Evaluation of AIRS Cloud-Thermodynamic-Phase Determination with CALIPSO

Atmospheric Infrared Sounder (AIRS) infrared-based cloud-thermodynamic-phase retrievals are evaluated using the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations ( CALIPSO ) cloud thermodynamic phase. The AIRS cloud phase is derived from spectral information contained within the 8–12- μ m window, and CALIPSO provides coincident pixel-scale observations of cloud phase using the depolarization capability of the 532-nm channel. Comparisons are performed between the AIRS and CALIPSO cloud-phase observations for single-layer (48.5% of all clouds), heterogeneous-layer (45.9%), and multilayered (5.6%) clouds. The AIRS ice phase is in agreement with CALIPSO for more than 90% of coincident observations globally, with the largest discrepancies found in high latitudes and multilayered clouds. AIRS water phase generally follows CALIPSO spatial patterns, but the frequency is lower by about a factor of 2. The ice and water phases of AIRS both show misclassifications about 1% of the time when compared with CALIPSO . Not all clouds demonstrate strong phase signatures in the AIRS spectrum, which leads AIRS to classify unknown phase to around 10% of CALIPSO ’s ice clouds and 60% of CALIPSO ’s water clouds. This study shows that the algorithm is capable of detecting ice clouds within the AIRS field of view and can be used as the first step in further retrievals of ice-cloud optical thickness and effective particle size.