Cirrus cloud classified by the split window and CALIOP observation

A simple split window (11 and 12 ƒÝm) technique to identify semi-transparent cirrus clouds was validated with observations by cloud lidar (Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP)) onboard CALIPSO. Unlike other clouds, high-level thin cirrus clouds which consist of ice crystal play different and important role in terms of radiation budget. Cloud amount of cirrus clouds shows different values depending on retrieval methods. Validation of cloud retrieval algorithm is one of the important issues in cloud climate study. Inoue (1985, 1989) demonstrated that the split window was effective in classifying cirrus clouds utilizing the brightness temperature difference (BTD=TBB11 -TBB12). Inoue and Ackerman (2002) validated the cirrus clouds classified by the split window using the Earth Radiation Budget Experiment (ERBE) data. With the advent of cloud lidar observation from space, we can retrieve 3D structure of cloud properties, especially optically thin clouds. In this study, we aim to compare cirrus clouds classified by the simple split window method with CALIOP observation. The split window data of MODIS onboard AQUA were used in this study, since time difference of observation is about 15 seconds between AQUA and CALIPSO. Here, we confined the study area over the tropical ocean area (30N-30S). We found that the 85% of thin cirrus clouds classified by the split window corresponded to the cloud signal higher than 8km in CALIOP observation. Some multiple layer cases of thin cirrus over thin cirrus indicate larger BTD, while some cirrus over low-level clouds indicate smaller BTD compared with single layer cirrus clouds.