Cloud thermodynamic-phase determination from near-infrared spectra of reflected sunlight

Abstract

A simple method for the determination of the thermodynamic phase of clouds over ocean from near-infrared spectra of reflected sunlight is presented. The method is based on thresholding the parameter S1.67 (in percent), which is defined as the ratio of the difference between the spectral reflectivities at 1.70 and 1.64 μm to the reflectivity at 1.64 μm. Radiative transfer calculations for different cloudy atmospheres over ocean are presented to show that S1.67 ≈ 0 for water clouds and S1.67 > O for ice clouds and mixed-phase clouds. It is shown that S1.67 is sensitive to the presence of ice particles in clouds, and depends primarily on ice-cloud optical thickness and crystal size. The method is relatively independent of viewing and solar geometry because it is based on spectral absorption properties rather than scattering properties of clouds. The method is thoroughly analyzed using near-infrared reflectivity spectra made by the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) over a well-defined cloud system of stratocumulus and cirrus situated over the Pacific Ocean. The location of water and ice clouds, derived from pilot information and from visual interpretation of the 0.87-μm (atmospheric window) and 1.38-μm (water vapor absorption band) images, is well reproduced by thresholding S1.67 according to the following scheme: S1.67 ≤ 2%, water cloud; 2% < S1.67 < 10%, optically thin ice cloud; and S1.67 ≥ 10%, optically thick ice cloud. On the basis of radiative transfer calculations it is shown that the method may lead to misclassifications in cases where optically thin clouds are present over snow. It is suggested that this also applies to minerals, rocks, and (dry) soils. On the other hand, it is shown that there is no fundamental difference between S1.67 cloud-phase determination over ocean and green vegetation. It is therefore expected that the method is suitable for application to measurements made over large parts of the globe by spaceborne spectrometers, which are able to identify the shape of the reflectivity spectrum around 1.67 μm, such as the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY), which is scheduled for launch on the European Space Agency's (ESA) Environmental Satellite (ENVISAT) in 2002.