THE INFLUENCE OF CLOUD STRUCTURE AND DROPLET CONCENTRATION ON THE REFLECTANCE OF SHORTWAVE RADIATION

The effects of cloud shadowing, channelling, cloud side illumination a nd droplet concentration are investigated with regard to the reflectio n of shortwave solar radiation. Using simple geometric clouds, coupled with a Monte Carlo model the transmission properties of idealized clo ud layers are found. The clouds are illuminated with direct solar radi ation from above. The main conclusion reached is that the distribution of the cloud has a very large influence on the reflectivity of a clou d layer. In particular, if the cloud contains vertical gaps through th e cloud layer in which the liquid water content is zero, then, smaller more numerous gaps are more influential on the radiation than fewer, larger gaps with equal cloud fraction. At very low solar zenith angles channelling of the radiation reduces the reflection expected on the b asis of the percentage cloud cover. At high solar zenith angles the il lumination of the cloud edges significantly increases the reflection d espite the shadowing of one cloud by another when the width of the gap s is small. The impact of droplet investigated. It is found that at lo w solar zenith angles concentration upon the reflection of cloud layer s is also where channelling is important, the lower concentrations inc rease the transmission. Conversely, when cloud edge illumination is do minant the cloud distribution is found to be more important for the hi gher concentrations.