A global view of midlevel liquid-layer topped stratiform cloud distribution and phase partition from CALIPSO and CloudSat measurements

The first 2 year measurements from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar and CloudSat radar were analyzed to study the distribution and phase partition of midlevel liquid-layer topped stratiform clouds (MLTSC, top higher than 2.5 km above the Earth’s surface and top temperature warmer than -40°C) globally. The global mean MLTSC occurrence was ~7.8% and the global mean MLTSC percentage fraction related to all midlevel clouds was ~33.6%. Strong seasonal and day-night variations of MLTSC occurrence were observed over different latitude regions. In the polar regions, the maximum occurrence was in summer, while the minimum occurred in winter, with small day-night differences. In the tropics, a high MLTSC occurrence band shifted southward from June – July – August to December – January – February with significantly more MLTSC during the nighttime. The global mean MLTSC top height and temperature were ~4.5 km above the surface and -13.6°C. Overall, 61.8% of MLTSCs were mixed phase and 12.4% were supercooled liquid (contains only liquid phase or with ice below the detection limit). The fraction of mixed-phase MLTSC increased as the cloud top temperature decreased, with a sharp increase between -10 and -15°C and a noticeable latitude difference. This temperature dependence indicated that ice nucleation is active at -10°C in these clouds. The global mean ice water path (IWP) of mixed-phase MLTSCs, estimated based on an empirical temperature – radar reflectivity – ice water content relationship, was ~13.4 g/m2, and the IWP increased as cloud top temperature decreased. To improve MLTSC parameterizations in global climate models, further studies are needed to better understand the latitude dependence of MLTSC distributions and microphysical properties and how aerosol and water phase cloud properties affecting ice generation in MLTSCs.