Total Shortwave Radiative Characteristics of Absorbing Finite Clouds

Abstract

A model which includes the effects of water vapor and droplet absorption in finite cloud radiative transfer calculations is described. The effect of absorption on the directional reflectance values of finite clouds and infinite clouds of equal optical thickness is examined in the 0.8–8.0 μm portion of the solar spectrum. Absorption of solar radiation in finite clouds is compared to absorption in equal volume elements of horizontally infinite clouds. The 0.8–8.0 μm values of directional reflectance and absorption are combined with previous research, in order to examine the impact of finite cloud radiative characteristics on the total spectral (0.3–0.8 μm), space-time averaged radiative budget of a region partially covered by finite clouds. The added contribution of the vertical cloud dimension of the finite cloud in this type of calculation may be expressed as an “effective cloud cover,” which is greater than the geometric cloud cover by the factor (1+tanθ), where θ is the solar zenith angle. Additionally, it is shown from purely geometrical considerations that the vertical extent of finite clouds, through the exaggeration of cloud-cover estimates obtained from satellite imagery, may have a significant impact on the calculated radiative budget of a region partially covered by finite clouds.