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

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 S-1.67 (in percent), which is defined as the ratio of the difference between the spectral reflectivities at 1.70 and 1.64 mum to the reflectivity at 1.64 mm. Radiative transfer calculations for different cloudy atmospheres over ocean are presented to show that S-1.67 approximate to 0 for water clouds and S-1.67 > 0 for ice clouds and mixed-phase clouds. It is shown that S-1.67 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-mum (atmospheric window) and 1.38-mum (water vapor absorption band) images, is well reproduced by thresholding S-1.67 according to the following scheme: S-1.67 less than or equal to 2%, water cloud; 2% < S-1.67 < 10%, optically thin ice cloud; and S-1.67 greater than or equal to 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 S-1.67 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 mum, 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.