A study of Antarctic clouds using CloudSat and CALIPSO measurements

Antarctic clouds play an important role in the Antarctic climate and global change issues through their influence on the hydrological and radiative balance. However, harsh climatic conditions, perennial ice cover and very low surface temperatures in the region have affected reliable observations of the Antarctic clouds using ground-based and passive satellite remote sensing measurements. Satellite-based active remote sensors, which are not significantly affected by surface ice cover and low temperatures, provide better observation capabilities in the region. Radar and lidar aboard NASA A-train CloudSat and CALIPSO satellites provide unprecedented opportunities to study Antarctic clouds. Here we use CloudSat and CALIPSO measurements from June 2006 to August 2008 to study the seasonal and inter-annual variation of Antarctic cloud physical and microphysical properties. CloudSat level 2 2B-GEOPROF data are used to identify cloudy bins (cloud mask >30) and their reflectivity factors. CALIPSO level 1 data products are collocated and averaged to CloudSat horizontal resolution and 180-m vertical resolution. Due to different sensitivities and attenuation of CloudSat radar and CALIPSO lidar measurements, CloudSat and CALIPSO cloud masks are combined to describe cloud vertical distribution over the Antarctic region. Based on cloud base height and thickness, clouds are classified as high, middle and low level clouds as well as deep clouds for statistical analyses. The seasonal variations of their occurrence, thicknesses and mean radar reflectivity factors and the zonal mean vertical distribution of cloud are analyzed. The results show a marked seasonal variation in cloud occurrence and their physical properties. The western part of Antarctica has higher cloud occurrence than the eastern part. Low-level clouds are the major cloud type throughout the year and have a mean annual occurrence of 44 % with 49.5±2.1 and 38.1±2.1 % for summer (December – February) and winter (June – August) respectively. However, high-level clouds also contribute significantly to total cloud cover in the interior continental Antarctica. The mean cloud thickness for high clouds and deep clouds show significant seasonal dependency with thicker clouds in late winter/early spring (July – September) than in late summer/early fall (January – March). The mean thickness for January-March and July-September for high clouds are 1.0±0.2 and 1.9±0.3 km respectively. Similarly, the mean thickness for deep clouds are 6.4±0.2 and 7.0±0.3 km for January-March and July-September respectively. The distribution of mean maximum radar reflectivity factor also shows marked seasonal variations with larger values in summer months than in winter months. There is a higher occurrence of high-level clouds in the winter season than in summer; however, summer season has the high-level clouds with larger crystal sizes than the winter season.