Estimations of Cloud Optical Thickness from Ground-Based Measurements of Incoming Solar Radiation in the Arctic

Abstract

A technique for evaluation of cloud optical thickness (plant-parallel, homogeneous layer) from ground-based measurements of incoming solar irradiance using a simple radiation model is introduced. The sensitivities of downward and upward fluxes of shortwave irradiance to surface albedo and the equivalent radius of cloud drops are analyzed. We show that the incoming solar irradiance at the surface during overcast conditions depends primarily on cloud optical thickness and surface albedo. The feasibility of using the integral shortwave albedo of the snow surface for shortwave flux determination is demonstrated. Cloud optical thickness (τ), derived from hourly mean values of incoming and reflected solar irradiance measured at the surface during April-August 1988 at Barrow, Alaska, is presented. The statistical characteristics of τ are provided on a mouth-by-month basis. These show that cloud optical properties vary significantly throughout the season, reflecting the varying physical properties of the air man from which they originated.