Effect of remote clouds on surface UV irradiance

Clouds affect local surface UV irradiance, even if the horizontal distance from the radiation observation site amounts to several kilometers. In order to investigate this effect, which we call remote clouds effect, a 3-dimens ional radiative transfer model is applied. Assuming the atmosphere is subdi vided into a quadratic based sector and its surrounding, we quantify the in fluence of changing cloud coverage within this surrounding from 0% to 100% on surface UV irradiance at the sector center. To work out this remote clou ds influence as a function of sector base size, we made some calculations f or different sizes between 10 km x 10 km and 100 km x 100 km. It appears th at in the case of small sectors (base size less than or equal to 20 km x 20 km) the remote clouds effect is highly variable: Depending on cloud struct ure, solar zenith angle and wavelength, the surface UV irradiance may be en hanced up to 15% as well as reduced by more than 50%. In contrast, for larg er sectors it is always the case that enhancements become smaller by 5% if sector base size exceeds 60 km x 60 km. However, these values are upper est imates of the remote cloud effects and they are found only for special clou d structures. Since these structures might occur but cannot be regarded as typical, different satellite observed cloud formations (horizontal resoluti on about 1 km x 1 km) have also been investigated. For these more common cl oud distributions we find remote cloud effects to be distinctly smaller tha n the corresponding upper estimates, e.g., for a sector with base size of 2 5 km x 25 km the surface UV irradiance error due to ignoring the actual rem ote clouds and replacing their influence with periodic horizontal boundary conditions is less than 3%, whereas the upper estimate of remote clouds eff ect would suggest an error close to 10%.