Evaluation of ice cloud parameterizations in general circulation models require accurate determination of ice water path (IWP) and ice particle size at regional scales across the globe. Existing satellite remote sensing techniques for cirrus, such as solar reflectance or infrared emission methods, measure ice water path rather indirectly and with limited accuracy. Submillimeter-wave radiometry is an independent method of cirrus remote sensing based on ice particles scattering the upwelling radiance emitted by the lower atmosphere. The submillimeter brightness temperature reduction depends strongly on wavelength, and hence multiple frequencies may be used to retrieve IWP and median mass diameter (D_me).

The Far Infrared Sensor for Cirrus (FIRSC) operated autonomously on the Proteus high altitude aircraft in an experiment above the Oklahoma ARM site in December 2000. The FIRSC employs a Fourier Transform Spectrometer (FTS), which measures the upwelling radiance from 10 to 33 cm^-1 (300 to 990 GHz) at 0.1 cm^-1 resolution. Adequate sensitivity for nadir only viewing is obtained with a cryogenically cooled bolometer. Retrievals of cirrus cloud IWP and D_me will be shown and comparisons made with radar reflectivity from the ARM millimeter wavelength radar.

There are two developing technologies that could be used for space-borne submillimeter radiometers: far infrared bolometers with spectral filters and microwave heterodyne receivers. The relative advantages of these two technologies and their current space readiness will be discussed.