SOME EFFECTS OF SPATIAL VARIATIONS OF WATER-CONTENT ON THE REFLECTANCE OF CLOUDS

A simple Monte Carlo model is used to calculate the effects of spatial variations of cloud liquid water content on the directional reflectance of clouds at 0.45-mu-m, neglecting the effects of vapour absorption. The variations in water content are considered to arise either from variations in the droplet size distribution or from variations in the droplet concentration, the assumed variations being representative of those observed. The results suggest that uncertainties in the directional reflectance of layer clouds in excess of 10% are due to uncertainties in the distribution of water within the cloud. Even larger uncertainties are found in the case of isolated clouds due to neglecting the region close to the cloud edge, where the water content increases from zero to its typical in-cloud value. as well as the effects of internal variations in the liquid water content.