NASA - National Aeronautics and Space Administration

Ice crystal shapes in tropical ice clouds are estimated with two different remote sensing methods and compared with measurements from an in situ cloud aerosol spectrometer (CAS) during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE) campaign conducted in Florida during July 2002. The remote sensing techniques use dual-satellite reflectances and lidar linear depolarization rates. The ice crystal shape is derived from CAS measurements of forward and backscattered light from individual particles in the size range from 1 to 45 mm. The remote sensing and in situ retrievals are based on ice crystal optical models, which incorporate the scattering phase functions integrated over the collection angles used by the CAS, the view angles from dual-satellites retrievals, and the complete scattering matrix for lidar. Owing to the space and time collocation constraint between in situ and remote sensing techniques, data from only 1 day are used to evaluate the dual-satellite technique (11 July) and from 3 days for the lidar (23, 26, and 29 July). Data from 23 and 29 July are also used to compare the two remote sensing techniques. In total, 40 shape retrievals were obtained for 20 different cloud areas, allowing paired comparisons of the methods. The results show consistent particle shapes for half of the cloud areas studied. The discrepancies for the other cases can be explained by insufficient spatial-temporal collocations of the data or limitations of the CAS that constrain its range to particles <45 mm, whereas the remote sensing techniques are influenced by particles outside the size range of the CAS.