Ring Effect Studies for a Cloudy Atmosphere Using Gome Data

The fact that the depth of solar Fraunhofer lines in scattered light is less than those observed in direct sunlight, was discovered by Shefov [1959] [17] and Grainger and Ring [1962] [6] and is known as the ”Ring Effect” or ”Filling-in”. Several publications analysed this effect and its origins, showing that rotational Raman scattering provides the dominant contribution to the Ring Effect [1, 10, 4, 5, 8, 3, 18]. The majority of these studies however concentrated on cloud-free conditions. Detailed investigations of the effect of trace gas absorption and particle and cloud scattering requires a radiative transfer model, including rotational Raman scattering. Vountas et al. [1998] [18] introduced a new method to calculate Ring spectra by multiple scattering radiative transfer calculations, using the radiative transfer model SCIATRAN (formerly GOMETRAN) [16]. Vountas et al. [1998] demonstrated that by taking rotational Raman scattering into account within the radiative transfer processing, including trace gas absorption and aerosol and Rayleigh scattering, the filling-in of Fraunhofer and gas absorption features, resulting from rotational Raman scattering, explains to a high accuracy the Ring effect for cloud-free conditions. SCIATRAN is also able to model clouds as layers and as reflecting boundaries [12]. The modelling accounts for scattering mechanisms like Rayleigh-, Mie-, and Raman-scattering, and as a consequence influences on the slant path of the light (e.g. due to multiple scattering), strongly affecting filling-in of spectral structures. Excluding horizontal inhomogenities and polarisation, which can not yet be modelled with SCIATRAN, this enables the Ring Effect arising from clouds to be extensively investigated for both ground and satellite geometry. In this study radiances between 390 nm and 400 nm were calculated including Ring in the cloudy atmosphere, and were compared to those from satellite borne observations where distinct Ring features appear. An investigation of the sensitivity of the retrieval of NO2 from modelled spectra using the widely spreaded DOAS (Differential Optical Absorption Spectroscopy) technique [15] has been used to investigate different cloud scenarios with varying cloud types and heights. The objective of the study is to support that our current understanding of physical processes, which determine the Ring Effect, is adequate to explain the observations from satellite spectrometers under cloudy conditions. Further such investigations are needed to gain a more general picture of the Ring Effect in the presence of clouds.