UV radiation transfer through broken cloud fields: Modeling and comparison with measurements

A method and results of calculations are presented for the solar UV radiation fluxes on the ground for different values of cumulus cloud amounts. Cloud fields are modeled by a normal random field with a fixed lower boundary. This field is incorporated into a standard stratified model of a cloudless atmosphere. The parameters of the correlation function of the field are determined depending on the cloud amount and the mean diameter and thickness of clouds. The calculation of the fluxes were calculated using the Monte Carlo method based on the construction of photon trajectories in a plane-parallel horizontally inhomogeneous atmosphere. The spectral fluxes of the total UV radiation normalized to the case of a cloudless atmosphere are obtained for different cloud amounts. A weak zenith-angle dependence of UV attenuation by clouds is noted. The calculated relative fluxes are compared with the results of many-year (1981-1990) measurements of the total UV radiation at wavelengths shorter than 380 nm at the Moscow State University (MGU) meteorological observatory. It is shown that the calculation results are in satisfactory agreement with the measured results.