Abstract
Abstract. Solar radiation has a crucial role in photosynthesis, evapotranspiration and other biogeochemical processes. The amount of solar radiation reaching the Earth's surface is a function of astronomical geometry and atmospheric optics. While the first is deterministic, the latter has a random behaviour caused by highly variable atmospheric components such as water and aerosols. In this study, we use daily radiation data (1978–2014) from 37 FLUXNET sites distributed across the globe to inspect for climatic traits in the shape of the probability density function (PDF) of the clear-day (c) and the clearness (k) indices. The analysis was made for shortwave radiation (SW) at all sites and for photosynthetically active radiation (PAR) at 28 sites. We identified three types of PDF, unimodal with low dispersion (ULD), unimodal with high dispersion (UHD) and bimodal (B), with no difference in the PDF type between c and k at each site. Looking for regional patterns in the PDF type, we found that latitude, global climate zone and Köppen climate type have a weak relation and the Holdridge life a stronger relation with c and k PDF types. The existence and relevance of a second mode in the PDF can be explained by the frequency and meteorological mechanisms of rainy days. These results are a frame to develop solar radiation stochastic models for biogeochemical and ecohydrological modelling.
Highlights
Solar radiation drives most physical, chemical and biological processes at the Earth’s surface
Inspection of the probability density function (PDF) of all sites led us to define three types of PDF according to the shape of the functions, namely unimodal with low dispersion (ULD), unimodal with high dispersion (UHD) and bimodal (B)
We identified three types of statistical behaviour according to the PDF shape, namely ULD, UHD and B
Summary
Solar radiation drives most physical, chemical and biological processes at the Earth’s surface. It is the primary energy source for photosynthesis, evapotranspiration and other biochemical processes (Wu et al, 2016; Mercado et al, 2009). Atmospheric attenuation of light is strongly affected by atmospheric constituents such as molecular gases, aerosols, water vapour and clouds by reflecting, absorbing and scattering processes (Platt et al, 2012; Wallace and Hobbs, 2006). Water vapour and clouds are highly variable in space and time. Uncertainty is unavoidable when calculating surface solar radiation because of the high space and time variability of aerosols, water vapour and clouds (Li and Trishchenko, 2001; Chen et al, 2000)
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