Estimation of Cirrus Optical Thickness from Sun Photometer Measurements

Abstract

A method is proposed to estimate the optical thickness of cirrus clouds from ground-based sun photometry. Transfer calculations of solar radiation in ice clouds were made by the Monte Carlo method. A scattering phase function presented by Takano and Liou was employed for ice clouds. Simulations of sun photometry, which include strong forward scattering into the instrument's field of view, give a simple relationship between the true and apparent optical thicknesses. The correction method was applied to Sun photometer measurements for cirrostratus clouds observed at Tsukuba, Japan. The relationship between the visible optical thickness and the broadband solar flux transmittance obtained from observations agreed well with that theoretically expected for cloud optical thickness up to about 10.