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Atmospheric Environment 45 (2011) 4815e4821

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Atmospheric Environment

journal homepage: www.elsevier .com/locate/atmosenv

Short-term variability of experimental ultraviolet and total solar irradiancein 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

aCentro Andaluz de Medio Ambiente (CEAMA), Av. del Mediterráneo s/n, 18006 Granada, SpainbDepartamento 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

Article history:Received 19 February 2011Received in revised form24 May 2011Accepted 9 June 2011

Keywords:Ultraviolet irradianceTotal solar irradianceBroadband UV radiometerShort-term variability

* Corresponding author. Departamento de FísicGranada, Granada, Spain. Tel.: þ34 924 289536; fax:

E-mail address: mananton@unex.es (M. Antón).

1352-2310/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.atmosenv.2011.06.020

a b s t r a c t

This paper quantifies the very short-term variability of the total solar irradiance and the ultravioleterythemal irradiance (UVER) averaged over 1-min intervals at Granada (Southeastern Spain). A statisticalanalysis for a four-year period (January 2006eDecember 2009) under different cloudiness and charac-terized by the amount of cloud cover (oktas) retrieved from an All-Sky Imager located next to theradiometers is presented. Very short-term variability of the total solar irradiance was larger than UVERfluctuations under cloudy conditions (above three oktas), in accordance with previous works found inthe literature. Nevertheless, for cloud cover bellow three oktas the opposite was true; the median relative1-min fluctuation was larger for UVER than for total solar irradiance. Moreover, while the coefficient ofvariation (CV) for UVER presented a clear dependence on the solar zenith angle (SZA) under completelycloud-free conditions (from 1.5% for SZA ¼ 20� to 9.5% for SZA ¼ 65�), the CV of the total solar irradiancewas under 1.3% with a more stable behaviour for the entire range of SZA. Large differences were found forcloud cover of seven oktas, where the median diurnal 1-min variability for total solar irradiance was3.9% min�1 compared to 2.5% min�1 for UVER data. Additionally, an episode with surface total solarirradiance higher than its corresponding extraterrestrial value is analyzed.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The energy budget of the eartheatmosphere system is mainlycontrolled by the solar radiation received at ground level. Thusa detailed knowledge of its temporal variability is truly valuable formany topics such as soilevegetationeatmosphere energy budgetmodels, validation of climate models, studies focussing on the use ofsolar radiation as a source of energy, etc. In addition, the study of thetemporal variability of the ultraviolet (UV) radiation (100e400 nm)at the Earth’s surface becomes a high priority in scientific researchsince it affects many biological, ecological and photochemicalprocesses, often being harmful for living organisms (Diffey, 1991,2004).

Cloud cover and aerosols present a high temporal variabilitythat, especially in the former case, is responsible for high variabilityin the solar radiation at short-term scales, from significantenhancements to almost total reduction (Kasten and Czeplak, 1980;

a Aplicada, Universidad deþ34 924 289651.

All rights reserved.

Nann and Riordan, 1991; Beyer et al., 1994; Frederick and Steele,1995; Bartlett et al., 1998). In addition, it is well known that UVradiation variability is also mainly controlled by cloudiness atshort-term scales (Frederick and Snell, 1990; Lubin and Frederick,1991; Wang and Lenoble, 1996; Matthijsen et al., 2000; Alados-Arboledas et al., 2003; Calbó et al., 2005; Mateos et al., 2010).Cloudiness variability may reduce, cancel or even reverse theexpected UV radiation increase caused by the reduction in theamount of ozone (Glandorf et al., 2005; WMO, 2007). Therefore, itis of high interest to analyze the short-term variation of solarradiation at ground level.

Several studies have evaluated the variability of the total solarradiation (entire solar spectrum) at very short-time scales, usuallyquantifying its probability distribution in the order of minutes orless (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 quan-tifying of solar irradiance fluctuations at short-time scales usingspectral analysis (Woyte et al., 2007) or evaluating its fractaldimension (Harrouni et al., 2005). Recently, Tomson (2010) studiedthe dynamic processes of fast-alternating solar radiation using 1-ssampling period in Tallinn (Estonia). Alternatively, the short-term

M. Antón et al. / Atmospheric Environment 45 (2011) 4815e48214816

variations in UV radiation data have also been extensively investi-gated. Several works have focused on evaluating very short-termenhancement of UV radiation reaching the Earth’s surface undercertain cloudiness conditions with respect to an equivalent cloud-free scenario (Nack and Green, 1974; Seckmeyer et al., 1994; Mimsand Frederick, 1994; Estupiñan et al., 1996; Sabburg and Wong,2000; Sabburg et al., 2001; Lovengreen et al., 2005; Sabburg andParisi, 2006). Moreover, Varo et al. (2005) analyzed instantaneousprobability distributions using 5-min UV radiation measurementsfor several ranges of relative optical air mass in Córdoba (Spain), andBorkowski (2008) studied the UV radiation variability both at short-and long-timescales in Belsk (Poland) for the period 1976e2006using wavelet multi-resolution decomposition.

However, very few studies related to the simultaneous evalua-tion of short-term variability in UV and total solar radiation arefound in the literature (Seckmeyer, 1989; Cede et al., 2002; Lucciniet al., 2003). Therefore, this paper aims to study the daily variabilityof the total solar irradiance (310e2800 nm) and the UV erythemalirradiance (UVER) based on high frequency data, averaged over1-min intervals at Granada (Southeastern Spain) for a four-yearperiod (January 2006eDecember 2009). The UVER is generallyquantified by weighing the solar UV radiation (280e400 nm) withthe erythemal spectral response proposed in its final form byMcKinlay and Diffey (1987) and adopted as a standard by theCommission Internationale de l’Éclairage (CIE). This study partic-ularly focuses on relating the fluctuations of these two variables fordifferent cloudiness conditions in order to better understand theshort-term variability of both the total solar irradiance and UVER.

This paper is structured as follows: Section 2 presents the maincharacteristics of instruments and data used. Section 3 describesthe methodology followed to establish the very short-term vari-ability of UVER and total solar irradiance. Section 4 presents anddiscusses the results obtained and, finally, Section 5 summarizesthe main conclusions.

2. Instruments and data

Total solar irradiance and UVER were measured at the radio-metric station located on the rooftop of the Andalusian Center forEnvironmental Studies (CEAMA, 37.17� N, 3.61� W, 680 m a.s.l.)operated by the Atmospheric Physics Group (GFAT) of the Univer-sity of Granada.

Granada is a non-industrialized medium-sized city with a pop-ulation of 300,000 inhabitants that increases up to 600,000 whenthe metropolitan area is included. The city is located in a naturalbasin surrounded by mountains with elevations between 1000 and3500 m a.s.l. Near continental conditions prevailing at this site areresponsible for large seasonal temperature differences, providingcool winters and hot summers. The diurnal thermal oscillation isquite high throughout the year, often reaching up to 20 �C.

The ground-based station is equipped with a broadband UVradiometer, model UVB-1, manufactured by Yankee EnvironmentalSystems, Inc. (Massachusetts, US), measuring UVER data anda CM-11 pyranometer manufactured by Kipp & Zonen (Delft, TheNetherlands) measuring total solar irradiance. In order to guaranteethe simultaneity of UVER and total solar irradiance data, both vari-ables were recorded with the same frequency (every minute) by thesame data-logger (CR10-X model, manufactured by CampbellScientific, Inc). The CM-11 pyranometer complies with the specifi-cations for the first-class WMO classification of this instrument(resolution better than �5 W m�2), and the calibration factorstability has been periodically checked against a reference CM-11pyranometer. On the other hand, output voltages provided by theUV radiometer were converted to UVER values applying conversionfactors obtained from the “two-steps” calibrationmethod; using the

information derived from the first Spanish calibration campaign ofbroadband UV radiometers which took place at the “El Arenosillo”INTA station in Huelva (Spain) during September 2007 (Vilaplanaet al., 2009). The two-steps method involves two stages; initially,the output signal of the broadband UV radiometer is compared withthe effective irradiance from the reference Brewer spectroradi-ometer, then, the effective response values are converted to eryth-emal units. A complete description of this calibrationmethod can befound in Seckmeyer et al. (1997a), Webb et al. (2006), Hülsen andGröbner (2007) and Antón et al. (2011a). Particularly, Antón et al.(2011b) compared data provided by the UVB-1 radiometerinstalled in Granada using this calibration method with those esti-mated by a multilayer transfer model; their results provided a highreliability of the UVER data used in this paper.

The GFAT developed an instrument that provides images of thewhole sky dome during daytime at 5 min intervals. From theseimages, the instrument, called All-Sky Imager, characterizes thecloud cover in oktas (i.e., eighths of the sky obscured by clouds).This All-Sky Imager is a custom adaptation of a scientific CCDcamera with a fish-eye lens (180� FOV) pointing to the zenith. Inaddition, the camera is environmentally protected and a solar-shadow system is used to avoid direct incidence of the Sun beamon the lens. The instrument is normally used in research activitiesrelated to radiative transfer in the atmosphere (Cazorla et al., 2008,2009).

3. Methodology

Rotation of the Earth around its axis induces diurnal changes insolar irradiace at the Earth’s surface (Iqbal, 1983). Thus, trans-missivity is used instead of irradiance in order to work witha variable whose short-term changes can be exclusively attributedto the atmospheric variability.

The transmissivity for total solar irradiance, also called clearnessindex (kt), and the UVER (kUV) are derived from the following twoexpressions:

kt ¼ EETOA

(1)

kUV ¼ UVUVTOA

(2)

where E and UV are the total solar and erythemal irradiancemeasured at the surface, and ETOA and UVTOA are the respectiveirradiances at the top of the atmosphere (extraterrestrial irradi-ance). The calculation of this extraterrestrial irradiance is based ona set of algorithms, depending on the hour, day of the year and thelatitude of the location under study (Iqbal, 1983).

Fluctuations of total solar irradiance over consecutive 1-minintervals were quantified using the following expression:

DEi ¼ 100� jkiþ1t � kit jkit

(3)

where kit and kiþ1t are the clearness index measured at two

consecutive minutes.An analogous expression is used to quantify the relative 1-min

changes for the UVER:

DUVi ¼ 100� jkiþ1UV � kiUV jkiUV

(4)

where kiþ1UV and kiUV are the transmissivity for UVER recorded at two

consecutive minutes.

M. Antón et al. / Atmospheric Environment 45 (2011) 4815e4821 4817

DE and DUV are expressed as percentage per minute (% min�1).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 anglesmaller than 70� were selected in order to remove sunrise andsunset periods, as they present fast changing irradiances undercloud-free conditions.

4. Results and discussion

4.1. Daily variability

In this work, the median was used instead of the mean to avoidthe influence of possible outliers. Median values of the relative1-min changes derived from expressions 3 and 4 for each day wereobtained. These daily median values are hereafter called totalirradiance and UVER daily variability and they characterize thedaily average of the very short-term variability for the total irra-diance and UVER data.

To characterize the daily cloud cover over Granada, dailymedianvalues from the oktas dataset recorded with the All-Sky Imagerwere calculated. Fig. 1 shows the median values of the total irra-diance and UVER daily variability for each fraction of cloud cover inorder to analyze in detail the influence of the cloud cover on thedaily variability of the total solar irradiance and UVER data. Errorbars correspond with the semi-interquartile range (differencebetween the 75th and 25th percentile divided by two) which areonly plotted for UVER in the interest of clarity. The size of the barsindicates the spread of UVER daily variability for a given fraction ofcloud cover. As can be observed, cloud fractions above three oktasshow great scatter which may be partly due to the variety of cloudtypes and the location of the clouds with respect to the sun diskincluded by each of these fractions. The same behaviour is alsoobserved for the error bars of total irradiance variability (notshown). In addition, these error bars are larger than the UVER barssuggesting that under a specific cloud fraction the total irradiancevariability is larger than the UVER variability. Fig. 1 also shows thattotal irradiance and UVER daily variability clearly increases asa function of cloudiness (from null to seven oktas) over the ground-

0 2 4 6 8

01

23

4

Cloud cover (oktas)

Dai

ly v

aria

bilit

y (%

/min

)

Total irradianceUVER

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 coverand error bars correspond with the semi-interquartile range.

based station. However, it can be observed a notable decrease oftotal irradiance and UVER variability when the sky is completelycovered by clouds (i.e., eight oktas) with respect to the cloudinesscondition of seven oktas. Under a cloud cover of eight oktas, theabsence of broken clouds and the likelihood of more homogenouscloud cover lead to a substantial reduction of solar variabilityreaching the surface (Tomson and Tamm, 2006).

It is well documented that besides the variation in solar zenithangle, cloudiness is the main attenuation factor responsible formost part of the variability presented by both UVER and total solarirradiance (Cede et al., 2002; Serrano et al., 2006). As indicated fromFig. 1, the influence of cloud cover was significantly higher for totalsolar irradiance than for UVER. The total irradiance daily variabilityranged from 0.2% min�1 (null oktas) to 3.9% min�1 (seven oktas).The amplitude of this dependence was reduced by almost 50% forthe UVER since its daily variability changed between 0.5% min�1

(null oktas) to 2.5% min�1 (seven oktas). The different degree of theshort-term variability for UVER and total irradiance under cloudyconditions may be related to the fact that fluctuations in tropo-spheric transmission are often strong at visible wavelengths butdampened somewhat at UV wavelengths due to the already highcontribution frommolecular (Rayleigh) scattering. It is well knownthat the Rayleigh scattering is more efficient at shorter wavelengths(inversely proportional to the fourth power of the radiationwavelength), resulting in a greater diffuse component in UV than intotal solar radiation. For instance, a small cloud blocking the sunmay extinguish most of the visible irradiance, but only partiallydecrease UV irradiance since much of it comes already from diffusesky radiance. Crawford et al. (2003) showed that the amplitude ofthese fluctuations increases generally with wavelength underbroken clouds. Additionally, cloud absorption by liquid watercontent is more intense in the visible and near-infrared regionsthan in the ultraviolet one; cloudiness attenuates total solar radi-ation more than UV solar radiation (Lenoble, 1993).

Fig. 1 also shows that the UVER daily variability is higher thantotal irradiance daily variability for days with cloud cover belowthree oktas (completely cloud-free cases together with almostcloud-free conditions). Under these conditions, the observedbehaviour could be associated with several factors. Firstly, it is wellknown that the actual ozone amount (slant ozone) crossed by thesolar radiation presents a marked diurnal cycle associated with thediurnal pattern of the relative optical air mass. In this sense, Antónet al. (2009) showed that while the UVER transmissivity presentsa significant diurnal change opposite to the diurnal cycles of boththe slant ozone column and the relative optical air mass, the totalirradiance transmissivity is not sensitive to these cycles. In addition,the diurnal fluctuations of the total ozone amount could alsocontribute to this behaviour. Antón et al. (2010) showed that atMadrid (Spain), about 90% of days presented non-negligible diurnaltotal ozone variability. This behaviour is likely to occur by thediurnal photochemical processes in the lower troposphere relatedto the formation of tropospheric ozone near the Earth’s surface atpopulated urban locations. The very short-term variability of theaerosol load could also have greater influence for the diurnalvariability of UVER than for total solar irradiance. Several authorshave shown that aerosols attenuation is more intense for the UVradiation than for total solar radiation (Ogunjobi and Kim, 2004;Antón et al., 2008). This higher attenuation of UV radiation maybe partly attributed to an enhancement in the optical path becauseof the scattering processes, with an amplification of absorption byboth aerosols and tropospheric ozone (Kaskaoutis et al., 2006;Badarinath et al., 2007). Additionally, the bowl-like topography inthe Granada basin together with the Mediterranean climate favourwinter-time inversions and the dominance of low wind speeds.This, in combination with pollutant emissions from anthropogenic

Table 1Median values of the daily variability for total solar irradiance and UVER datarecorded at Granada ranged by months. Errors correspond with the semi-interquartile range. Percentage of days with a median value equal or lower thantwo oktas for each month are also shown.

Total irradiance dailyvariability (% min�1)

UVER dailyvariability (% min�1)

Days with two orless oktas (%)

January 0.97 � 0.26 0.76 � 0.21 57February 1.23 � 0.25 0.98 � 0.18 47March 1.17 � 0.24 0.93 � 0.19 57April 1.66 � 0.25 1.49 � 0.19 34May 1.21 � 0.19 1.13 � 0.14 45June 0.29 � 0.15 0.66 � 0.09 63July 0.16 � 0.07 0.49 � 0.04 62August 0.16 � 0.12 0.51 � 0.07 78September 1.22 � 0.17 1.07 � 0.09 39October 0.93 � 0.22 0.82 � 0.14 54November 0.89 � 0.20 0.74 � 0.13 61December 1.38 � 0.23 0.73 � 0.19 51

M. Antón et al. / Atmospheric Environment 45 (2011) 4815e48214818

activities produce a clear diurnal patter in the aerosol absorptionand scattering coefficients for all seasons (Lyamani et al., 2010); andtherefore, could also have a significant influence on UVER diurnalvariation under cloud-free conditions.

To investigate the proportionality and similarity of the totalirradiance and UVER daily variability, Fig. 2 shows the relationshipbetween these two variables obtained during the whole period ofstudy (1274 pairs of data points). The elevated concentration of datapoints in the lower left corner of the figure (small daily fluctua-tions) corresponds to measurements recorded under cloud-freeconditions. In these cases, the daily variability for UVER data isslightly higher than the daily variability for total irradiance data.The other data points show a large scatter connected to varyingcloudy conditions, being the total irradiance variability significantlyhigher than the UVER variability. Selecting days with cloud coverabove three oktas, only 13% of them present a daily variability forUVER higher than for total solar irradiance. Thus, the UVER vari-ability can be assumed as a lower threshold for the total irradiancevariability under cloudy conditions. These results are also consis-tent with what is shown in Fig. 1.

Arbitrarily, it has been considered very small short-term fluc-tuation in UVER and total solar irradiancewhen the daily variabilitywas smaller than 1% min�1, while large short-term fluctuation wasconsidered when the daily variability reaches values over 5%min�1.Thus, the percentage of days with large daily variability was 10.5%for total solar irradiance and 5.1% for UVER. This result emphasizesthe different behaviour of the very short-term variability for bothradiative variables. In contrast, the percentage of cases with smalldaily variability was very similar; 56% and 59% for the total solarirradiance and UVER respectively. The high percentage of smallfluctuations indicates the large number of cloud-free conditions atthe site.

Both total irradiance and UVER daily variabilities were calculatedfor each month of the year in order to analyze seasonal changes ofthese fluctuations. Monthly averages of median values of the dailyvariability (hereafter called as monthly daily variability) for the totalsolar irradiance and UVER at Granada are shown in Table 1. The errorscorrespond with the previously defined semi-interquartile range. Inaddition, the percentage of dayswith two oktas or less for eachmonth

0 2 4 6 8 10 12

02

46

810

12

UVER variability (%/min)

Tota

l irra

dian

ce v

aria

bilit

y (%

/min

)

Fig. 2. Relationship between daily variability for total solar irradiance and UVER dataobtained during the monitoring period (2006e2009). The solid black line representsthe unit slope.

has also been added into the table. It can be seen that the monthlydaily variability was lower than 2% min�1 for all months. These smallvalues were due to the high percentage of days with completely oralmost cloud-free conditions over the site. The highest monthly dailyvariability (1.6% min�1 and 1.5% min�1 for total solar irradiance andUVER, respectively) was obtained for April where dayswith two oktasor lesswere “only” recorded for 38% of the time. From Table 1 it can beobserved that the total irradiance daily variability was higher than theUVER daily variability for all months except for the summer(JuneeJulyeAugust), where the latter variability wasmore than twicethe total irradiance daily variability. This may be associated witha high percentage of days showing two or less oktas (between 62%and 78%), which produce a larger daily variability for UVER than forthe total irradiance (Fig. 1).

The effect of solar elevation over the short-term variability fortotal solar irradiance and UVER was studied using only completelycloud-free days (zero oktas). Total solar irradiance and UVERtransmissivity 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 ratioof the standard deviation to the mean, was calculated. Fig. 3 shows

20 30 40 50 60

02

46

810

Solar Zenith Angle (º)

Coe

ffcie

nt o

f var

iatio

n (%

)

Total irradianceUVER

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

10 11 12 13 14 15

0.00

0.10

0.20

UVE

R (W

/m²)

020

040

060

080

010

00

Tota

l sol

ar ir

radi

ance

(W/m

²)

UVERTotal irrad. (ground)Total irrad.(top)

10 11 12 13 14 15

0.00

00.

010

0.02

00.

030

UVE

R tr

asnm

issi

vity

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Tota

l irra

dian

ce tr

asnm

issi

vityUVER

Total irradiance

10 11 12 13 14 15

050

100

150

Hour

One

−min

ute

varia

bilit

y (%

/min

)

UVERTotal solar irradiance

Fig. 4. Top: Daily evolution of total solar irradiance and UVER data recorded on3 February 2009 at Granada. The dotted black curve represents the extraterrestrialsolar irradiance. Middle: Daily evolution of atmospheric transmissivity for the totalsolar irradiance and UVER on 3 February 2009. The dotted black line represents theunit value for the total transmissivity. Bottom: The relative 1-min variability for totalsolar irradiance and UVER data on 3 February 2009.

M. Antón et al. / Atmospheric Environment 45 (2011) 4815e4821 4819

median CV (�the semi-interquartile range) for each 5� bins of SZAusing all CV calculated for selected cloud-free days during thestudied period. The CV for UVER experiences a strong increase asa function of SZA, from 1.5% for SZA ¼ 20� to 9.5% for SZA ¼ 65�. Incontrast, the curve associated with the CV for total solar irradianceshowed a significant more stable behaviour for the entire range ofSZA, with small CV between 0.3% for SZA ¼ 20� and 1.3% forSZA ¼ 65�. The different evolution of these two curves may beassociated with the diurnal variation of the slant ozone column asthe solar radiation crosses the relative optical air mass related tothe diurnal pattern. The UVER transmissivity is strongly affected bythe changes of the slant ozone, while the total irradiance trans-missivity is not. The amplitude of the slant ozone values for each 5�

bins increased as a function of SZA, and, therefore, its influence onthe UVER transmissivity variability also increased with SZA undercloud-free conditions.

4.2. “Extreme enhancement” episodes

It is well known that clouds can have a brightening effect(commonly called cloud enhancement) which consists in short-term enhancement of solar radiation measured at the surfaceunder broken cloud fields compared to equivalent cloud-freeconditions. These enhancements are observed for both the totalsolar irradiance (Pfister et al., 2003) and the UV radiation (Estupiñanet al., 1996; Sabburg and Parisi, 2006). It has been documented thatthe total solar irradiance at the surface can reach levels even higherthan 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 itshigh atmospheric absorption by the stratospheric ozone column.Fig. 4 (top) shows the evolution of the total solar irradiance andUVER data recorded on 3 February 2009. The dotted black curverepresents the extraterrestrial solar irradiance. It can be seen thatthere are some periods where the total solar irradiance at thesurface was clearly higher than the corresponding irradiance at thetop of the atmosphere. The overall duration of these periods for thisparticular day was 41 min Fig. 4 (middle) shows the evolution of theatmospheric transmissivity for the UVER and total solar irradiance.The dotted black line represents the unit value for the total trans-missivity above which these “extreme enhancement” events occur.The UVER transmissivity was always lower than 0.015, which showsthat the UVER data recorded at the surface was almost 70-timeslower than the UVER at the top of the atmosphere. This highlightsthe strong absorption effect of atmospheric ozone pointed outpreviously. Finally, Fig. 4 (bottom) shows the relative 1-min vari-ability for the two radiative variables. It can be observed that thevariability of the total solar irradiance was notably larger than forthe UVER data; median daily values were 2.4% min�1 and1.8% min�1, respectively. The extreme values for the total solarirradiance (larger than 100% min�1) were clearly higher than theUVER extreme values. As a result, their mean values also showedlarge differences reaching 10.2% min�1 for the total solar irradianceand only 4.8% min�1 for UVER.

The physics of the cloud enhancement is well understood. Itoccurs during partly cloudy conditions generally when clouds donot occlude the solar disk, which induce two different contribu-tions (Madronich, 1987; Mims and Frederick, 1994; Cede et al.,2002; Calbó et al., 2005): (1) multiple reflection of direct solarradiation at cloud borders and, (2) increased forward scattering dueto the photons scattered inside clouds and reflected again fromcloud sides. Both processes increase the diffuse component of solarradiation at the surface but without attenuation of the directcomponent. Fig. 5 shows six images taken with the All-Sky Imagerbetween 11:00 and 11:25 LT on 3 February 2009 in which the Sun

was surrounded by cumulus clouds and “extreme enhancement”events were observed (Fig. 4). Before 11:00 LT, the sky was char-acterized by cloud-free conditions while after 11:25 LT, overcastconditions prevailed until approximately 12:00 LT, with a persistentreduction in both total solar irradiance and UVER data.

The four-year data recorded at Granada showed that there were76 days with at least 5 min of “extreme enhancement” episodes,and in 26, 13 and 5 days these events lasted more than 10, 15 and20 min, respectively. Seckmeyer et al. (1997b) reported that thiseffect can even enhance the daily sum. These large increments

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

M. Antón et al. / Atmospheric Environment 45 (2011) 4815e48214820

could have a direct impact on materials or energy conversiontechnology systems exposed to the outside, as well as potentialbiological effects.

5. Conclusions

The results from this work show that the relative 1-min changesfor the UVER and total solar irradiance monotonously increase asa function of the cloud fraction, except for overcast conditions(eight oktas) where these very short-term fluctuations slightlydecrease. In addition, the cloudiness effect (above three oktas) issignificantly higher for the short-term changes of the total solarirradiance than for UVER short-term changes. Thus, for a cloudcover of seven oktas, the daily variability for total solar irradianceand UVER was 3.9% min�1 and 2.5% min�1, respectively. In contrast,under a cloud cover below three oktas, the relative 1-min fluctua-tionwas larger for UVER than for total solar irradiance. For example,for completely cloud-free cases (zero oktas), the daily variability forUVER was 0.5% min�1 compared to 0.2% min�1 for the total solarirradiance. In addition, the CV for UVER experienced a strongincrease (from 1.5% to 9.5%) as a function of SZA under completelycloud-free conditions, while CV for total solar irradiance was lowerthan 1.3% for the entire range of SZA.

The analysis of one day with a long “extreme enhancement”episode (total solar irradiance at the surface higher than at the topof the atmosphere) showed that the relative 1-min changes couldreach pick values larger than 100% min�1 for the total solar irra-diance, being significantly smaller than the simultaneous short-term fluctuations for UVER. These events were associated withthe presence of cumulus clouds around but not covering the Sun.

Acknowledgements

Manuel Antón thanks Ministerio de Ciencia e Innovación andFondo Social Europeo for the award of a postdoctoral grant (Juan de laCierva). This workwas partially supported by the Andalusian RegionalGovernments through project P10-RNM-6299 and P08-RNM-3568,the Spanish Ministry of Science and Technology through projectsCGL2010-18782 and CSD2007-00067, and by the European Unionthrough ACTRIS project (EU INFRA-2010-1.1.16-262254).

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