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 Δ 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 Δ Z corresponds to an optical depth of ∼1, which based on observed ice‐particle sizes yields an average cloud‐top IWC of ∼0.015 gm −3 , a value consistent with in situ measurements. The analysis indicates potential for deriving cloud‐top IWC using dual‐satellite data.