Temporal structure of the solar radiation field in cloudy conditions: Are retrievals of hourly averages from space possible?

The authors examine the temporal variability of measured 415- and 611.7-nm spectral solar irradiance in cloudy stratocumulus conditions. This is accomplished by normalizing measured data by the equivalent irradiance for cloudless conditions and the same solar zenith angle. Spectral and other analyses of the time series exhibit fractal behavior in agreement with the multifractal model of Schertzer and Lovejoy. A three-dimensional cloud model with dimensions of 6 km X 38 km is constructed that has these fractal properties, and a Monte Carlo radiative code is applied to obtain irradiances in 50-m grid elements at the surface. Model output is used to test the ability of satellites to calculate hourly irradiance with one, two, three, four, five, and six observations per hour. Root-mean-square errors are substantial-between 17% and 43% for one single satellite observation per hour. The smallest errors of around 5% are obtained with six scans per hour. These results argue that a higher frequency of satellite observations is needed to estimate hourly surface solar irradiances with acceptable accuracy.