Estimating the top altitude of optically thick ice clouds from thermal infrared satellite observations using CALIPSO data

The difference between cloud-top altitude Z(top) and infrared effective radiating height Z(eff) for optically thick ice clouds is examined using April 2007 data taken by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and the Moderate-Resolution Imaging Spectroradiometer (MODIS). For even days, the difference Delta Z between CALIPSO Z(top) and MODIS Z(eff) is 1.58 +/- 1.26 km. The linear fit between Z(top) and Z(eff), applied to odd-day data, yields a difference of 0.03 +/- 1.21 km and can be used to estimate Z(top) from any infrared-based Z(eff) for thick ice clouds. Random errors appear to be due primarily to variations in cloud ice-water content (IWC). Radiative transfer calculations show that Delta Z corresponds to an optical depth of similar to 1, which based on observed ice-particle sizes yields an average cloud-top IWC of similar to 0.015 gm(-3), a value consistent with in situ measurements. The analysis indicates potential for deriving cloud-top IWC using dual-satellite data.