We present several case studies of thermal observations of cirrus using the Along Track
Scanning Radiometer (ATSR) nadir and 55° forward view aboard ERS-1 (Earth Resources
Satellite). We examine the potential of ATSR for retrieval of thermal optical depth at. 3.7, 10.8
and 11.9 tm and cloud temperature at 11.9 jm under absorption and scattering approximations.
Ice crystal size is estimated by applying Mie theory under the assumption of spheres
and Anomalous Diffraction theory (ADT), assuming hexagonal columns, plates, rosettes and
planar polycrystals. The estimated particle sizes are compared using the two theories under
assumed conditions of absorption and scattering.
The potential use of the dual look capability of ATSR-2, with its additional visible channels
at 0.55, 0.66, and 0.865 m, and advanced ATSR (AATSR) for crystal shape and size determination
at 0.55 and 1.6 jm is demonstrated. For semi-transparent cirrus the difference in
backscattered solar radiation at nadir and forward views is sensitive to ice crystal habit which
is demonstrated using representative ice crystal phase functions (columns and polycrystals).
The sensitivity to ice crystal size is examined using the forward view at 0.5 and 1.6 ,am for
columns and polycrystals.