Spectral imaging of clouds using a digital array scanned interferometer

Spectral images of various cloud fields have been made from the ground using a prototype imaging interferometer. The instrument, called DASI (digital array scanned interferometer), is under development as an alternative technique for terrestrial remote sensing. Our paper describes the unusual characteristics of DASI's that make them promising candidates for both ground and aircraft based measurements of clouds. These characteristics include superior signal-to-noise potential, design simplicity and compactness. The prototype DASI had a wavelength sensitivity range of 4550-9090 cm^-1 (1.1-2.2 μm), a spectral resolution of 300 cm^-1, a spatial field of view of 5 degrees, and 256 transverse spatial elements for each exposure frame with exposure rates up to 1 frame/s. Spectrally resolved images of cirrus and cumulus clouds over the San Francisco Bay area were obtained by multiple frame scanning over the field of view. The DASI measurements are compared with radiative transfer calculations using LOWTRAN-7. Modification of the LOWTRAN code was necessary so that high spectral resolution data for ice optical properties could be used. With these modifications, the agreement between measured and model results is good in some cases. However, quantitative interpretation of these preliminary measurements was not generally possible because of various sources of uncertainty. Finally, a discussion is given on the potential applications and science yields of DASI instruments in the framework of coordinated field missions such as project FIRE. Such applications include the use of ground and airborne based DASI's to retrieve microphysical properties of clouds and to investigate current problems in atmospheric radiation.