Cloud Optical Thickness Estimation from Irradiance Measurements

Abstract Radiative transfer algorithms are developed to estimate the optical thickness of clouds using an irradiance detector located above, deep within, and beneath a cloud. Both monodirectional and diffuse illumination cases are considered. For each detector position, one algorithm is derived from asymptotic radiative transfer theory and another is derived from transport-corrected diffusion theory. The algorithms for an above-cloud or below- cloud detector with monodirectional illumination can be utilized without radiative transfer calculations and are much less sensitive to the phase function than a related algorithm for an above-cloud detector that requires measurements of downward and upward radiances. Radiative transfer calculations with the FN method are used to numerically test the above-cloud detector algorithms for monodirectional illumination for the Haze-L and Fair Weather Cumulus cloud models. Both algorithms show good agreement with the FN results for optically thick clouds; the transport-co...