Spectral properties of backscattered UV radiation in cloudy atmospheres

[1] We use a Plane-Parallel Cloud (PPC) model to illustrate how Mie scattering from cloud particles interacts with Rayleigh scattering in the atmosphere and produces a complex wavelength dependence in the top-of-the-atmosphere (TOA) reflectances measured by satellite instruments that operate in the ultraviolet (UV) part of the spectrum. Comparisons of the PPC-model-derived spectral dependence of reflectances with the Total Ozone Mapping Spectrometer ( TOMS) measurements show surprisingly good agreement over a wide range of observational conditions. The PPC model results also are compared with the results of two other cloud models: Lambert Equivalent Reflectivity ( LER) and Modified LER (MLER) that have been used to analyze satellite data in the UV. These models assume that clouds are opaque Lambertian reflectors rather than Mie scattering particles. Although one of these models ( MLER) can be adjusted to agree reasonably well with the TOMS data, the adjustments are somewhat arbitrary and may not be suitable for interpreting satellite data if one desires high accuracy. We also use the PPC model to illustrate how clouds can perturb tropospheric O-3 absorption in complex ways that cannot be explained by models that treat them as reflecting surfaces rather than as volume scatterers.