Depolarization ratio and attenuated backscatter for nine cloud types: analyses based on collocated CALIPSO lidar and MODIS measurements

Abstract

This paper reports on the relationship between lidar backscatter and the corresponding depolarization ratio for nine types of cloud systems. The data used in this study are the lidar returns measured by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud- Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite and the collocated cloud products derived from the observations made by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard Aqua satellite. Specifically, the operational MODIS cloud optical thickness and cloud-top pressure products are used to classify cloud types on the basis of the International Satellite Cloud Climatology Project (ISCCP) cloud classification scheme. While the CALIPSO observations provide information for up to 10 cloud layers, in the present study only the uppermost clouds are considered. The layer-averaged attenuated backscatter (gamma') and layer-averaged depolarization ratio (delta) from the CALIPSO measurements show both water- and ice-phase features for global cirrus, cirrostratus, and deep convective cloud classes. Furthermore, we screen both the MODIS and CALIPSO data to eliminate cases in which CALIPSO detected two- or multi-layered clouds. It is shown that low gamma' values corresponding to uppermost thin clouds are largely eliminated in the CALIPSO delta-gamma' relationship for single-layered clouds. For mid-latitude and polar regions corresponding, respectively, to latitude belts 30 degrees -60 degrees and 60 degrees -90 degrees in both the hemispheres, a mixture of water and ice is also observed in the case of the altostratus class. MODIS cloud phase flags are also used to screen ice clouds. The resultant water clouds flagged by the MODIS algorithm show only water phase feature in the delta-gamma' relation observed by CALIOP; however, in the case of the ice clouds flagged by the MODIS algorithm, the co-existence of ice- and water-phase clouds is still observed in the CALIPSO delta-gamma' relationship.