Stochastic Model for Shortwave Radiation Transfer in a Multiple-Layered Atmosphere: Surface Global Radiation in Cloudless Conditions

A two-flux method is presented, which describes propagation of solar radiation in the atmosphere as a random walk of diffuse photons among several atmospheric layers. Results are obtained in terms of absorption probabilities in each layer, at ground and within sky, allowing to easily introduce and to analyze influence of profiles of the main atmospheric absorbers (ozone, aerosols and water vapor). Global radiation is obtained by integration of monochromatic irradiances. Application of this model to cloudless situations in the extreme cases of rural environment and high aerosol load by burning biomass shows good results when compared with SBDART code (they have a systematic deviation of +10 W.m(-2)), and both differ from ground measurements of global radiation within 30 to 50 W.m(-2). This difference could be lowered having a better definition of aerosol load during daytime. It is observed that the stochastic model performance is five times faster than SBDART.