Abstract
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-dimensional radiative transfer model is applied. Assuming the atmosphere is subdivided into a quadratic based sector and its surrounding, we quantify the influence of changing cloud coverage within this surrounding from 0% to 100% on surface UV irradiance at the sector center. To work out this remote clouds influence as a function of sector base size, we made some calculations for different sizes between 10 km à 10 km and 100 km à 100 km. It appears that in the case of small sectors (base size < 20 km à 20 km) the remote clouds effect is highly variable: Depending on cloud structure, solar zenith angle and wavelength, the surface UV irradiance may be enhanced up to 15% as well as reduced by more than 50%. In contrast, for larger sectors it is always the case that enhancements become smaller by 5% if sector base size exceeds 60 km à 60 km. However, these values are upper estimates of the remote cloud effects and they are found only for special cloud structures. Since these structures might occur but cannot be regarded as typical, different satellite observed cloud formations (horizontal resolution about 1 km à 1 km) have also been investigated. For these more common cloud distributions we find remote cloud effects to be distinctly smaller than the corresponding upper estimates, e.g., for a sector with base size of 25 km à 25 km the surface UV irradiance error due to ignoring the actual remote clouds and replacing their influence with periodic horizontal boundary conditions is less than 3%, whereas the upper estimate of remote clouds effect would suggest an error close to 10%.Key words: Atmospheric composition and structure (transmission and scattering of radiation) - Meteorology and atmospheric dynamics (radiative process)
Highlights
Clouds have considerable impact on the ultraviolet (UV) radiation reaching the ground
Special remote cloud con®gurations are taken into account, ensuring maximum eects of the surface UV irradiance attenuation and intensi®cation mechanisms
The remote clouds eect (RCE) is obtained as a function of atmospheric sector base size for the two dierent solar zenith angle (SZA) 30 and 60, for the two dierent wavelengths 300 nm and 330 nm and for dierent cloud structures within the sector
Summary
Clouds have considerable impact on the ultraviolet (UV) radiation reaching the ground. Because it is impossible to specify the true horizontal boundary conditions (this would require an a priori knowledge of the radiative transfer within the whole atmosphere), they are frequently de®ned as specular or periodic. This de®nition implies an atmosphere of some regular structure, which generally is an unrealistic assumption for cloudy conditions (Zhu et al, 1992; Lee et al, 1994). In order to investigate the impact of this potential source of error on surface UV irradiance, in the current study the eect of remote clouds is systematically analyzed and quanti®ed by means of a 3d radiative transfer model (Spherical Harmonics Discrete Ordinate Method or SHDOM, Evans, 1998).
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