While pure water clouds cause the normalized transmission to decrease with wavelength from 320 nm through the visible, aerosol particles cause the opposite behavior. The results show that when clouds are superimposed on an aerosol profile with the cloud drops and aerosol particles externally mixed, the shape of the normalized transmission spectrum is dominated by the effect of the cloud drops, unless the optical depth of the aerosols begins to approach the optical depth of the cloud. This is the case for an optically thin stratus cloud and an urban aerosol profile. When cloud drops and aerosol particles are internally mixed through coagulation, the shape of the normalized transmission spectrum is again dominated by the effect of the cloud drops, unless there is an unrealistically high volume fraction of strongly absorbing aerosols embedded in the droplets. The model results are also shown to be consistent with Brewer spectrophotometer irradiance measurements under cloudy and hazy conditions.
Special attention focuses on the spring and summer seasons.
An algorithm for estimating biologically active UV irradiance from satellite measurements of ozone and reflectivity is presented. In contrast to previous work, this technique treats bulk radiative properties of clouds explicitly, assuming that clouds are reflecting surfaces. To address this issue, an Eddington radiative transfer model is used to calculate not only ground-level irradiance, but also an estimate of the backscattered radiance in the nadir direction using the formal solution to the radiative transfer equation. While this model does not simulate detailed angular distributions of radiation scattered off clouds, it is found that this model retrieves average measured biologically active surface UV irradiance around solar noon quite well and that different combinations of parameters yield strikingly similar estimates of surface UV irradiance.