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 Centro Andaluz de Medio Ambiente (CEAMA), Av. del Mediterráneo s/n, 18006 Granada, Spain b Departamento de Física Aplicada, Universidad de Granada, Campus de Fuentenueva s/n, 18071 Granada, Spain article info Article history: Received 19 February 2011 Received in revised form 24 May 2011 Accepted 9 June 2011 Keywords: Ultraviolet irradiance Total solar irradiance Broadband UV radiometer Short-term variability abstract 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 charac- terized 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% min 1 compared to 2.5% min 1 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. 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; 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; Alados- Arboledas 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 quan- tifying 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 * Corresponding author. Departamento de Física Aplicada, Universidad de Granada, Granada, Spain. Tel.: þ34 924 289536; fax: þ34 924 289651. E-mail address: mananton@unex.es (M. Antón). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.06.020 Atmospheric Environment 45 (2011) 4815e4821