Application of CALIOP Measurements to the Evaluation of Cloud Phase Derived from MODIS Infrared Channels

In this study, Moderate Resolution Imaging Spectroradiometer (MODIS) infrared-based cloud thermodynamic phase retrievals are evaluated using Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) retrievals for the 6 months from January to June of 2008. The CALIOP 5-km cloud-layer product provides information on cloud opacity, cloud-top height, midlayer cloud temperature, and cloud thermodynamic phase. Comparisons are made between MODIS IR phase and CALIOP observations for single-layer clouds (54% of the cloudy CALIOP scenes) and for the top layer of the CALIOP scenes. Both CALIOP and MODIS retrieve larger fractions of water clouds in the single-layer cases than in the top-layer cases, demonstrating that focusing on only single-layer clouds may introduce a water-cloud bias. Of the single-layer clouds, 60% are transparent and 40% are opaque (defined by the lack of a CALIOP ground return). MODIS tends to classify single-layer clouds with midlayer temperatures below -40 degrees C as ice; around -30 degrees C nearly equally as ice, mixed, and unknown; between -28 degrees and -15 degrees C as mixed; and above 0 degrees C as water. Ninety-five percent of the single-layer CALIOP clouds not detected by MODIS are transparent. Approximately 1/3 of transparent single-layer clouds with temperatures below -30 degrees C are not detected by MODIS and close to another 1/3 are classified as ice, with the rest assigned as water, mixed, or unknown. CALIOP classes nearly all of these transparent cold clouds as ice.