A New Method to Retrieve Thin Cloud Optical Thickness from a Ratio of Scattering to Global Solar Irradiance

Abstract

hree kinds of ?visible? cloud optical thickness τ?matching shortwave direct, global, and scattering solar irradiances (Ids, Igs and Iss)?are defined, which are marked as τd, τg, and τs, respectively. It is found from radiation calculations that a ratio of Iss to Igs in the small-τ case has a unique characteristic: strong sensitivity to τ but weak sensitivity to the cloud scattering phase function. On the basis of this characteristic, a method to retrieve Iss-equivalent τs from the ratio is proposed. This method is validated by way of simulation and application tests, in which the Discrete Ordinate Radiative Transfer model (DISORT) is used to calculate irradiances. As shown in simulations with τ < 2, there may be unrealistically negative or grossly overestimated τ values from Igs, owing to the difference between τs and τd, while the new method can lead to a very good agreement of τs retrieval with its input. Furthermore, this method is used to retrieve small τ from the pyrheliometer and pyranometer measurements in Lhasa during 2006. It is found that τ retrieved from Igs was often negative because of cloud inhomogeneity, while the application of the new method resulted in stable yet reasonable τs values. The Iss calculations using 1293 sets of τs retrievals fit well into the Iss determinations from pyrheliometer and pyranometer measurements with an annual-mean deviation of 0.18%, but the deviation was raised to 46.4% when using τg retrievals.