated values of direct and diffuse photosynthetically active photon flux density, Q pb and Q pd, indicates the absence of given ranges with special tendencies to over or underestimate the fluxes. The use of data sets from two different climatic conditions enables the verification of the local independence of the proposed technique.

Ground based ultraviolet (290–385 nm) and broadband solar radiation measurements in south-eastern Spain

Presented here is the first analysis of hourly solar ultraviolet irradiance (290‐385 nm) and broadband global irradiance data, registered in a radiometric station located in the outskirts of Granada (37.18°N, 3.58°W, 660 m a.m.s.l.), an inland location in south-eastern Spain, during a 2-year period. According to the prevailing cloudless conditions, the results show that the highest UV radiation levels are received in June or July and the lowest in December. Hourly monthly means of the ratio UV to broadband solar radiation covers a range from a minimum of ca. 3% to a maximum of 5%. The higher values of this ratio are associated to cloudy situations. In this way for 1994, the lowest values of this ratio are encountered in December, and the highest value appears in May, associated with a higher frequency of cloudy days. The study of the ratio UV to global radiation hourly monthly means reveals a clear effect of the optical air mass and of the cloud cover. It is found that, the UV to broadband global radiation ratio increases with decreasing optical air mass and increasing cloud cover. © 1998 Royal Meteorological Society.