Abstract
A Monte Carlo model is used to investigate the effects on the reflectance of low cloud layers of organized cellular structure, which is often observed in such layers. It is assumed that the cloud is composed of a series of cells each of which comprises a core region surrounded by a wall region of reduced water content. It is shown that the sensitivity of reflectance to the structure is largest for wavelengths that are not absorbed by liquid water. The reflectance of the cloud layer is reduced when the water content in the regions of low water content is less than about 30% of the water content in the remainder of the layer. The effect on the cloudβlayer reflectance of organized structure in the water content is large compared with the effect of changes in the effective radius at cloud top. It is further shown that, for narrow (βΌ100 m) regions of low water content, estimations of the layer average reflectance based on the area weighted average of the reflectance of layer clouds with high and low water contents are significantly in error. For small solar zenith angles the regions of low water content tend to permit incident radiation to pass through the cloud, reducing the reflectance, while for large solar zenith angles the reflectance is increased by the effects of cloud shadowing.