The distribution of solar radiation in the earth's atmosphere: The effects of ozone, aerosols and clouds

Abstract

We have developed a detailed model of solar radiation in the atmosphere as it is affected by atmospheric constituents, aerosols, clouds and the surface characteristics of the earth. Such a model is the foundation for studying global change and atmospheric chemistry under natural and disturbed conditions. The model includes radiative transfer processes for solar ultraviolet and visible wavelengths (290 ∼700 nm) under different environmental conditions. It calculates the optical properties of aerosols and cloud droplets as well as the direct, diffuse, net, and actinic fluxes for different wavelengths, altitudes, and zenith angles at a relatively high computational speed and accuracy. It only takes about three and a half minutes to calculate all the optical properties and radiative fluxes in a cloudy air (including all the properties and fluxes in 100 sub-layers inside a cloud), and about 20 seconds in a clear sky and clean air condition at a SUN SPARCStation 10 50 (with single SPARC CPU running at about 50 MHz). We show that local environmental conditions, particularly in the lower atmosphere, can greatly alter the actinic flux throughout the atmosphere. This feature is especially apparent in the wavelengths with weak or no O 3 absorption, as multiple scattering dominates the atmospheric radiative transfer. Compared to the actinic flux under clear sky and clean air conditions, for example, the actinic flux around 400 nm at zero zenith angle decreases by a factor of 5 at the earth's surface while increasing by more than 100% at the top of the atmosphere when a one-km altostratus cloud is added to the middle troposphere. According to our calculations, the radiation field outside a cloud is mainly controlled by the total liquid water content of the cloud; however, the actinic flux inside a cloud is very sensitive to the macro structure of the cloud. Readers may acquire the computer model from the authors.