Spectral and total albedo to solar radiation of ice and water clouds - experimental results from ASPIRE

Abstract

An extensive experiment (the Aspendale ISCCP Regional Experiment) to study the radiative and microphysical properties of mixed-phase clouds yielded aircraft profiles from 3 fully iced cirrus decks, 2 liquid water decks of stratocumulus and 2 mixed-phase decks of altostratus. The short-wave solar radiation results are presented here. The reflection spectra from 1.1 to 2.5µm suffered badly from instrumental noise and the detail of infrared absorption could not be examined. However, the broadest average of cloud reflectivity as a function of wavelength was consistent with the theoretical work of Wiscombe et at. (1984). Analysis of total spectrum albedo A as measured by Eppley pyranometers yielded fairly consistent relations between A and total water or ice path W for solar zenith angles of the order of 60º: namely,

A = 0.07+0.037(logW)²

for the stratocumulus, and

A = 0.0079(logW)²

for the cirrus. The albedo of the mixed-phase altostratus sat nicely between the two extremes as an apparently simple function of the fraction of liquid water to the total. The unexpected consistency of the cirrus relation is discussed and attributed to the relatively constant mean particle radius (≈ 200µm) of the clouds examined. A parameterization to take account of mean effective particle radius is proposed.