Modeling of gaseous, aerosol, and cloudiness effects on surface solar irradiance measured in Brazil's Amazonia 1992-1995

The effects of water vapor, aerosol, and cloudiness on the incident surface solar irradiance continuously measured in Brazil's Amazonia at six observa tional sites from 1992 to 1995 are examined by means of a clear-sky broadba nd radiative transfer model. The aerosol optical depth and precipitable wat er, both retrieved from Sun photometer measurements, serve as inputs to the model. Computed monthly mean values of clear-sky surface irradiance are an alyzed in conjunction with the monthly mean values of all-sky surface irrad iance measured on the ground. To assess the effect of cloudiness, we presen t the cloud radiative forcing and cloud radiative forcing ratio at the surf ace, both widely employed in cloud radiation budget studies, By its definit ion the monthly mean cloud radiative forcing (cloud radiative forcing; rati o) is the difference (ratio) between surface solar irradiances under all-sk y an clear-sky conditions. The analysis of the irradiances, as computed and as measured, shows that during the wet season the gaseous and cloudiness e ffects on the! solar radiation attenuation in the atmosphere are comparable , while the aerosol influence is much smaller. The aerosol effect increases and cloudiness effect decreases in the second half of the dry season. Thus during the biomass burning period ill southern Amazonia, the water vapor a nd aerosol effects become comparable, while the cloudiness impact is 2-3 ti mes smaller. Both cloudiness and aerosol effects have strong seasonal varia tions, while the gaseous effect changes slightly throughout the year.