Short-term variability of experimental ultraviolet and total solar irradiance in Southeastern Spain

Atmospheric Environment 45 (2011) 4815e4821

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Short-term variability of experimental ultraviolet and total solar irradiance in Southeastern Spain M. Antón a, b, *, J.E. Gil a, b, A. Cazorla a, b, J. Fernández-Gálvez a, b, I. Foyo-Moreno a, b, F.J. Olmo a, b, L. Alados-Arboledas a, b a b

Centro Andaluz de Medio Ambiente (CEAMA), Av. del Mediterráneo s/n, 18006 Granada, Spain Departamento de Física Aplicada, Universidad de Granada, Campus de Fuentenueva s/n, 18071 Granada, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 February 2011 Received in revised form 24 May 2011 Accepted 9 June 2011

This paper quantifies the very short-term variability of the total solar irradiance and the ultraviolet erythemal irradiance (UVER) averaged over 1-min intervals at Granada (Southeastern Spain). A statistical analysis for a four-year period (January 2006eDecember 2009) under different cloudiness and characterized by the amount of cloud cover (oktas) retrieved from an All-Sky Imager located next to the radiometers is presented. Very short-term variability of the total solar irradiance was larger than UVER fluctuations under cloudy conditions (above three oktas), in accordance with previous works found in the literature. Nevertheless, for cloud cover bellow three oktas the opposite was true; the median relative 1-min fluctuation was larger for UVER than for total solar irradiance. Moreover, while the coefficient of variation (CV) for UVER presented a clear dependence on the solar zenith angle (SZA) under completely cloud-free conditions (from 1.5% for SZA ¼ 20 to 9.5% for SZA ¼ 65 ), the CV of the total solar irradiance was under 1.3% with a more stable behaviour for the entire range of SZA. Large differences were found for cloud cover of seven oktas, where the median diurnal 1-min variability for total solar irradiance was 3.9% min1 compared to 2.5% min1 for UVER data. Additionally, an episode with surface total solar irradiance higher than its corresponding extraterrestrial value is analyzed. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Ultraviolet irradiance Total solar irradiance Broadband UV radiometer Short-term variability

1. Introduction The energy budget of the eartheatmosphere system is mainly controlled by the solar radiation received at ground level. Thus a detailed knowledge of its temporal variability is truly valuable for many topics such as soilevegetationeatmosphere energy budget models, validation of climate models, studies focussing on the use of solar radiation as a source of energy, etc. In addition, the study of the temporal variability of the ultraviolet (UV) radiation (100e400 nm) at the Earth’s surface becomes a high priority in scientific research since it affects many biological, ecological and photochemical processes, often being harmful for living organisms (Diffey, 1991, 2004). Cloud cover and aerosols present a high temporal variability that, especially in the former case, is responsible for high variability in the solar radiation at short-term scales, from significant enhancements to almost total reduction (Kasten and Czeplak, 1980;

* Corresponding author. Departamento de Física Aplicada, Universidad de Granada, Granada, Spain. Tel.: þ34 924 289536; fax: þ34 924 289651. E-mail address: [email protected] (M. Antón). 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.06.020

Nann and Riordan, 1991; Beyer et al., 1994; Frederick and Steele, 1995; Bartlett et al., 1998). In addition, it is well known that UV radiation variability is also mainly controlled by cloudiness at short-term scales (Frederick and Snell, 1990; Lubin and Frederick, 1991; Wang and Lenoble, 1996; Matthijsen et al., 2000; AladosArboledas et al., 2003; Calbó et al., 2005; Mateos et al., 2010). Cloudiness variability may reduce, cancel or even reverse the expected UV radiation increase caused by the reduction in the amount of ozone (Glandorf et al., 2005; WMO, 2007). Therefore, it is of high interest to analyze the short-term variation of solar radiation at ground level. Several studies have evaluated the variability of the total solar radiation (entire solar spectrum) at very short-time scales, usually quantifying its probability distribution in the order of minutes or less (Suehrcke and McCormick, 1988; Skartveit and Olseth, 1992; Tovar et al., 1998, 1999; Varo et al., 2006; Tomson and Tamm, 2006; Soubdhan et al., 2009). Other studies have focused on quantifying of solar irradiance fluctuations at short-time scales using spectral analysis (Woyte et al., 2007) or evaluating its fractal dimension (Harrouni et al., 2005). Recently, Tomson (2010) studied the dynamic processes of fast-alternating solar radiation using 1-s sampling period in Tallinn (Estonia). Alternatively, the short-term

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variations in UV radiation data have also been extensively investigated. Several works have focused on evaluating very short-term enhancement of UV radiation reaching the Earth’s surface under certain cloudiness conditions with respect to an equivalent cloudfree scenario (Nack and Green, 1974; Seckmeyer et al., 1994; Mims and Frederick, 1994; Estupiñan et al., 1996; Sabburg and Wong, 2000; Sabburg et al., 2001; Lovengreen et al., 2005; Sabburg and Parisi, 2006). Moreover, Varo et al. (2005) analyzed instantaneous probability distributions using 5-min UV radiation measurements for several ranges of relative optical air mass in Córdoba (Spain), and Borkowski (2008) studied the UV radiation variability both at shortand long-timescales in Belsk (Poland) for the period 1976e2006 using wavelet multi-resolution decomposition. However, very few studies related to the simultaneous evaluation of short-term variability in UV and total solar radiation are found in the literature (Seckmeyer, 1989; Cede et al., 2002; Luccini et al., 2003). Therefore, this paper aims to study the daily variability of the total solar irradiance (310e2800 nm) and the UV erythemal irradiance (UVER) based on high frequency data, averaged over 1-min intervals at Granada (Southeastern Spain) for a four-year period (January 2006eDecember 2009). The UVER is generally quantified by weighing the solar UV radiation (280e400 nm) with the erythemal spectral response proposed in its final form by McKinlay and Diffey (1987) and adopted as a standard by the Commission Internationale de l’Éclairage (CIE). This study particularly focuses on relating the fluctuations of these two variables for different cloudiness conditions in order to better understand the short-term variability of both the total solar irradiance and UVER. This paper is structured as follows: Section 2 presents the main characteristics of instruments and data used. Section 3 describes the methodology followed to establish the very short-term variability of UVER and total solar irradiance. Section 4 presents and discusses the results obtained and, finally, Section 5 summarizes the main conclusions. 2. Instruments and data Total solar irradiance and UVER were measured at the radiometric station located on the rooftop of the Andalusian Center for Environmental Studies (CEAMA, 37.17 N, 3.61 W, 680 m a.s.l.) operated by the Atmospheric Physics Group (GFAT) of the University of Granada. Granada is a non-industrialized medium-sized city with a population of 300,000 inhabitants that increases up to 600,000 when the metropolitan area is included. The city is located in a natural basin surrounded by mountains with elevations between 1000 and 3500 m a.s.l. Near continental conditions prevailing at this site are responsible for large seasonal temperature differences, providing cool winters and hot summers. The diurnal thermal oscillation is quite high throughout the year, often reaching up to 20  C. The ground-based station is equipped with a broadband UV radiometer, model UVB-1, manufactured by Yankee Environmental Systems, Inc. (Massachusetts, US), measuring UVER data and a CM-11 pyranometer manufactured by Kipp & Zonen (Delft, The Netherlands) measuring total solar irradiance. In order to guarantee the simultaneity of UVER and total solar irradiance data, both variables were recorded with the same frequency (every minute) by the same data-logger (CR10-X model, manufactured by Campbell Scientific, Inc). The CM-11 pyranometer complies with the specifications for the first-class WMO classification of this instrument (resolution better than 5 W m2), and the calibration factor stability has been periodically checked against a reference CM-11 pyranometer. On the other hand, output voltages provided by the UV radiometer were converted to UVER values applying conversion factors obtained from the “two-steps” calibration method; using the

information derived from the first Spanish calibration campaign of broadband UV radiometers which took place at the “El Arenosillo” INTA station in Huelva (Spain) during September 2007 (Vilaplana et al., 2009). The two-steps method involves two stages; initially, the output signal of the broadband UV radiometer is compared with the effective irradiance from the reference Brewer spectroradiometer, then, the effective response values are converted to erythemal units. A complete description of this calibration method can be found in Seckmeyer et al. (1997a), Webb et al. (2006), Hülsen and Gröbner (2007) and Antón et al. (2011a). Particularly, Antón et al. (2011b) compared data provided by the UVB-1 radiometer installed in Granada using this calibration method with those estimated by a multilayer transfer model; their results provided a high reliability of the UVER data used in this paper. The GFAT developed an instrument that provides images of the whole sky dome during daytime at 5 min intervals. From these images, the instrument, called All-Sky Imager, characterizes the cloud cover in oktas (i.e., eighths of the sky obscured by clouds). This All-Sky Imager is a custom adaptation of a scientific CCD camera with a fish-eye lens (180 FOV) pointing to the zenith. In addition, the camera is environmentally protected and a solarshadow system is used to avoid direct incidence of the Sun beam on the lens. The instrument is normally used in research activities related to radiative transfer in the atmosphere (Cazorla et al., 2008, 2009).

3. Methodology Rotation of the Earth around its axis induces diurnal changes in solar irradiace at the Earth’s surface (Iqbal, 1983). Thus, transmissivity is used instead of irradiance in order to work with a variable whose short-term changes can be exclusively attributed to the atmospheric variability. The transmissivity for total solar irradiance, also called clearness index (kt), and the UVER (kUV) are derived from the following two expressions:

kt ¼

E ETOA

kUV ¼

(1)

UV UV TOA

(2)

where E and UV are the total solar and erythemal irradiance measured at the surface, and ETOA and UVTOA are the respective irradiances at the top of the atmosphere (extraterrestrial irradiance). The calculation of this extraterrestrial irradiance is based on a set of algorithms, depending on the hour, day of the year and the latitude of the location under study (Iqbal, 1983). Fluctuations of total solar irradiance over consecutive 1-min intervals were quantified using the following expression:

DEi ¼ 100 

jktiþ1  kit j kit

(3)

where kit and kiþ1 are the clearness index measured at two t consecutive minutes. An analogous expression is used to quantify the relative 1-min changes for the UVER:

DUVi ¼ 100 

iþ1 jkUV  kiUV j

kiUV

(4)

i where kiþ1 UV and kUV are the transmissivity for UVER recorded at two consecutive minutes.

M. Antón et al. / Atmospheric Environment 45 (2011) 4815e4821

DE and DUV are expressed as percentage per minute (% min1). Time series of both total and erythemal irradiance data extended for four full years, from January 2006 to December 2009. Total solar irradiance and UVER data recorded for solar zenith angle smaller than 70 were selected in order to remove sunrise and sunset periods, as they present fast changing irradiances under cloud-free conditions. 4. Results and discussion 4.1. Daily variability

4

In this work, the median was used instead of the mean to avoid the influence of possible outliers. Median values of the relative 1-min changes derived from expressions 3 and 4 for each day were obtained. These daily median values are hereafter called total irradiance and UVER daily variability and they characterize the daily average of the very short-term variability for the total irradiance and UVER data. To characterize the daily cloud cover over Granada, daily median values from the oktas dataset recorded with the All-Sky Imager were calculated. Fig. 1 shows the median values of the total irradiance and UVER daily variability for each fraction of cloud cover in order to analyze in detail the influence of the cloud cover on the daily variability of the total solar irradiance and UVER data. Error bars correspond with the semi-interquartile range (difference between the 75th and 25th percentile divided by two) which are only plotted for UVER in the interest of clarity. The size of the bars indicates the spread of UVER daily variability for a given fraction of cloud cover. As can be observed, cloud fractions above three oktas show great scatter which may be partly due to the variety of cloud types and the location of the clouds with respect to the sun disk included by each of these fractions. The same behaviour is also observed for the error bars of total irradiance variability (not shown). In addition, these error bars are larger than the UVER bars suggesting that under a specific cloud fraction the total irradiance variability is larger than the UVER variability. Fig. 1 also shows that total irradiance and UVER daily variability clearly increases as a function of cloudiness (from null to seven oktas) over the ground-

2 0

1

Daily variability (%/min)

3

Total irradiance UVER

0

2

4

6

8

Cloud cover (oktas) Fig. 1. Daily variability for total solar irradiance and UVER as a function of cloud cover. Values shown correspond with the 50th percentile for each fraction of the cloud cover and error bars correspond with the semi-interquartile range.

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based station. However, it can be observed a notable decrease of total irradiance and UVER variability when the sky is completely covered by clouds (i.e., eight oktas) with respect to the cloudiness condition of seven oktas. Under a cloud cover of eight oktas, the absence of broken clouds and the likelihood of more homogenous cloud cover lead to a substantial reduction of solar variability reaching the surface (Tomson and Tamm, 2006). It is well documented that besides the variation in solar zenith angle, cloudiness is the main attenuation factor responsible for most part of the variability presented by both UVER and total solar irradiance (Cede et al., 2002; Serrano et al., 2006). As indicated from Fig. 1, the influence of cloud cover was significantly higher for total solar irradiance than for UVER. The total irradiance daily variability ranged from 0.2% min1 (null oktas) to 3.9% min1 (seven oktas). The amplitude of this dependence was reduced by almost 50% for the UVER since its daily variability changed between 0.5% min1 (null oktas) to 2.5% min1 (seven oktas). The different degree of the short-term variability for UVER and total irradiance under cloudy conditions may be related to the fact that fluctuations in tropospheric transmission are often strong at visible wavelengths but dampened somewhat at UV wavelengths due to the already high contribution from molecular (Rayleigh) scattering. It is well known that the Rayleigh scattering is more efficient at shorter wavelengths (inversely proportional to the fourth power of the radiation wavelength), resulting in a greater diffuse component in UV than in total solar radiation. For instance, a small cloud blocking the sun may extinguish most of the visible irradiance, but only partially decrease UV irradiance since much of it comes already from diffuse sky radiance. Crawford et al. (2003) showed that the amplitude of these fluctuations increases generally with wavelength under broken clouds. Additionally, cloud absorption by liquid water content is more intense in the visible and near-infrared regions than in the ultraviolet one; cloudiness attenuates total solar radiation more than UV solar radiation (Lenoble, 1993). Fig. 1 also shows that the UVER daily variability is higher than total irradiance daily variability for days with cloud cover below three oktas (completely cloud-free cases together with almost cloud-free conditions). Under these conditions, the observed behaviour could be associated with several factors. Firstly, it is well known that the actual ozone amount (slant ozone) crossed by the solar radiation presents a marked diurnal cycle associated with the diurnal pattern of the relative optical air mass. In this sense, Antón et al. (2009) showed that while the UVER transmissivity presents a significant diurnal change opposite to the diurnal cycles of both the slant ozone column and the relative optical air mass, the total irradiance transmissivity is not sensitive to these cycles. In addition, the diurnal fluctuations of the total ozone amount could also contribute to this behaviour. Antón et al. (2010) showed that at Madrid (Spain), about 90% of days presented non-negligible diurnal total ozone variability. This behaviour is likely to occur by the diurnal photochemical processes in the lower troposphere related to the formation of tropospheric ozone near the Earth’s surface at populated urban locations. The very short-term variability of the aerosol load could also have greater influence for the diurnal variability of UVER than for total solar irradiance. Several authors have shown that aerosols attenuation is more intense for the UV radiation than for total solar radiation (Ogunjobi and Kim, 2004; Antón et al., 2008). This higher attenuation of UV radiation may be partly attributed to an enhancement in the optical path because of the scattering processes, with an amplification of absorption by both aerosols and tropospheric ozone (Kaskaoutis et al., 2006; Badarinath et al., 2007). Additionally, the bowl-like topography in the Granada basin together with the Mediterranean climate favour winter-time inversions and the dominance of low wind speeds. This, in combination with pollutant emissions from anthropogenic

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M. Antón et al. / Atmospheric Environment 45 (2011) 4815e4821 Table 1 Median values of the daily variability for total solar irradiance and UVER data recorded at Granada ranged by months. Errors correspond with the semiinterquartile range. Percentage of days with a median value equal or lower than two oktas for each month are also shown. Total irradiance daily variability (% min1) January February March April May June July August September October November December

           

0.26 0.25 0.24 0.25 0.19 0.15 0.07 0.12 0.17 0.22 0.20 0.23

0.76 0.98 0.93 1.49 1.13 0.66 0.49 0.51 1.07 0.82 0.74 0.73

           

0.21 0.18 0.19 0.19 0.14 0.09 0.04 0.07 0.09 0.14 0.13 0.19

Days with two or less oktas (%) 57 47 57 34 45 63 62 78 39 54 61 51

has also been added into the table. It can be seen that the monthly daily variability was lower than 2% min1 for all months. These small values were due to the high percentage of days with completely or almost cloud-free conditions over the site. The highest monthly daily variability (1.6% min1 and 1.5% min1 for total solar irradiance and UVER, respectively) was obtained for April where days with two oktas or less were “only” recorded for 38% of the time. From Table 1 it can be observed that the total irradiance daily variability was higher than the UVER daily variability for all months except for the summer (JuneeJulyeAugust), where the latter variability was more than twice the total irradiance daily variability. This may be associated with a high percentage of days showing two or less oktas (between 62% and 78%), which produce a larger daily variability for UVER than for the total irradiance (Fig. 1). The effect of solar elevation over the short-term variability for total solar irradiance and UVER was studied using only completely cloud-free days (zero oktas). Total solar irradiance and UVER transmissivity were grouped for each selected day over 2.5 intervals of SZA centred at 20 , 25 , 30 , 35 , 40 , 45 , 50 , 55 , 60 and 65 . For each day and SZA interval, the CV, defined as the ratio of the standard deviation to the mean, was calculated. Fig. 3 shows

12 4

6 0

0

2

4

6

8

Coeffcient of variation (%)

8

10

Total irradiance UVER

2

Total irradiance variability (%/min)

0.97 1.23 1.17 1.66 1.21 0.29 0.16 0.16 1.22 0.93 0.89 1.38

UVER daily variability (% min1)

10

activities produce a clear diurnal patter in the aerosol absorption and scattering coefficients for all seasons (Lyamani et al., 2010); and therefore, could also have a significant influence on UVER diurnal variation under cloud-free conditions. To investigate the proportionality and similarity of the total irradiance and UVER daily variability, Fig. 2 shows the relationship between these two variables obtained during the whole period of study (1274 pairs of data points). The elevated concentration of data points in the lower left corner of the figure (small daily fluctuations) corresponds to measurements recorded under cloud-free conditions. In these cases, the daily variability for UVER data is slightly higher than the daily variability for total irradiance data. The other data points show a large scatter connected to varying cloudy conditions, being the total irradiance variability significantly higher than the UVER variability. Selecting days with cloud cover above three oktas, only 13% of them present a daily variability for UVER higher than for total solar irradiance. Thus, the UVER variability can be assumed as a lower threshold for the total irradiance variability under cloudy conditions. These results are also consistent with what is shown in Fig. 1. Arbitrarily, it has been considered very small short-term fluctuation in UVER and total solar irradiance when the daily variability was smaller than 1% min1, while large short-term fluctuation was considered when the daily variability reaches values over 5% min1. Thus, the percentage of days with large daily variability was 10.5% for total solar irradiance and 5.1% for UVER. This result emphasizes the different behaviour of the very short-term variability for both radiative variables. In contrast, the percentage of cases with small daily variability was very similar; 56% and 59% for the total solar irradiance and UVER respectively. The high percentage of small fluctuations indicates the large number of cloud-free conditions at the site. Both total irradiance and UVER daily variabilities were calculated for each month of the year in order to analyze seasonal changes of these fluctuations. Monthly averages of median values of the daily variability (hereafter called as monthly daily variability) for the total solar irradiance and UVER at Granada are shown in Table 1. The errors correspond with the previously defined semi-interquartile range. In addition, the percentage of days with two oktas or less for each month

0

2

4

6

8

10

12

UVER variability (%/min)

20

30

40

50

60

Solar Zenith Angle (º) Fig. 2. Relationship between daily variability for total solar irradiance and UVER data obtained during the monitoring period (2006e2009). The solid black line represents the unit slope.

Fig. 3. Coefficient of variation for total solar irradiance and UVER as a function of solar zenith angle. Error bars correspond with the semi-interquartile range.

0 15

0.020 0.010 0.000

UVER trasnmissivity

14

12

13

14

15

13

14

15

UVER Total solar irradiance

0

50

100

150

11

Total irradiance trasnmissivity

13

0.0 0.2 0.4 0.6 0.8 1.0 1.2

12

UVER Total irradiance

10 One−minute variability (%/min)

Total solar irradiance (W/m²)

600 400 200

0.10 0.00

11

0.030

10

4.2. “Extreme enhancement” episodes It is well known that clouds can have a brightening effect (commonly called cloud enhancement) which consists in shortterm enhancement of solar radiation measured at the surface under broken cloud fields compared to equivalent cloud-free conditions. These enhancements are observed for both the total solar irradiance (Pfister et al., 2003) and the UV radiation (Estupiñan et al., 1996; Sabburg and Parisi, 2006). It has been documented that the total solar irradiance at the surface can reach levels even higher than its value at the top of the atmosphere (Piacentini et al., 2003, 2010). However, it was not possible to find these events (called “extreme enhancement” in this work) for the UV radiation due to its high atmospheric absorption by the stratospheric ozone column. Fig. 4 (top) shows the evolution of the total solar irradiance and UVER data recorded on 3 February 2009. The dotted black curve represents the extraterrestrial solar irradiance. It can be seen that there are some periods where the total solar irradiance at the surface was clearly higher than the corresponding irradiance at the top of the atmosphere. The overall duration of these periods for this particular day was 41 min Fig. 4 (middle) shows the evolution of the atmospheric transmissivity for the UVER and total solar irradiance. The dotted black line represents the unit value for the total transmissivity above which these “extreme enhancement” events occur. The UVER transmissivity was always lower than 0.015, which shows that the UVER data recorded at the surface was almost 70-times lower than the UVER at the top of the atmosphere. This highlights the strong absorption effect of atmospheric ozone pointed out previously. Finally, Fig. 4 (bottom) shows the relative 1-min variability for the two radiative variables. It can be observed that the variability of the total solar irradiance was notably larger than for the UVER data; median daily values were 2.4% min1 and 1.8% min1, respectively. The extreme values for the total solar irradiance (larger than 100% min1) were clearly higher than the UVER extreme values. As a result, their mean values also showed large differences reaching 10.2% min1 for the total solar irradiance and only 4.8% min1 for UVER. The physics of the cloud enhancement is well understood. It occurs during partly cloudy conditions generally when clouds do not occlude the solar disk, which induce two different contributions (Madronich, 1987; Mims and Frederick, 1994; Cede et al., 2002; Calbó et al., 2005): (1) multiple reflection of direct solar radiation at cloud borders and, (2) increased forward scattering due to the photons scattered inside clouds and reflected again from cloud sides. Both processes increase the diffuse component of solar radiation at the surface but without attenuation of the direct component. Fig. 5 shows six images taken with the All-Sky Imager between 11:00 and 11:25 LT on 3 February 2009 in which the Sun

0.20

UVER Total irrad. (ground) Total irrad.(top)

UVER (W/m²)

median CV (the semi-interquartile range) for each 5 bins of SZA using all CV calculated for selected cloud-free days during the studied period. The CV for UVER experiences a strong increase as a function of SZA, from 1.5% for SZA ¼ 20 to 9.5% for SZA ¼ 65 . In contrast, the curve associated with the CV for total solar irradiance showed a significant more stable behaviour for the entire range of SZA, with small CV between 0.3% for SZA ¼ 20 and 1.3% for SZA ¼ 65 . The different evolution of these two curves may be associated with the diurnal variation of the slant ozone column as the solar radiation crosses the relative optical air mass related to the diurnal pattern. The UVER transmissivity is strongly affected by the changes of the slant ozone, while the total irradiance transmissivity is not. The amplitude of the slant ozone values for each 5 bins increased as a function of SZA, and, therefore, its influence on the UVER transmissivity variability also increased with SZA under cloud-free conditions.

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800 1000

M. Antón et al. / Atmospheric Environment 45 (2011) 4815e4821

10

11

12

Hour Fig. 4. Top: Daily evolution of total solar irradiance and UVER data recorded on 3 February 2009 at Granada. The dotted black curve represents the extraterrestrial solar irradiance. Middle: Daily evolution of atmospheric transmissivity for the total solar irradiance and UVER on 3 February 2009. The dotted black line represents the unit value for the total transmissivity. Bottom: The relative 1-min variability for total solar irradiance and UVER data on 3 February 2009.

was surrounded by cumulus clouds and “extreme enhancement” events were observed (Fig. 4). Before 11:00 LT, the sky was characterized by cloud-free conditions while after 11:25 LT, overcast conditions prevailed until approximately 12:00 LT, with a persistent reduction in both total solar irradiance and UVER data. The four-year data recorded at Granada showed that there were 76 days with at least 5 min of “extreme enhancement” episodes, and in 26, 13 and 5 days these events lasted more than 10, 15 and 20 min, respectively. Seckmeyer et al. (1997b) reported that this effect can even enhance the daily sum. These large increments

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The analysis of one day with a long “extreme enhancement” episode (total solar irradiance at the surface higher than at the top of the atmosphere) showed that the relative 1-min changes could reach pick values larger than 100% min1 for the total solar irradiance, being significantly smaller than the simultaneous shortterm fluctuations for UVER. These events were associated with the presence of cumulus clouds around but not covering the Sun. Acknowledgements Manuel Antón thanks Ministerio de Ciencia e Innovación and Fondo Social Europeo for the award of a postdoctoral grant (Juan de la Cierva). This work was partially supported by the Andalusian Regional Governments through project P10-RNM-6299 and P08-RNM-3568, the Spanish Ministry of Science and Technology through projects CGL2010-18782 and CSD2007-00067, and by the European Union through ACTRIS project (EU INFRA-2010-1.1.16-262254). References

Fig. 5. Images taken with the All-Sky Imager on 3 February 2009 at 11.00 UTC (top left), 11:05 UTC (top right), 11:10 UTC (middle left), 11:15 (middle right), 11:20 (bottom left) and 11:25 UTC (bottom right).

could have a direct impact on materials or energy conversion technology systems exposed to the outside, as well as potential biological effects.

5. Conclusions The results from this work show that the relative 1-min changes for the UVER and total solar irradiance monotonously increase as a function of the cloud fraction, except for overcast conditions (eight oktas) where these very short-term fluctuations slightly decrease. In addition, the cloudiness effect (above three oktas) is significantly higher for the short-term changes of the total solar irradiance than for UVER short-term changes. Thus, for a cloud cover of seven oktas, the daily variability for total solar irradiance and UVER was 3.9% min1 and 2.5% min1, respectively. In contrast, under a cloud cover below three oktas, the relative 1-min fluctuation was larger for UVER than for total solar irradiance. For example, for completely cloud-free cases (zero oktas), the daily variability for UVER was 0.5% min1 compared to 0.2% min1 for the total solar irradiance. In addition, the CV for UVER experienced a strong increase (from 1.5% to 9.5%) as a function of SZA under completely cloud-free conditions, while CV for total solar irradiance was lower than 1.3% for the entire range of SZA.

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