On the intrinsic time-scales of temporal variability in measurements of the surface solar radiation

Abstract

Abstract. This study is concerned with the intrinsic temporal scales of the variability of the surface solar irradiance (SSI). The data consist of decennial time-series of daily means of the SSI spanning ten years, obtained from high quality measure- ments of the broadband solar radiation impinging on a horizontal plane at ground level, issued from different Baseline Surface Radiation Network (BSRN) ground stations around the world. First, embedded oscillations roughly sorted by ranges of in- creasing time-scales of the data are extracted by empirical mode decomposition. Next, Hilbert spectral analysis is applied to obtain an amplitude modulation–frequency-modulation (AM–FM) representation of the data. The time-varying nature of the characteristic time-scales of variability, along with the variations of the signal intensity, are thus revealed. A novel, adaptive null-hypothesis based on the general statistical characteristics of noise is employed, in order to discriminate between the different features of the data, those that have a deterministic origin and those being realisations of various stochastic processes. The data has a significant spectral peak corresponding to the yearly variability cycle and features quasi-stochastic high-frequency "weather noise", irrespective of the geographical location or of the local climate. Moreover, the amplitude of this latter feature is shown to be modulated by variations of the yearly cycle, indicative of non-linear multiplicative cross-scale couplings. The study has possible implications on the modelling and the forecast of the surface solar radiation, by clearly discriminating the deterministic from the quasi-stochastic character of the data, at different local time-scales.