PNNL

Abstract

A ratio method for estimating cloud fractional cover from spectral radiation measurements has been developed. It is based on spectral characteristics of clouds and clear-sky aerosols to partition sky fraction. As illustrated in sensitivity study and demonstrated in real measurements, the transmittance ratio at selected wavelengths is insensitive to solar zenith angle and major atmospheric gaseous absorption. With a localized baseline procedure, retrievals of this ratio method are independent of absolute calibration and weakly sensitive to changes of cloud and aerosol optical properties, and thus substantially reduce the retrieval uncertainty. The uncertainty of this ratio method, estimated through sensitivity study and intercomparison, is less than 10%. With globally deployed narrowband radiometers, this simple ratio method will substantially enhance current capability of monitoring cloud fractional cover in large geographic distribution.