The Earth's climate system is strongly affected by clouds through their influence on the incoming solar and outgoing thermal radiation. The degree of affect depends on their properties such as cloud top height, thickness and cloud phase/particle size. Unlike other clouds, cirrus is the only cloud type which has the possibility of warming the atmosphere. The study of optical properties of cirrus cloud has been performed using the passive satellite data, since the simple cloud type classification using the split window data (11 and 12 micron). Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) was launched on 28 April, 2006 together with CloudSat. The cloud lidar and cloud radar onboard these satellite open a new era in retrieving vertical profile of cloud properties.

In this study, high-level cloud classified by the simple technique using the split window was compared with cloud height observed by Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). High-level clouds including thin cirrus and cumulonimbus type clouds mostly correspond to the clouds signal higher than 8km by CALIOP observations. High-level cloud classified by the split window also reasonably agreed with the ice phase or mixed phase identified by the use of the 8.7 micron channel of SEVIRI.

The advantage of CALIOP observation is effective in identifying the multi-layer cloud. Double layer cirrus cloud observed by CALIOP corresponded to smaller brightness temperature (TBB) difference between the split window (BTD=TBB11-TBB12) than that of single layer cirrus type cloud. A simple double layer case was simulated by radiation code of RSTAR. It indicates smaller BTD, which corresponds to N-type cloud in the split window cloud type classification method (Inoue, 1987). Some of the N-type cloud by the split window was suggested as multi layer cloud from both CALIOP observations and simulation.