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We compare three different methods for determining an average ‘effective‧ UV albedo. These methods are applied to spectral irradiance data from a measurement campaign held in the German Alps during the spring of 1999. The first method is based on the comparison of measurements of absolute levels of UV irradiance with model calculations. The second method takes advantage of changes in the spectral slope of spectral UV irradiance, which is a function of the surface albedo. In the third method, the surrounding area is partitioned into snow-covered and snow-free regions, and the effective albedo estimated by applying a higher or lower reflectivity to each facet before integrating over the surroundings. We present the differences and the correlations between the various methods as well as the results for the different locations.

Seven spectroradiorneters measured simultaneous surface UV irradiances at six different sites in the vicinity of Garmisch-Partenkirchen, Germany, during spring 1999. The measured clear-sky irradiance variability between the sites was analyzed with respect to altitude, aerosol optical depth, solar zenith angle, effective albedo, and tropospheric ozone. For conditions of low aerosol loading the increase of irradiance per 1000 m altitude difference was 9% at 400 nm. 20% at 320 nm, and 30% at 300 nm in this season. Effective albedo differences of 0.15, 0.29, and 0.65 were found between the snow-covered stations and the snow-free ground station with the higher effective albedo values determined at the two mountain stations. Clean continental aerosols with a single-scatter albedo of 0.95 were observed during this campaign. The measurements and the observed variations between the sites should enable more accurate modeling studies to be performed for an Alpine environment.

There exists in the literature many methods of determining the Angstrom parameters for the measurement of the optical thickness of aerosols. This thickness is required in cases such as when the spectral beam solar irradiance at ground level has to be assessed. The present work studies the differences between some of these methods and establishes the relationships between their results. A systematic study investigating 509 direct solar irradiance spectra was undertaken. As a result, a new wavelength pair for filter measurements is proposed to calculate the Angstrom parameters with the Voltz technie, namely, the pair (0.400 μm, 0.750 μm). This new pair is the one which best describes the solar beam attenuation in the atmospheric transmittance models. For the considered atmospheric conditions (continental urban aerosols), the mean value of the Angstrom parameter α is found to be 1.0±0.3, which is relatively close to the conventional α=1.3 if taking into account the variance. The wavelength range explored is from 0.290 μm to 0.900 μm, which concerns many applications in bioscience.

A set of 509 direct solar irradiance spectra, carefully measured over one year, is checked against spectral irradiances computed from five updated transmittance models. The wavelengths under investigation range from 290 to 900 nm, with a 5- or 10-nm step. The parameters explored include the solar altitude angle, with a range from 13° to 68°, and the standard Linke turbidity factor, with a range from 2.0 to 6.0. Measurement devices and experimental processes are described in detail in the paper. The comparison between measured and computed values is carried out by means of the relative mean bias error and the mean absolute relative error. These coefficients are applied to ultraviolet-B and ultraviolet-A total irradiances, and to visible and near-IR spectral irradiances. No clear and systematic sensitivity of the models or measurements to the solar altitude and the turbidity parameters is observed. Of the five models tested, three of them give mean coefficient values between 7% and 16% in UV bands ...

The concept of standardized Linke's turbidity factor TLAM2 allows the conversion of the usual TL values, which exhibit virtual diurnal variations into new ones, which are strictly representative of the atmospheric turbidity caused by both water vapor and aerosol. The TLAM2 are TL values reduced at relative air mass 2 on the basis of a spectral TL model recently developed. The TLAM2 values are used to analyze the variations of atmospheric turbidity in a steep-sided urban alpine valley over a 3-year period. It appears that the dependence of the turbidity on altitude is related to the annual cycle of high and low turbidity periods. The TLAM2 concept is required to detect the instantaneous changes of atmospheric turbidity during the course of a given day, but it can also reveal the general trends in diurnal variations of turbidity, in relation to the season and to the altitude.

In spectral diffuse solar irradiance measurements, when diffusing devices used are neither perfectly Lambertian nor have an ideal cosine response, significant errors may spoil the collected data. An optical method permits the determination of a spectral correction factor (SCF) which fully compensates for the diffusers' imperfection when the sky radiance is isotropic. A study of the errors introduced when using such a isotropic SCF in anisotropic radiance conditions is presented for two common flat-plate diffusers fitted with a shadow-ring or a tracking disk. The wavelength band explored is 0.29–0.90 μm and the clear sky radiance model used is Kittler's. The relevance of several diffuse irradiance spectra measured on clear sky days with isotropic SCF is analysed by comparison with Brine-Iqbal model spectra and total diffuse measurements. A remarkable coherence is found for small solar zenith angles when using a tracking disk and a diffuser with an isotropic SCF smaller than 1.2.

A model of Linke's turbidity factor, TL, is developed by means of updated spectral extraterrestrial irradiances and extinction coefficients of gaseous absorbers. It is shown that the new values of TL are clearly different from those obtained by Kasten's formula which parameterizes the optical thickness of the clean dry atmosphere. The model is used to investigate the dependence of TL on the relative optical air mass and to elucidate the relationships linking TL to Angstrom's turbidity coefficient and to the water vapor content. For any TL, the corresponding value related to the air mass 2.0 can be determined. Such a standardized value is independent of solar elevation and is therefore strictly representative of the atmospheric turbidity. It can be linked to Angstrom's turbidity coefficient. Practical procedures and algorithms for computing the standard Linke's turbidity factor and determining Angstrom's turbidity coefficient are described. A relationship for converting the TL values obtained by Kasten's formula into the new values is proposed.

Simultaneous measurements of horizontal global solar irradiance were done over three years, at altitudes ranging from 200 m to 1,700 m in an industrial alpine valley, for studying the clearness index variations versus height. Estimated values of the Angstrom coefficient suggested that the main cause of depletion of the clear sky clearness index k c at the lower levels was the scattering by the aerosols. For irradiances, a depletion term D = 1 − k c was empirically assumed to be a function of the altitude z , as simple as D = ( Mz + N ) −1 . A study carried out over 8,000 10-minute sequences of measurement, during clear sky periods, showed that the coefficient M was sensible to the season only, while N was mainly a function of the sun elevation h such as N = A ( h − C ) 0.5 + B ; from 240 m to 1,100 m, k c was found increasing by about 12% for h = 30° and 7% for h = 60°. The yearly global horizontal irradiation, although including many cloudy sequences, appeared linearly related to ln( z ), its relative increase between 200 m and 1,700 m rising 10%; whereas, for shorter periods, the monotony of the vertical profiles of the clearness index for irradiation was often significantly affected by local stationary clouds or seasonal mists.

As a result of sky radiance anisotropy, significant errors spoil diffuse solar irradiance data obtained by means of noncosine-true instruments, which happens in spite of knowledge of the relative cosine response. To reduce these errors we recommend a method that involves an additional measurement of diffuse irradiance from a horizon band. The efficiency of the so-called double-measurement method is found to be optimum for a 25° horizon band elevation. We confirm the feasibility of the method with relevance tests that use diffuse solar spectral measurements carried out on cloudless skies by means of an UV spectroradiometer.

Routine measurements of global and diffuse UV irradiances at Briançon station (1310 m a.s.l.) are used to retrieve the direct solar irradiance and the aerosol optical depth (AOD), for cloudless days. Data of three years (2003, 2004, 2005) are analyzed; the results confirm those of a preliminary analysis for 2001, 2002. The atmosphere is very clear in winter, with AODs between 0.05 and 0.1. The turbidity increases slowly in spring, starting end of February, with AODs around 0.2–0.3 in mid summer, some values reaching 0.4. A similar behaviour is observed for all years, with somewhat higher values in late summer for the year 2003.

The diurnal and annual variability of solar UV radiation in Europe is described for different latitudes, seasons and different biologic weighting functions. For the description of this variability under cloudless skies the widely used onedimensional version of the radiative transfer model UVSPEC is used. We reconfirm that the major factor influencing the diurnal and annual variability of UV irradiance is solar elevation. While ozone is a strong absorber of UV radiation its effect is relatively constant when compared with the temporal variability of clouds. We show the significant role that clouds play in modifying the UV climate by analyzing erythemal irradiance measurements from 28 stations in Europe in summer. On average, the daily erythemal dose under cloudless skies varies between 2.2 kJ m-2 at 70N and 5.2 kJ m-2 at 35N, whereas these values are reduced to 1.5–4.5 kJ m-2 if clouds are included. Thus clouds significantly reduce the monthly UV irradiation, with the smallest reductions, on average, at lower latitudes, which corresponds to the fact that it is often cloudless in the Mediterranean area in summer.

El Libro “Radiación Solar en el sistema Tierra-Atmósfera” es traducción de la versión original francesa titulada “Le Rayonnement Solaire dans l’ Environment Terrestre”, realizada por el Dr. Alain Chiron de la Casinière. Queremos resaltar aquí dos puntos importantes: uno de ellos es el relativo al carácter educativo y científico del mismo como elemento básico para el estudio de la radiación solar y el otro a la oportunidad de editar un libro de estas características en lengua española. El Sol es la única fuente de energía del planeta Tierra y por tanto el que determina su clima en toda su extensión. La energía radiada por el Sol que llega a nuestro planeta es una cantidad gigantesca de energía que ha de aprovecharse como prototipo de las “limpias”, en el momento en el que la sociedad se enfrenta al reto de la descarbonización como política base para enfrentar el Cambio Climático, considerado ya uno de los problemas fundamentales de la Sociedad. El libro se presenta con una estructura muy académica, obviamente al estilo francés, donde la repetición y rigor es un elemento fundamental en la enseñanza. El libro se estructura en cuatro amplios capítulos. El primero es una introducción a las nociones básicas de radiometría. El segundo capitulo está conformado en dos partes, una dedicada los conceptos propios de la astronomía terrestre y solar. La otra parte se dedica a estudiar ampliamente los procesos básicos de interacción radiación-atmósfera: absorción y dispersión o scattering, que conforman el proceso de atenuación o extinción de la radiación a través de la atmósfera mediante la ley de Beer-Lambert-Bouguer. El tercer capítulo se dedica a estudiar los modelos de radiación solar, desde los que evalúan la insolación a las irradiancias o flujos solares que incluyen tanto los modelos “Broadband” como espectrales, hasta llegar a los llamados Códigos de Cálculo de Transferencia Radiativa. El último capítulo se dedica a estructurar y clasificar los diferentes instrumentos de medida de la radiación y a los principios de calibración y medida, con un breve apunte sobre la acción biológica y química de la radiación solar. Todos estos capítulos se acompañan con una serie de observaciones a los conceptos fundamentales definidos, así como con una serie de problemas y ejercicios y su solución. El segundo punto a resaltar es la importancia de disponer en español de un libro de estas características pues apenas existen obras de este tipo, al ser comunes de uso las escritas en el idioma Inglés. El nivel académico/científico del libro es notable y en él aparecen toda una gama de términos que proceden en su mayoría de la terminología científica original del inglés, aunque en este caso particular además vienen de un libro en francés, es por ello que la obra original contiene una nomenclatura y un amplio índice de términos con un léxico en francés-inglés. Este libro debería de servir a la amplia comunidad de habla hispana como una herramienta elemental para la introducción al estudio de la radiación solar en el sistema global Tierra-Atmósfera., Fotografías de la cubierta pertenecientes al Grupo de Óptica Atmosférica de la Universidad de Valladolid (GOA)

Received 18 March 2005; revised 25 July 2005; accepted 26 September 2005; published 17 December 2005. (1) A differential absorption method, developed by the Laboratoire d'Optique Atmospherique (LOA), is applied to retrieve the total ozone column from UV global irradiance spectra under clear and cloudy sky conditions. Contrarily to clear sky, cloudy sky generates a high uncertainty in the retrieved ozone column. This study shows that under cloudy conditions the daily ozone mean is a rather good estimation of the true value. The standard deviation allows us to estimate the relative uncertainty of this mean value, i.e., about 7%. Results for all conditions from 3 years at Villeneuve d'Ascq are presented, as well as a comparison with TOMS (Total Ozone Mapping Spectrometer) data. The results are very similar to those obtained for clear days: In 75% of cases both values agree to within 5%; however, some cases present larger relative differences. Over the 3 years considered, there is a bias less than 3% (LOA > TOMS). This method is applied to five other European sites involved in the EDUCE (European Database for UV Climatology and Evaluation) project. Three of them are Brewer stations. The results of the comparison between the LOA-retrieved ozone and TOMS data show that a relative difference smaller than 5% is obtained in 63-80% of the cases, depending on site and year. Biases smaller than 3% are observed (LOA > TOMS). The comparison between the LOA-retrieved ozone and total ozone data from direct sun and zenith sky observations of the Brewer instruments shows better agreement. For more than about 80% of cases, except at one site, the relative difference is smaller than 5%, and the biases are smaller than 1%. The LOA method allows us to obtain a complementary data set and hence to provide time series of reliable measurements of total ozone column under all sky conditions.

Direct ultraviolet spectral solar irradiance is regularly obtained by the difference between global and diffuse irradiances at the French Alpine station of Briancon; the data of years 2001 and 2002 are analyzed in this paper. Comparison with modeled values is used for cloud screening, and an average UV-A aerosol optical depth is used as an index of turbidity; it is found to be around 0.05 for the clear winter days and around 0.2 in summer. Langley plots are used to verify the instrument calibration; they confirm the expected uncertainty smaller than 5%. The ozone total column amount is estimated with an uncertainty between -3 and +5 Dobson units; comparisons with TOMS (Total Ozone Mapping Spectrometer) overpass values shows agreement within the expected uncertainties of both instruments.

A measurement campaign was carried out in February 1998 at Briançon Station, French Alps (44.9 degrees N, 6.65 degrees E, 1,310 m above sea level) in order to determine the UV effective snow albedo that was retrieved for both erythemal and UV-A irradiances from measurements and modeling enhancement factors. The results are presented for 15 cloudless days with very variable snow cover and a small snowfall in the middle of the campaign. Erythemal irradiance enhancement due to the surface albedo was found to decrease from approximately +15% to +5% with a jump to +22% after the snowfall, whereas UV-A irradiance enhancement decreased from 7% to 5% and increased to 15% after the snowfall. Thesevalues fit to effective surface albedos of 0.4, 0.1, and 0.5 for erythemal, and to effective albedos of 0.25, 0.1, and 0.4 for UV-A irradiances, respectively. An unexpected difference between the effective albedos retrieved in the two wavelength regions can be explained by the difference of the environment contribution.

Albedo inversion techniques are investigated in this work. Several methods are applied to spectral irradiance data from a measurement campaign held in the German Alps during the spring of 1999. One first method is based on the comparison of measurements of absolute levels of UV irradiance with model calculations. The second method takes advantage of changes in the spectral slope of spectral UV irradiance, which is a function of the surface albedo. In the third method, the surrounding area is partitioned into snow- covered and snow-free regions, and the effective albedo estimated by applying a higher or lower reflectivity to each facet before integrating over the surroundings. We present sensitivity analysis, the differences and the correlations between the various methods as well as the results for the different locations.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A measurement campaign was organized in March 1999 in the Bavarian Alps as part of the European project, Characteristics of the UV Radiation Field in the Alps (CUVRA), to analyze the effect of altitude, aerosols, and snow cover on ground-level UV spectral irradiance. We present the results of simultaneous measurements of aerosol optical depth (AOD) made at various sites on two cloudless days in March 1999. The two days exhibited different aerosol conditions. Results derived from spectral measurements of UV irradiance are compared with data from filter radiometer measurements made at discrete wavelengths extending from the UV to the near IR. The different methods generated values for the AOD that were in good agreement. This result confirms that one can use either method to retrieve the AOD with an uncertainty of approximately 0.03-0.05. On 18 March, high turbidity was observed at low altitude (400-nm AOD approximately 0.5 at 700 m above sea level), and the AOD decreased regularly with altitude; on 24 March, the turbidity was much less (0.11 at 700 m above sea level). On both days very low AODs (0.05-0.09) were measured at 3000 m above sea level. The spectral dependence of the AOD is often parameterized by the angstrom relationship; the alpha parameter is generally difficult or impossible to retrieve from spectral measurements because of the relatively narrow wavelength range (320-400 nm), and only one of the spectro-radiometers used during the campaign permits this retrieval. In most cases, during this field campaign, alpha was found by filter sunphotometers to be 1.1-1.5.

A new method for correcting the wavelength misalignment in measured UV spectra is presented. It is based on a comparison between measured irradiances and irradiances computed from a radiative transfer code for a set of given atmospheric and solar conditions (250 < Dobson units < 450, 30 degrees < solar zenith angle < 75 degrees ). Results of tests run with spectra recorded on a clear-sky day by two spectroradiometers in a French UV spectral network station are analyzed. Applying the method once reduces shift to less than 0.05 nm. The smoothing included in the method enables detection of aberrant irradiance values and then completion of an initial quality control of measured spectra. A technique for assessing the instruments' slit function is also presented. The key algorithms needed to build a computer code based on this method are given.

The relative importance of the principal elements of the thermal balance above snow during a period of melting has been evaluated using recent studies of the turbulent transfer in the boundary layer. Several anomalies in the profiles prove the continuing presence, near to the surface, of a thick layer of air apparently re-heated by the radiative flux. It is shown that the upper surface of this “exchange layer” ought to be considered as the new level for the origin of profiles, and taken as the exchange surface. The importance of nocturnal refreezing has been quantified. Excellent correlations have been found between mean air temperature and total energy balance; they have allowed the establishment of an approximate, non-linear expression for the effective daily ablation of snow as a function of mean air temperature. This relation gives satisfactory results at high and intermediate altitudes, in horizontal uncovered terrain. The measurements were made in the French Alps at 3550 m and in Spain at the latitude of Madrid à 1860 in.