Analysis of the solar radiation/atmosphere interaction at a Mediterranean site: The role of clouds

Abstract

This study is the last of a series of three research papers analysing the solar radiation and its interaction with the atmospheric components spanning an eleven-year period (2008–2018) at a mid-latitude urban site in the Mediterranean basin. During the previous works a detailed characterization of the photosynthetically active radiation (PAR; 400–700 nm), as well as analysis the aerosol and clouds effects on PAR was carried out. This research work addresses an analysis of solar radiation on the total solar irradiance spectrum (TSI; 280–3000 nm) in terms of the effects of different atmospheric conditions on it, considered by the clearness index and the cloud cover, bringing very valuable findings from the long-term comparative analysis of radiative effects of clouds on TSI and PAR wavelength ranges. The average values in the entire period of the study for the global solar radiation in the total solar irradiance (TSIGlobal) found to be 450 ± 100 Wm-2 and 530 ± 110 Wm-2 under all and clearsky conditions, respectively. Meanwhile, the average values for the diffuse component (TSIDiffuse) are 141 ± 21 Wm-2 and 130 ± 21 Wm-2 (all and clear skies, respectively), with a relatively low interannual variation up to 11% for both global and diffuse TSI, as well as for both sky conditions. Analysis on the total cloud cover (TCC) shows that the clearness index is not a good parameter to discriminate between all and clear sky conditions, since there is a marked overlap in the ranges of the kt values for the different categories of TCC. Additionally, the cloud radiative forcing (CRF) are computed as the difference in solar radiation measured under all and clear sky conditions. A high seasonal variability is found for CRF, where CRFTSI,Global ranges between -37.6 Wm-2 and -137.4 Wm-2, while CRFTSI,Diffuse from 4.4 Wm-2 to 22.6 Wm-2. The positive sign implies increase in solar radiation at the surface, while the negative one implies the opposite, i.e., less availability of solar radiation on the surface. Finally, the analysis of the annual evolution of CRF reveals a downward trend on CRFTSI and CRFPAR, for both global and diffuse This relevant finding implies that clouds are exerting less cooling effects over time at this Mediterranean site.