Transmittance of a cloud is wavelength-dependent in the UV-range. Physical interpretation

In the recently published GRL paper by Seckmeyer et al. [1996] an example of a cloud that has a wavelength dependent effect on the surface UV-radiation is given. Through careful and accurate radiative transfer modelling, the present paper aims to give a physical interpretation of the reported wavelength dependence of this particular cloud. The modelling shows that the transmission of the cloud alone does not vary significantly with wavelength in the UV. However, the cloud gives a wavelength dependent effect in the surface UV-radiation due to radiation scattered upwards from the cloud and then scattered downwards again, effectively trying to make it through the cloud more than once. The number of photons this happens to is a function of the wavelength dependent Rayleigh scattering and ozone absorption cross sections.