Cloud ice and water vapor profile retrievals with submillimeter-wave radiometry

Submillimeter-wave radiometry is a developing technique for retrieving ice mass and particle size over a wide range of ice clouds. Submillimeter-wave cloud ice imagers are being considered for the NASA Aerosol-Cloud-Ecosystem mission and future EUMETSAT Polar System satellites. The Compact Scanning Submillimeter-wave Imaging Radiometer (CoSSIR) flew on the NASA ER-2 aircraft during the Tropical Composition, Cloud, and Climate Coupling (TC4) experiment in July and August 2007. During TC4, CoSSIR measured with 11 channels from 183 to 874 GHz, scanning conically and through nadir. Brightness temperature images show the expected increase in sensitivity to thinner ice clouds of the higher frequency channels. Significant 640 GHz polarization signatures are seen in anvil cloud regions, indicating oriented non-spherical ice particles. Comparison with cloud lidar shows that CoSSIR is sensitive enough to detect thin anvil cloud (optical depth depth around 1), though not thin cirrus with small particles.

A new Bayesian algorithm has been developed to retrieve profiles of ice water content (IWC), a mass-based particle size (Dme), and relative humidity from CoSSIR brightness temperatures and (optionally) radar reflectivity profiles. The algorithm can also incorporate infrared brightness temperatures to improve retrievals of thin cirrus. The algorithm uses a-priori statistics of ice cloud properties from in-situ optical cloud probes and radar reflectivity profiles. Retrieval evaluation with nadir viewing 94 GHz radar profiles show that submillimeter radiometry has skill at retrieving low resolution profiles of ice cloud properties, in addition to achieving good accuracy for ice water path.