Abstract
Abstract. All-sky cameras are frequently used to detect cloud cover; however, this work explores the use of these instruments for the more complex purpose of extracting relative sky radiances. An all-sky camera (SONA202-NF model) with three colour filters narrower than usual for this kind of cameras is configured to capture raw images at seven exposure times. A detailed camera characterization of the black level, readout noise, hot pixels and linear response is carried out. A methodology is proposed to obtain a linear high dynamic range (HDR) image and its uncertainty, which represents the relative sky radiance (in arbitrary units) maps at three effective wavelengths. The relative sky radiances are extracted from these maps and normalized by dividing every radiance of one channel by the sum of all radiances at this channel. Then, the normalized radiances are compared with the sky radiance measured at different sky points by a sun and sky photometer belonging to the Aerosol Robotic Network (AERONET). The camera radiances correlate with photometer ones except for scattering angles below 10∘, which is probably due to some light reflections on the fisheye lens and camera dome. Camera and photometer wavelengths are not coincident; hence, camera radiances are also compared with sky radiances simulated by a radiative transfer model at the same camera effective wavelengths. This comparison reveals an uncertainty on the normalized camera radiances of about 3.3 %, 4.3 % and 5.3 % for 467, 536 and 605 nm, respectively, if specific quality criteria are applied.
Highlights
The knowledge of sky radiance is a fundamental problem of the radiative transfer in the atmosphere or other media where absorption, emission and scattering processes occur (Coulson, 1988)
The spectral sky radiance reaching Earth’s surface under cloud-free conditions is basically the solar irradiance scattered by gases and aerosols; the knowledge of the spectral sky radiance at different angles is a footprint of the aerosol properties; it implies that the sky radiance contains useful information that can be used for the retrieval of aerosol optical and microphysical properties (Nakajima et al, 1996; Dubovik and King, 2000)
The ratio of two broadband measurements which are taken under different conditions but with the same instrument is equal to the ratio of the same measurements taken with an instrument which is only sensitive at the effective wavelength (Kholopov, 1975)
Summary
The knowledge of sky radiance is a fundamental problem of the radiative transfer in the atmosphere or other media where absorption, emission and scattering processes occur (Coulson, 1988). Restricted to the case of solar radiation in the atmosphere–surface of Earth, sky radiance depends on the Sun’s position in the sky, and its angular distribution is mainly controlled by the light scattering caused by atmospheric gases through Rayleigh scattering (responsible for the colour of the blue sky under clear conditions) and caused by aerosols and clouds through Mie scattering. Mainly those used by satellite platforms, are based on upward sky radiance measurements formed by the radiation reflected by Earth’s surface and scattered by the atmosphere, allowing us to determine the different atmospheric compounds
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