Fourier transform infrared (FTIR) spectroradiometer measurement of cloud radiative and microphysical properties

Abstract

Spectrally resolved radiometric measurements of middle infrared atmospheric emission can be used in conjunction with detailed radiative transfer calculations to retrieve cloud emissivity, and to estimate cloud liquid water path (LWP), optical depth, and equivalent radius of the droplet size distribution. Using a discrete-ordinates radiative transfer formulation, an algorithm has been developed to retrieve these cloud properties from FTIR data. The algorithm has been successfully applied to a four month Antarctic data set provided by the CalSpace FTIR Spectroradiometer. Radiative transfer calculations were performed to estimate spectral cloud emissivity for a range of cloud optical depth, liquid water content, and equivalent radius, sufficient to bracket values expected in the field. These calculations made use of bi-modal droplet size distributions actually observed in Antarctic clouds. Using a least- squares algorithm, a theoretical cloud emission spectrum is chosen which best reproduces a given measured brightness temperature spectrum. The results show marked differences in cloud emissivity between high and low overcast layers, and between clouds with and without precipitation. The results also suggest that the emissivity of a maritime Antarctic cloud deck should be smaller for a given LWP than the parameterization frequently used in general circulation models.