Pf. Coley et Pr. Jonas, THE INFLUENCE OF CLOUD STRUCTURE AND DROPLET CONCENTRATION ON THE REFLECTANCE OF SHORTWAVE RADIATION, Annales geophysicae, 14(8), 1996, pp. 845-852
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.